Might try opening the mix screws 1/2 turns to see if goes away. At least you’ll know what direction to go next.
Try raising the floats to get more overlap on the idle/booster transition.
The Power valve might be fluttering.

 

I will try the idle mixture screws and see how it does. As far as the floats goes they are as high as I can go as I have the sight plug type bowls on right now and it is set to where when you take the plug out and its running it is just barely dribbling out the site hole when you just faintly bump the vehicle. Also on the power valve I don't know how it would be that as this does not happen on the highway and its only under high vacuum conditions when its doing it especially in town. I can't feel it happening while slowing down coming off at high speeds like at 55 mph but only at 35 mph or less.

At 25 mph I am cruising at about 1300 rpm and am about at 1400 at 30 mph and the throttle is hardly open and I let off completely and then brake and as the rpm drops its smooth for a second then studders rough for about two seconds then is completely smooth and fine. Then I take back off and no issues getting back up to speed regardless if I floor it or do it slowly getting back up to 25 to 30 mph. I did try the mixture screws at 1.5 turns out and it did not difference but I will try at 1 3/4 out and see. Normally the 1 1/4 setting has been the best setting for the leanest wise without effecting anything else such as going into gear and the idle in park and off idle.

I will hook up my vacuum gauge and see what my reading is and see how that does as well as I have not hooked it up yet but normally I am fine without using one for setting the idle as I go by feel and response and it has never failed me yet. Thanks johnsongrass1 appreciate your time.

 

You only have 3 circuits to work with. I have a feeling that the idle and boosters aren’t overlapping enough and it going lean. You don’t feel this on throttle because the squirted is helping cover it.
Floats at the bottom of the plug is just a generic starting point and not an absolute. The higher the level the sooner the boosters pick up the fuel. That’ll help the transition by starting it sooner even when off throttle.
Power valves aren’t on or off. They aren’t switches. The gradually open and gradually close. The rating is the starting point in which they are supposed to start opening. If you get a tester and start checking then you’ll find cheap ones all over the place as far as rating goes. I’m suggesting the power valve is fluttering under higher vacuums and adding fuel when it shouldn’t be.
Do what ya want with this, it’s just another opinion and certainly not trying to start an argument but instead get you thinking about the carb functions in more detail since the problem is very specific.

 

If you are running a vacuum advance, I am wondering if the high vacuum at Decel is pulling too much advance in??
Just a theory, if you don't have a vac advance limiter set-up on it.
Or similarly, it pulls so much advance in that you get spark scatter or a rotor/reluctor phasing issue maybe??

Carbs get blamed for a lot of things that end up being ignition related.

 

I had the same thing happen.
figured out it was the vac pot hooked to manifold vac and the cure was by simply swapping over to ported Vac.

 

Seems like a lot of vacuum for that cam! What RPM is that reading taken at?

Bogie

 

That reading of the vacuum is in park with 900 rpm idle. Setting the idle lower and the engine does not like it and this was on the previous 377 I had as well even with a victor intake. I re read my notes and I had jotted down that my last vacuum reading was 17 inches and I must have did a typo. Yes I do have a vacuum advance limiter plate on my hei distributor and is limited to about 12 degrees give or take a hair.

As far as the power valve goes I installed a new one and I have a power valve tester tool and tested it with my vacuum hand pump and it starts to open up pretty close to its rating and held just fine. I agree johnsongrass1 and am not trying to sound like I am arguing here as strange things have happened with carbs before that one would not think it could happen. I know that float setting is not etched in stone as well and only a guideline on settings etc just like the mixture screws.

As far as the power valve fluttering I would not know really how to know if its that unless I change it out. My power valve still seems to be good as I know if one is normally blown it would cause an excessive rich idle mixture and I by turning in my idle mixture screws will kill my engine pretty quick and it will shake and quite as it still gets super lean so I don't think it a problem of a blown power valve.

I do have a power valve plug and could install it temporary and see if the problem is still there and eliminate that as a possibility. As far as the distributor goes it should be fine as it worked just fine on my previous build and is not old at all even though I know that does not mean anything as stuff can go bad. I checked my plug wires and they turned out fine and I installed a new set of plugs just last week and yesterday was the first time running on them.

My previous set was from my 377 build and was low mileage and we had some first start up issues when breaking in my 350 and the engine ended up flooded and had a backfire and the timing was guessed and had to be adjusted until it got a good starting point till I got it home to set the timing correct. Before the timing was set even with vacuum advance hooked up it showed only about 15 degrees give or take a few so I knew that was contributing to some issues that was found before I was able to get this thing leak free and back up to run and set and adjust things.

I set my timing like above and verified everything and I matched my timing tab with my balancer and with my timing chain when it was being assembled and also with the piston checked at TDC so I know the marks are correct on that. I am going to pull some plugs and see how they look and try the suggestions above. Going to hook my vacuum gauge up and see hot its reading again and check some other things and readjust and see how it goes. I will post back on what I find.

I hope I have not missed anything on any feedback and have answered things and have not put myself in a bad spot. This one little blip is not major but enough it needs to be found and fixed. It has to be something minor and nothing major here. If all the stuff above does not work and the problem is still there then I will put my idle feed restrictor size up from .031 back up to .033 in the rear like I had it on my previous 377 build and see how it responds. I can't see why it would not work right where it is now when it did on previous builds with not much difference in specs but weirder things have happened before.

With me being able to start this thing up even before it was up to temperature after sitting for the last two plus months and not needing any choke and thought I would have to have it with it being only 45 degrees out and thought it would struggle to stay running even before having my timing set, the carb fired right up and ran and even stayed running although not perfect it did not struggle hardly at all and allowed me enough time to get out of my truck and I held my throttle by hand to warm it up and then proceeded to set and adjust my timing so I could not seeing the idle circuit being to lean at least in that situation. I know driving and being in park are two different things.

Sorry for all the stuff just trying to get everything covered and provide as much information as possible so you can understand where I am coming from. Even though I have as much experience as I do I can still miss something as I am only human and don't have tons of experience like many of you guys have.
Thanks guys

 

Another thought, the secondary side could also be fluttering or chattering the plates under vacuum.
It seems like a stretch but I’m just watering the brain with coffee here.
I’m thinking one of the circuits is going lean and the engine is dropping cylinders for a second. The only way to know for sure is get outside and get to playing with it because typing on the internet about should and shouldn’t isn’t gonna fix it. I’d fatten it up and see if it goes away, if it does, you at least know what circuits to work on to get a more desirable fuel curve.

 

How about installing an AFR gauge? I don't see how anybody gets a carb correct without one.

 

That is part of my plan johnsongrass1 as even though it does not make sense compared to previous builds. I don't know what it is with these dart builds, but for some reason what was normal when having a GM factory block 383 or 350 build whatever worked on those has not worked on the two Dart builds I have done.

These Dart shp builds have seemed to have been a lot more picky and sensitive to tuning vs previous GM builds. Seems pretty weird to say that but that is the only difference is the block and all else things being the same. I am going to try some simple things first and see how it does and if no change then I will enrich the idle circuit up a notch and see how it does.

I will check the secondary butterflies as well. I know they are working as could feel them gradually open up with the silver spring. Will check a few other things out as well that I am going through my mind.

Yeah I know an o2 air fuel ratio gauge will help in tuning a carburetor but its not the be all end all solution as it still takes older methods of tuning and always trying to get certain numbers won't always work in my research on folks who have used them. I have been able to dial in things quite well without one for the last ten years and have had good results but have stumbled across a few minor things here and there that I have been able to figure out after a few minor tests and errors and using old school methods and engine response etc for tuning.

Maybe someday I will get one installed but the thing with small block chevy motors is all eight cylinders will have different readings and some will be leaner or richer then the others and the dual plane intake or even a open plane intake can make a difference in tuning things.

I had a Holley HP double pumper on a previous Dart 377 build using an Edelbrock victor jr intake and it was a four corner idle and using a one inch tappered spacer I had it tuned so well that all plugs were almost identical with a very minor difference between the different cylinders and all were a nice tan color and ran very clean and crisp and felt like fuel injection.

Since switching to an Edelbrock RPM air gap intake on my 377 with this milder cam I did have and carrying it over to my 350 build I have not been able to gain that back regardless with what carb I have used. I also have used a non air gap intake dual plane on previous builds and same results and could only get so close without letting one cylinder being to lean and another being on the two rich side within a hair. Seems the dual plane is very picky compared to a single plane.

Will be trying a few changes today and see how it does.

 

I have noticed a similar problem. I don't think it is pronounced as eric 32's problem, though. I get a very slight stutter on decel from low speed. Mine seems as though the engine is speeding up very slightly. Just enough to put slack in the drivetrain. I was thinking mine is a too rich condition. I am interested in what you find to fix it.

 

Well it seems as if you're having quite a bit of invested time and although I respect your vast experience in all those Holly modifications y'all keep talking about, an afr gauge tells you exactly what's going on at idle or at light throttle or at full throttle or at any other condition that would pinpoint whether you do have a fuel problem or not with no guess work. I threw one on The Judge just to see what was going on since my gas mileage wasn't what I expected (although the 383 with an air gap was running perfectly since break-in) and I found that I was running rich all over the place on my Edelbrock. So with a quick reference to the published charts and a pair of new rods and a couple of sets of jets and 15 minutes of my time she now runs exactly between 12 and 15 no matter what. Simple Simon easy peasy! No slot measurements no grinding no float level concerns no secondary concerns no fiddling with it no guesswork! Perfect!

 

I agree 49Ford Coupe that an air fuel ratio gauge will tell you a lot more then just using old school methods for tuning and will get the best tune possible but even without one I have been able to get 19 mpg out of my chevy truck on the previous 377 and the 350 I had before that and for a carbureted vehicle and my previous 377 having a 3000 plus stall converter along with my other truck I used to have that I sold several years back had a Proform Holley style 650 double pumper on it with a little wild 282/290 231/239@50 535/550 lift 110 lsa hydraulic roller lunati voodoo camshaft and a 3500 stall converter and 3:73 rear gears and only 12 inches of vacuum was pretty dang good. It took time and the old 86 s10 I sold never fouled plugs and did not burn pig rich and did not smell like a bad fuel smell so many talk about when it idled.

For not having an O2 gauge that is not to bad but I am sure with a air fuel ratio gauge I might have been able to get maybe a hair more but nothing that would have been a night and day difference. Ok guys now onto what I have found today. I have not gotten it out to drive but will be doing so tomorrow and this is what I have remembered the time I had it out yesterday and what I had down on my notes and thinking back on my problem.

First off I figured why it would run without the choke engaged and forgot to write down what I had but remembered it while thinking over things I did yesterday. Because the timing was way to slow where it was initially set at for guessing while breaking it in, I had to turn the idle down quite a bit and it was around 1100 rpm or a hair more and when I just completely undid the idle screw to see where it was at so I did not go past the max amount I can have it opened to not expose the transfer slot to much, I had to turn it quite a bit so the transfer was exposed too much on initial startup yesterday and that is why it would stay and idle fine as the transfer slot was exposed to much thus allowing for a rich enough idle for it to run off of while warming up.

After I had read the timing before I set it even with vacuum advance hooked up it was only at 14 degrees so I barely had 2 degrees initial plus the 12 from the vacuum advance. Not enough to run this thing by any means. I figured that part out and that would explain how it ran with out the choke on cold startup. After timing was set to where it needed to be and before I readjusted the idle speed it was around 1200 rpm before I did it. I looked at my notes I jotted down before I put the carb on months back and readjusted my idle speed screw and my transfer slot is now set right at .030 showing.

So it being too rich on the idle circuit is ruled out. Looking at what I had written down yesterday I missed a few things and it came back to memory today. I could go further out on my mixture screws and I had no changed in the one symptom I said I had. For the most part the truck ran fine at 25 mph which I did about 95 percent of that in town until I hit the highway and the one tiny issue happened all the time I let off the gas completely at estimated 1400 rpm and started breaking and one it hit around 1200 rpm it would do that.

I just remembered that there was only one time I was in an area before I hit the highway at 35 mph and was only about a 1/4 mile or less and I was at a steady 1700 rpm and I noticed my engine was not bucking or surging but did notice it was like it had a steady studder to it but not severely bad enough to cause it to be horrible but just enough to make it seem like it was a tiny lean condition just in that part of the transition circuit. I forgot to jot that down on my notes.

All highway driving while it was pulling from the mains at 2500 plus rpm and hitting wide open throttle and shifting down to second gear to do like I was passing and all it ran clean and crisp without any problems. So I did check over some things today and the idle setting on the carb was correct and I am using currently an open spacer on it and thinking things over it seems the problem is on the transition circuit with the throttle angle just when it starts to open and when it gets to the point it goes back closed and it creates that one tiny studder for two seconds.

So I took out my spark plugs and checked them over and I had driven about ten plus miles on them yesterday and looking at them the number 2 cylinder is borderline too rich and several other cylinders were pretty good and I had a few that was on the borderline to lean side. They have not been ran long enough to get a better picture of things but close enough starting off wise that I have a few plans of action.

First off I am going to try the idle mixture screws out a half turn more and see if any difference, if not then at that point I am going to take out my one inch open spacer and install my one inch four hole spacer and that alone will affect the carb signal way differently and create a stronger signal to my carb and should tell me a better tale of things on the idle circuit calibration. If the problems go away after just that change then I know using the one inch spacer open spacer my idle circuit was just calibrated just a hair to lean and if the four hole spacer works just fine then it then should be spot on.

If putting on the four hole spacer does not get rid of all of it completely but make a vast improvement on those two things or eliminates at least most of at least one problem then I know the idle circuit needs to be stepped up just a notch richer. Now the hard part since this is a two corner idle I am wondering should I change out the primary from .031 to a .033 ifr or should I leave it alone at .031 and change out the rear to .033 and what change would give me the better tune for it to be just enough right for everything without it being to rich on the number two cylinder?

Now as a last ditch check as johnsongrass1 mentioned about the power valve if after the above changes on the spacer nets me no change at all in the symptoms then I will want to do the idle feed restrictor change first before testing the power valve theory of it fluttering by chance which I have never had happen before but weirder things have happened before. From my experience I am thinking the idle circuit calibration is really close but just still a hair off.

This is one of the few times being so close and with just a minor hickup and determining what exact little change that is needed to give that little bit of what it wants without being to much past the sweet spot. Would I have a better adjustability and better tune with the front ifr change and leaving the secondary as is or would I get a better mixture with leaving the front as is and changing the secondary ifr instead? I had never though of that before of doing one versus the other.

On m previous 377 build I had it at .031 primary ifr and .033 secondary ifr and before I sold it I did have a few problems but only minor and never got around to seeing what it was but the number two cylinder was burning a bit to rich compared to all the others which had some just fine and one cylinder that I can't remember that was a tad rich but not as bad as number two with using a one inch four hole spacer and going to the one inch open spacer seemed to help clean it up some and I could not go any leaner on it from previous tuning adjustments while dialing it in.

I never got around to do any tweek adjustment on it and had a bit too cold of a driving temperature to get a good reading on thigns spark plug wise and because of health issues I had to take time off as did not do so well then by the time I could get to it the engine was pulled and sold since my 350 parts were done and ready. Other the tiny issue on the number two cylinder and forget the other one the tune was good and did not have any problems like this current build is having. I only changed out the rear ifr size from .033 to .031 as I figured the 350 would not need as rich of an idle since its less cubic inch and compression but maybe I was wrong.

Seems to be carrying over but not as bad since this is a smaller cubic inch build and that is why I posted all the information above as in the past my settings was spot on but this time it seems to be 95 percent there its just getting that last 5 that I need. Just never had it so close but yet have a tiny hickup like this but there is always a first time for everything. Seems there is something going on with this Edelbrock rpm air gap intake on the number two cylinder intake port versus all the others and one other intake port that is not as bad as number two but is still a bit richer versus all the other cylinder fuel distribution going on.Sorry for the long post.

 

It’s quite common to have mixture distribution issues with dual planes or any intake for that matter. The length and the turns and shapes of the cross section area as well as the paired holes pulling on the carb this sorta thing gets into the realm of the carb needing staggered settings is possible if not probable.

 

After reading this post it brings back to my mind the issues I had with too much vacuum advance. I have had best results on SBC when I limit the vacuum advance to about 7 degrees. If you need more put it in the initial. Keep in mind as the rotor is spark jumping to the distributor cap contacts and the rotor swings 6 degrees (distributor) for a 12 degree (crank) max the spark is off and has to jump farther. Weaker ignition is more the issue as opposed to the carburetor in most cases. Many ignition systems right out of the box are not phased correctly. Then advance makes it worse. GM in the later ‘96-‘97 small HEI distributors had a retard just to compensate for advance to retain correct rotor timing phase.

Saying this thread stated problem is at lower speeds just suggest fluctuation in the vacuum advancing. I would try more initial and less vacuum. Easy to do vs other more evasive tasks. Understand initial timing does not affect rotor to cap contact phasing.

 

Ok guys here is a slight update. I warmed my truck up today and I swapped out the open spacer for the one inch four hole spacer and while it started up it studdered some and seemed more worse but I knew it was cold so I was giving it some time warming up. I noticed it was sounding on the choppy side and normally my cam does not have that much of a lope to it so I know I had a slight exhaust leak in one pipe that I need to get fixed but with my previous engine and all things being the same minus the shortblock difference of 377 cubic inches vs 350 it never sounded like that at idle or cold startup.

Anyways while I was looking at my rear pipes and trying to listen to hear what was happening I noticed besides the white normal smoke or steam from condensation from the pipes being cold the right side looked like it had some faint smoke coming out of the right bank and the left bank was not as bad but there was still something definitely going on. I could start it up again without needing a choke and after just two pump shot starting up it chugged like it had to much fuel but two pump shots never has done that. It showed symptoms of to much fuel.

I know my idle and transfer slot are correct as I verified everything the last time I ran it last week. After it warmed up I noticed it started to studder a lot more versus using the open spacer and I know the 4 hole spacer increases the signal going to the carb in most instances on a dual plane intake. I got it warmed up and temperature got up to 190 and then I proceeded to look over things and all looked good and I drove through town and this time things are actually a bit worse then it previous was.

While cruising with the four hole spacer it did not matter what speed I was going at 25 or 35 it was a constant studder but yet I have dealt with a lean condition before and this seems to me to not be the case as I can give it pump shots and it would gurgle some but not fall on its face and is more like an over rich condition which over my years I have mostly have only a super lean condition and in the past it was not like this. It would buck or jolt a lot and shake back and forth. While cruising and letting off the gas it sill had the studdering part.

I then tried the half a turn more on the mixture screws and it did not change anything. This engine is running like its being over fed and not under fed and its like a dog with the four hole spacer. I only did about a 4 mile round trip and only in town. I got it up to about 40 just for a hair at one point and it starts to clean up at that point. I pulled over and reset my idle mixture screws and unhooked my vacuum advance and instantly naturally my idle went down about 300 rpm and if my engine was running to lean it should have quit as it always did in the past with a correct idle.

It seemed like it was struggling a hair but it would still run but like it was overloading itself though but I could still give it a pump shot and it would not fall flat on its face or anything. I then popped it into gear to see how it would react and expected it to quit and it still would stay running even at 500 rpm. It never did that before. I drove it with it unhooked and it seemed to still studder but naturally with no vacuum advance it was not as good.

So I got it in the garage and parked it and let it cool down and I took the number two plug out and have not checked some others yet but I knew that one was the one burning more richer then the others. I took out the number two plug and it was almost black around the ring and the last time I had checked it before today it was a lot cleaner. The inner porcelain was very black a lot more from the last check. I am going to pull some more plugs and will post what I find.

I then turned it off and I got out my vacuum pump to test that my vacuum advance canister was not going crazy and did not have a busted diaphragm and I also noticed while unhooking the vacuum advance canister hose that it looked like fuel vapors are somehow staying in the hose and it had a oil residue as its T eed into my vacuum modulator going to my transmission. I checked my vacuum modulator and its holding vacuum fine and no problems coming from it.

I have had this before many years back and the fuel vapors somehow end up over a long period of time would get into the line like that but I never had it cause any plug issues or anything being pulled into the intake as I checked plugs on older builds and they were never showing any signs or problems. I checked the hose at the point at the carb which is only about a 4 inch section from the T fitting and it was clean and dry so I don't believe that is any issue. The hose from my vacuum modulator from transmission was dry.

So putting things all together I am thinking this thing for some reason is just running pig rich and its just enough to make the number two cylinder want to foul out in short time versus the other cylinders which I have yet to get to and will have to wait till tomorrow. With the open spacer it was not as bad but with the four hole its definitely not doing better but worse. If it was two lean then the engine should have had a hard time starting up with out a choke and also not be able to keep running with vacuum advance unhooked. While I had it running before I shut it off I took my pcv valve off and with it making a massive vacuum leak it should have made it stumble some and maybe even quit but it picked up a few hundred rpm and smoothed out quite a bit but I only let it do that for a few seconds while checking that the pcv valve was working and allowing it to be sucked in at idle.

So I am thinking this thing needs to be brought down some and I don't know why it would be this rich on this cam but the previous builds had more overlap and a slight tighter lobe separation angle. The last 350 I had in this truck had the lunati voodoo hydraulic roller cam which had 270/278 219/227@50 with a 106/112 lsa. I mentioned it in a previous comment and it had 50 degrees of overlap seat to seat. I ran a 600 Holley on it and ran the .031 .031 idle feed restrictor size on it as a two corner idle and ran an dual plane intake high rise non air gap style with the four hole spacer.

It would run with the .028/.031 ifr size but was a little to lean and had to have the .031/.031 size to run clean and just rich enough but nowhere close to being to rich. This cam in my 350 I have now is not much different except it has 268/272 220/224@50 with a 110/114 lobe separation angle and only 42 degrees of overlap seat to seat and a little bit less duration but not much to affect things to much but apparently what works on one build does not always work on another. I think what I am going to do is I have one last day to try and adjust thing before I park it for the winter I have a 60 degree plus day coming and I am going to take my carb off and recalibrate the idle circuit a step leaner going back to a stock out of box .028 ifr in the front and also look over my carb to see if anything is out of the ordinary while its off and then put a set of new plugs on the right bank which seems to run a lot more richer vs the left bank which is a lot leaner but I have to pull the other four rear four plugs to see what is happening with the rest.

I am going to put a new set of plugs on the right bank since they are the worst and then see how it does for burning wise and I am going to also before doing that do a spark test to rule out my coil being bad. I checked it during the spring time with my 377 having some issues with the same carb on it being so picky. I don't know what it is with this camshaft versus some of my previous ones but this one is one picky cam to try to get a proper tune with and with everything I have done over the years this has been more challenging to me but I have dealt with about 90 percent of the time a carb being to lean and having to go slightly richer one step at at time till I hit the sweet spot versus about only ten percent of being overly to rich.

This the first time I have had a camshaft of these specs be what seems to want a more leaner fuel curve versus others with similar specs but with a slightly tighter lobe separation angle and a hair more overlap which I know the more ovlerap a cam has the more richer of a mixture it needs. If those changes net even worse results and I know going richer will only make things worse and I have checked my plug wires and they are all good and still look like new and my volt meter gave good readings on the resistance and the only thing left would be the coil and if it shows a good blue burn then I know something has to be wrong with the carb even though it was rebuilt earlier this year.

This is a first for me for it being like this but I feel like its just super rich but not to rich to make black smoke out the pipes but enough to be feeding it to much. There are no signs of flooding and the carb barrels are dry and have not shown any signs of flooding. Thanks guys for your help.

 

The vacuum line to the distributor is under negative pressure and shouldn’t get gas vapors/fuel inside it. Do you have a vacuum gauge that you can hook up while driving to verify vacuum? How does it run using all initial timing and divorce the vacuum line. If all is well it should haul ass as long as it don’t ping. I’ve always treated vacuum same as accelerator pump……. just over come the change. That’s why only a few degrees (7) is all that’s required. I can’t see where your cam is anything that requires exotic attention to run well. But I do think (IMHO for what it’s worth) the original IFR at .028“ s/b adequate. The IAB would then need to be readdressed.

Your methodical method of working this issue and complete information makes it interesting for anybody follow this thread. Too often others leave out so much.

 

57Nomad I did unhook the vacuum advance and it ran slightly worse but nothing bad and I could not smash the throttle as I had stuff in the back of my truck and my tail gate was down so I could not let it do anything but just cruising it would not ran worse. The three piece of rubber hoses that are all connected to my T fitting only one was slightly wet and it was the T fitting itself that had some residue on it. I have had this in the past and i researched and it seems that a vacuum line if it sits at a low angle which mine does can collect fuel vapors over time and end up eventually puddling in the line at the low point.

It has happened to where there is a C curve going into m vacuum modulator into my th350 collected fuel vapors and one day when I went to check things and I pulled the hose off some fuel cam puddling out and I had no vacuum leaks and no signs of anything wrong at that time and was only doing some slight tuning stuff and nothing was out of the normal of things. I researched this and I have read that back in the old days when 350 turbos were common GM used a steel line in between the connection between the carb and the transmission with a C curve in the middle of the line to catch the vapors. I know its negative system but it has happened to many folks and it is one of those things that I can't understand why it has happened but it has to me and others. I normally have taken my vacuum advance off and the small hose at the carb was always dry.

I had the same issue on my older truck I used too own as well but it would come from my transmission vacuum modulator connection hose. I replaced the modulator and tranny worked fine and shifted good and no signs of anything being out of the ordinary with it. Just one of those things. The hoses themselves were dry on all the end and even the final one going into my carb. Just the T fitting had some residue on it and was black and smelled of fuel. It could have bee leftover fuel from when it was first started up and broken in and the process ended up flooding my engine and had a big backfire after pumping so much fuel to get it started up and a few other factors. Don't know for sure though.

Its one of those things if you google it, and it comes up with some interesting things on that subject.

 

Back fire would create positive pressure. Best possible situation to reverse vacuum to pressure in line. Eventually the fuel/moisture should pull thru with vacuum restored.

Did you up the initial timing when you removed vacuum advance line?

Here is the setup on my 327 L79 Corvette w/270 Comp cam 224 @ .050”
Holley 3810 Remains as 2 corner idle
Epoxy sealed constant feed holes both primary & secondary holes were about .022”
(Weird having them in primaries)
Primary jet .065” changed to .066”
Secondary jet .068” (stamped #65) Secondary meter plate changed to meter block
Power valve 6.5 changed to 5.5
PVR .040 same as original
IAB .070” primary changed to .062”
IAB .031” secondary changed to .037”
MAB .031” primary
MAB .0225” secondary changed secondary MAB .031”
Idle Fuel Restrictions (IFR) .029” primary & .029” secondary in lower positions
Emulsion jets on both meter blocks all are .028” 2 per side same as original
Anti-Siphon air bleeds are .028” front & rear
IFR primary was .031” & rear was #22 meter plate w/.076” & .028” IFR
(Original meter plate p/n 34R9716-22 equals #65 main jet per Holley chart)
Accelerator pump squirter .025” w/no tubes changed to .025” w/brass tubes
Squirter cam red #240 but worn position #1 changed to white #218 position #1

 

With regard to one cylinder showing a fouled spark plug this is a strong indicator that a cam lobe is going away on that cylinder. Remove a valve cover to observe rocker motion.

Bogie

 

I currently have my Holley 600 set at .074 primary idle air bleed and .044 secondary. Primary ifr size is .031 and secondary the same. No metering plate being used. Power valve restriction is stock size of .046 give or take a hair and the idle discharge ports which it sounds like you epoxied closed are the stock setting on my carb and base plate has not been modified at all. Jetting is 67/73 and the carb was showing signs of being slightly rich on my previous 377 but not to where it was undriveable and I did not have time to get it worked out and gave up since the engine was going to be pulled for my 350 and I knew it would be to rich with the original .031 primary ifr and .033 secondary ifr size. I only went down to .031 in the rear metering block. I drilled and tapped the blocks to take brass allen screws and have them in the lower position like Holley used to do. Emulsion holes in both blocks are all .028 like they came and same with syphon holes.

This carb for some reason is just running so dang rich from my findings yesterday and will be pulling plugs here shortly and see what I find out. Seems like the right side bank is more richer versus the left bank but after doing some research on the dual plane intake I guess the left side always tends to be leaner versus the right side because the right side runners are higher and the fuel does not have to travel as far versus the left side to where it has a longer drop and then the turn into the cylinder head.

I found an interesting post on another forum that a guy had problems with an Edelbrock rpm air gap intake and it was a 383 and here is what he had.

"Dart 383 SHP block, 6” rods, AFR 195cc heads, KB pistons, CR set to 9.75, full roller, combo 1.6/1.5 rockers to open intake faster to take advantage of the great heads, Edelbrock Air-Gap, QFT 735 SS carb w/vac secondaries, and a custom designed and ground cam (213°/218°, .506/.525 lift, 114° lobe centers to give lot’s of vac). Specs do not include timing and lift change using 1.6 rockers on intake"

On his engine he had a problem that no matter what he did it had severe richness on the right bank vs the left bank and the air fuel ratio was all over the place. He had it on the dyno over 40 times give or take and he tried different spacers and the 4 hole spacer made things worse and really no improvement and the open spacer helped to equal things out some but not enough to make it go away. Here is the link and I am having similar problems like he is having though slightly different but very close. Unequal Fuel Distribution with Air-Gap Manifold

After the guy had enough of messing with his build and the fuel distribution problem that was not able to be solved he had another slightly bigger cam with less vacuum grounded for him and once he did the swap all of the previous issues he had was gone. I have never seen anything like that before until now with the same problem from my previous motor build too this one with it being worse since its a smaller cubic inch build and thus the carb would have a change in signal going to it vs the 377.

On my previous 377 with an Edelbrock victor jr intake and a slightly bigger cam of 276/280 228/232@50 5??/5?? lift with a 112 lsa and it had 54 degrees of overlap and 15 inches of vacuum and I ran a Holley 4777 double pumper as a two corner idle at first with .033 primary and .033 ifr sizes and .078 primary and .048 secondary idle air bleeds and converted it to a 4 corner idle and ran with .078 idle air bleeds front and rear and was able to then go down to .031 ifr size front and rear then ran a Holley HP double pumper as well with a four hole spacer and also with a tappered spacer as well and the fuel distribution was really close on all 8 cylinders and ran really well on the combo I had without any weird issues like I am having now and I swapped out to the smaller cam which I posted above and knew with it being smaller that the victor jr intake would not be a good match with it.

I swapped it out with the Edelbrock rpm performer air gap intake and had the smaller cam put in. Since day one I have had issues with carbs getting to run on it and now this build with the same cam and intake combo. Its way more pronounced rich since this build is smaller cubic inch.

Last late summer into fall my main carb I put together I had a 600 80457-1 Holley vacuum secondary that I customized and pulled out the straight leg boosters and installed down leg stepped boosters and I had it setup as a 2 corner idle and it was super lean running .078 primary ifr and secondary .052 estimate idle air bleeds. The original size on the air bleeds were I think .065 or so and .035 secondary give or take. On the upper dash number Holley 1850 carbs and 80457 Holley changed the the idle air bleeds to .078 primary and around .048 secondary over time and slight changes in jetting. I put that carb on and with .033 ifr size front and rear it would not hardly run at all. It would idle fine but off idle it would buck and surge and was just like gasping for air.

I thought I had the secondary idle air bleeds at .048 but I must have used the wrong drill bit and it measured at .052 five or take a hair and I figured out that was my problem as for some reason this cam regardless of being in my 377 or this new 350 build has been very picky to just even a very minor change.

I got tired of messing with it and did not know at first what was causing it to be so lean when in normal tuning over the years I have never had this much problems with fine tuning with such a mild cam and build. I did not know what was wrong with that carb at first until later on I looked over my notes and saw the specs I wrote down with the air bleeds and to where they were stock and what I drilled them out too.

I got a used 600 Holley 1850-3 and in stock trim with only adding a secondary metering block and changing out the ifr sizes up to .033 front and back starting off with on the previous 377 and it was showing signs of being on the too rich side especially on cylinder number 2 just like this build. I at first changed down the ifr size a hair from .031 in the front and left the rear at .033 and it helped some and I knew I was very close.

I then drilled out the idle air bleeds to .074 primary and .044 on the secondary side and it cleaned things up a bit more. I was able to drive it and my plugs were better but cylinder number 2 eventually got black and a few other cylinders did as well and some of the other plugs were showing a lot more better burn on that 377 build.

The engine was pulled and did not see if I could go leaner yet and just left it as it was. Now on this build being a lot smaller its just running so dang rich its horrible in the short time I have driven it and its been a nightmare. Just minor changes on the air bleeds has either made things too rich or just to lean between the two carbs and I have never in my ten plus years have such a picky engine like this and has only been like this with this camshaft in two different engines being like this.

On previous builds with similar specs as my current 350 build I was able to run a 600 or 670 Holley street avenger with that air bleed size as they came out of box and only had to change the ifr sizes to .031 primary and .031 secondary run with said build and had no problems getting a good tune. It had a weiand high rise dual plane intake with a four hole one inch spacer and even ran fine with an open spacer.

Heck I even had an old 350 build that I for a very short time had a comp cam big mutha thumper hydraulic roller cam with 243/257@50 with a 107 lobe separation angle and only 6 inches of vacuum in park and I had a Proform Holley style 650 vacuum secondary on it and it was only a two corner idle and an Edelbrock air gap intake manifold and I had no problems even tuning it unlike this and its driving me crazy.

Sorry for the long post but just putting some information out there in case someone else someday comes across this post and with the big amount of information they might be able to get some good from it. I am going to hit this thing Saturday before I put it away for the winter and then if I can't get it working well then I know that something is either wrong with this carb for some dang reason even though I rebuild the thing and its like new and is very clean and not all dirty and filth all over it and the air bleeds are clean and stuff then I might have to look at something just being wrong elsewhere and maybe for some reason this intake just does not like my camshaft for some dang reason and might have to swap it out but would hate to do that if I don't have to.

This is one of those things that has left me baffled and normal stuff is just not working yet to fix this problem.

 

Just saw your post Bogie and when my engine was put together during the summer I was there and looked over my billet hydrulic roller shaft and it still looked like new and the lobes were in excellent shape and only show just minor marks where the roller wheels make contact. All the valves were checked on the heads and the guides were in excellent shape and should be as I don't even have 3000 miles on those heads yet. I also checked my valve lash again after break in while I had my valve covers off lats month to fix my leaking gaskets and putting on my ARP valve cover studs and all was in good order.

 

It was after swapping out to this current camshaft and intake on day one on my 377 and it had the same problems like this build is having but it was not as bad since it was a bigger cubic inch build and such I figured with the larger bore and more from that engine the carb had a different effect on it and was not as bad but was still persistent and ran horrible lean with my one 600 custom holley with slightly bigger air bleed size. Running the Holley 600 with .078 and .048 idle air bleeds and .033/.033 ifr sizes it ran just horribly lean.

Putting on another 600 holley but with it having slightly smaller air bleed size caused it to run on the two rich side and on the lesser 350 and it being a smaller engine it is way more richer now then how it was on the 377 since it was a bigger build the carb most likely was slightly leaner on it just enough for it to not be as bad. Looking over my notes last night this had been carried on from both builds.

Previous build with victor jr intake and bigger cam I have excellent fuel distribution overall and clean plugs that looked good and normal and while I was tuning my carbs on that combo I could change out air bleeds and fine tune the idle and there was no big drastic change for super lean to super rich just by going up or down a small change in air bleed size. Once I swapped out to the milder cam and the Edelbrock rpm air gap intake this is when things just went really horrible and I did not have time to work it out last year and then it sat for the winter and once spring came I was working on trying to find out why the over richness but my poor health made it hard for me to have good enough days to get to do stuff on it.

Then it came time in late April to take my engine out and get my short block ready to sell and my parts all cleaned up for my 350 build which was done over the summer months. Then finally got it back in late September and then I had the problems with bad mechanical fan and leaks and having to replace a bunch of junk put me off from being able to try and get this thing running good and right. Looking over a set of spark plugs I had left over from the previous 377 build I saw the same thing happening but not as bad but I still went through a set of plugs in short order during the spring time and the new plugs I just put in last month on the number two its almost completely black after just two days of running on it.

I have never seen anything to where its so dang super sensitive to get a carb to tune on this combination and in all my years its making me want to pull my hair out. I know enough to be able under normal conditions get things to work well and this is the first time ever that normal stuff for some reason for tuning and adjusting is for some reason not working right. I can't see how just a minor cam swap and intake change would cause such a great deal. I though it was just my 377 that was picky but its been passed on to this build as I am wondering if the intake is the culprit here and does not like working with my cam for some odd reason.

I had an Edelbrock rpm air gap intake on my other s10 I used to own and it had a bigger sized cam and the plugs did differ some since it was a dual plane intake but not so much to have some cylinders super rich and others looking really lean. I am going to check spark again and try to lean out the idle circuit this coming weekend and see how it responds to the changes. I have done a compression test and its all good with about 160 to 170 psi on everything.

I am going to lean out the primary metering block down to a .028 ifr size and leave the rear size at .031 and this puts it back to stock setting on that part and if this still then keeps the same problem with no change then I will want to really go insane on this as after this I don't know what to do then as something is wrong here and I just can't figure it out and its driving me crazy and causing a lot of stress.

Makes me feel and wonder if its something with this intake that is the problem for some reason beyond my understanding and it does not want to work with this cam. I have had different combos over the years and in all my tuning experience and knowledge with different cams from mild to wild and different intake manifolds and in normal conditions a carb should not be reacting the way this is doing with such big swings as super lean then super rich with just a slight change in air bleed sizes.

Have never had a major tuning problem like this looking over things. In the past I had one quick fuel carb that was super rich at idle and a few size changes in the idle feed restrictors and it was good to go and was with a mild build really close like mine.

Basically in short detail.

377 build with slightly bigger cam and victor jr intake. Holley 650 double pumper two corner idle and then four corner idle and a Holley HP 750 and 650 double pumper all ran fine. Great fuel distribution and no problems in being able to tune. Changed air bleeds up and down just a hair such as nothing more then .004 leaner or richer regardless of two corner idle or four corner idle. Changing ifr sizes was normal as well and changed in tune within a normal range and no drastic big change in going from super rich to super lean. Ran excellent.

377 build changed to slightly smaller cam and edelbrock rpm air gap intake since the victor jr would not be wise for a smaller cam.

Holley 600 vacuum secondary custom with .078/.052 idle air bleeds and .033/.033 ifr size ran insanely lean. Tried upping the ifr sizes and hardly any change. Used Quick fuel and Holley metering blocks with out any difference in change and going up richer on the ifr sizes did not make hardly any noticeable difference. Took carb off and had not been able to try it again as it needs some parts that I don't have for it at the moment.

Backup 600 vacuum secondary Holley with .062/.035 idle air bleeds and .033/.033 ifr size and at very beginning ran really rich but nothing as bad like on current 350. Did not want to make a super lean condition like previous carb had so I opened up the idle air bleeds to .070/.040 and kept ifr at .033/.033. Still slightly too rich. Changed ifr in front to .031 and left kept at .033. Then slight change to primary idle air bleed at .074 and rear to .044. Not much of a change and still showing too rich.

Swapped out short block to 350. Backup 600 Holley on and is now way richer being on it versus the 377 and way worsening of the richness and major issues are worse but same symptoms of the carb passing over to this build and with the less cubic inch its worse I even started off by going down to .031/.031 ifr size to start off with and super rich.

With same model carbs, one carb and a slight hair in the air bleeds being leaner it was super lean and with the other carb its just being pig rich and just that slight difference in air bleed size should not in my experience make such a drastic change to super lean and can't run to just to rich and can't lean it out enough to be right.

I have never had anything like this before. Sorry for long posts just I want to be able to let you guys see what I am seeing and what all I have done. Unfortunately I don't have the Holley double pumpers anymore to test one of them.

 

In my findings IFR’s over .029” are to big. Start with the original .028” and go about .062” on primary IAB & .035“ - .038” on secondary IAB’s. Being this is a 2 corner carb. Big IFR and big IAB’s for me didn’t work. Too big air bleeds makes the carb lazy. While smaller air bleeds perks it up provided you don’t have IFR’s over .029”. This is for street performance with a reasonable cam as you have.

My findings thru trial and error the percentage of fuel/air (by area of the brass holes, pie R squared) needs to be right for a given engine. Thru the math I was in the realm of 18 to 22 to one with a 2 corner idle. Where as a 4 corner was better at 13 to 16. But this was on my engine carburetor.
AFR IAB %FUEL TO AIR

.028 .055 25.9%

.029 .062 21.9%

.028 .062 20.4%

.031 .070 19.5%

.028 .064 19.1%

.027 .062 18.9%

.028 .065 18.5%

.028 .066 18%

.026 .064 16.5%

.028 .070 16%

.026 .070 13.75%

I have plugged the constant idle ports (.022”) but that’s my choice. I feel they are more of a cover one’s ass by the mftg’er. In any event the slight feed they give can be accomplished with brass size selected.

 

57nomad I don't have time to drill and tap the carb for this weekend and be able to try different air bleeds. Getting ready to store it for the winter as in cold weather its hard to try and tune a carb.

In past 350 builds I never had a 600 holley vacuum secondary run so filthy rich with the current idle air bleed size and idle feed restrictor size with such a mild cam. The only difference is I did not have an air gap style intake but I ran one on my other truck I used to own but never had any problems with it. Its so odd that my other Holley 600 ran what seemed like super lean and this one is super rich and causing smoke to come out my pipes because its so bad.

This has just gotten me really confused as in all my years I never had to really touch air bleeds 95 percent of the time and i never had a carb just run so pig rich but one time but it had really big ifr size like .033 primary and .039 secondary and it had .070 primary idle air bleeds and .029 secondary idle air bleeds and it was a Quick fuel 600 slayer and it was so rich I could only put the idle mixture screws out a half a turn and it blew smoke out my pipes until I leaned it out. I ended up with somewhere about where I am at right now and it was clean and crisp after the change. Cam was similar in size I am currently using and it was mild.

I am about to just take the thing off and sling it up on ebay and get rid of it as I am sick of messing with it and looking over my notes from last night from my previous 377 build I went through two sets of plugs before going to this one. The carb naturally would have been slightly leaner on the bigger cubic inch build vs my current 350. Something is just not right here and I am going to pull the rest of my plugs today or tomorrow when I get some time and snap a photo and post it. Ever since the cam and intake swap regardless of it being the 377 and then my 350 the problem has been there and I checked the ignition stuff earlier this year right before I had the engine ready to be pulled. My coil tested good and my control module tested good.

I guess looking over my notes the plugs were fouling out over time and looking black over time and not as quick as its happening now. I had to end up putting the open spacer on the 377 to help lean things out some more and was going to get around to at least try to check things but I knew my engine was being pulled anyways so I just left it alone. Then once my 350 was close to being done I took my carb and looked it over and it was all good and I had rebuilt it with all new stuff earlier this year and I leaned out the idle circuit a hair to be really close and in all my years of experience with things it should be close and just need a fine tweak and not be running super rich like this.

My brother for pity sake had a late 88 chevy caprice with a bone stock chevy 305 and his Qjet had seen better days and I built a Holley 600 vacuum secondary for him and it was just all stock and it had .028/.031 ifr sizes and similar in size idle air bleeds like mine and it ran just fine and was nowhere close to being pig rich in any form and ran clean and crisp. I am thinking something is wrong here but I don't know what it could be. I am thinking something with the intake or maybe something was grounded wrong in the cam even though it was from a known good cam grinder as it was a custom grind.

I have had fuel distribution issues in the past with a dual plane intake but not to where a few cylinders are burning super rich and black while a few cylinders are on the far lean side and super white. It was within a acceptable reasonable amount and the richer burning cylinders would not go total black and foul out or run bad in anyway.

I am going to check my spark again especially on cylinder number two and the other two running rich and I will have to note which ones they are and then hook up my vacuum gauge which I should have done to start off with but I was so all over the place trying to figure this out and am bombed with all these issues that came up and stressed from having to replace my pulley and it going bad and then my fan hitting something and nicking my shroud and going through leak after leak and fixing that only for something else to leak I have gotten side tracked with it all.

I already did a compression check and its all good and I checked the plug wires and they all read good still.