IMO, it sounds/looks like the MAB's are a little too big, I have good luck with MAB's in the .028-.030" range. Lots of factors to consider and orifices to measure for a baseline before making any changes. Try here for GREAT Holley tuning info/tips, I have learned a great deal from the very helpful/knowledgeable folks there.
racingfuelsystems.myfunforum.org - Index page

 

Wow that's a really great thread.. I found a few articles that have enlightened me a bit, but also caused some confusion and more questions.

What I gathered is the reason my fuel curve goes so fat at the bottom but is fine everywhere else has to do with either my MAB size, or thr arrangement/sizes of my emulsion holes.

Question 1) would a larger or smaller MAB lean it out down low?? I was under the impression that smaller bleeds meant a richer circuit and that larger meant leaner... but what I'm reading is that there's the possibility that my larger MAB in conjunction with my emulsion circuit configuration may actually emulsify the fuel with more air which essentially helps "lift" the fuel and gets the main boosters to kick in early??

Question 2) I have the quick fuel metering block with the 4 interchangeable emulsion hole "jets". I haven't messed with them due to the fact that I have no idea what they do, so it's in stock configuration where they have no jets installed except for the second to bottom one which came from quick fuel with a blank jet installed there (no hole). Could this emulsion circuit be what needs fine tweaking? If so, would removing that plug richen my low end or lean it out?

I mean, I could try all of these things... which I probably will, but I'm not too fond of pulling the bowls too often due to how messy it is. So, if I could get a little clarification on this subject that will hopefully speed up my process!!

Thanks ranch.

 

The first picture is my quick fuel metering block, which has that same set up with the 2nd from the bottom emulsion hole plugged with a blank.

The second picture is one I found online which shows a different arrangement of the emulsion jets.. in this picture he blocked off 2 and 4 with a blank.. what effect might that have on the characteristics of my fuel curve?

 

I also read a lot of magazine articles about how a smaller MAB would cause the mains to start quicker AND be richer throughout their intended rpm range, what I found and/or discovered at the website I linked is that this info is somewhat erroneous. I found (through the info provided on this website)that a LARGER MAB will start the fueling process sooner, but will also have the fuel mixture taper off sooner(get leaner as rpm's rise). A smaller MAB will start a little later but will extend the fueling of the mains past what a larger MAB will. Basically from what I have read and/or experienced a larger MAB seems to spit/sputter it's fuel at a lower rpm, causing dips/spikes in the AFR readings, whereas the smaller MAB's will start a bit later in rpm but will do it at a much smoother and extended delivery. It was VERY hard for me to understand these findings when I first read through so many air bleed issues or threads, really eye opening! I will say the newer Holley's are manufactured to give "shade tree" tuners (such as myself) a fit in tuning, from the air bleed sizes, IFR's being in the top of the well in the metering block instead of the better location which is the bottom, too many emulsion bleeds or not the right size, too small or too large PVCR's and on & on. Your 750 should like the .070" IAB's and .028" MAB's, emulsion bleeds "should" be in the .0275-.028" range(for two emulsion bleed metering blocks, in 4 bleed blocks 1st-.028" , 2nd-plug. 3rd-.028", 4th-plug, the upper most bleed in the well is a kill bleed usually in the angled port but may be above it leave it open) it may have .0275-.028 kill bleeds. PVCR's are a "salt to taste" type of tune, whereas the main jets need to be chosen to give a good cruise AFR (14.0-15.0 for mileage-some engines like a little richer and some can tolerate a much higher AFR an d still not have drivability issues like surging). Once the main jets get the cruise rpm AFR you like, adjust the PVCR's to richen or lean WOT blasts. There are many threads dealing with the HP metering blocks on the website that was linked, usually find they are more troublesome than they are helpful, especially for a street driven cruiser. Tuner and Jmarkaudio have a wealth of carb tuning knowledge. Hope this helps.

 

I believe you may have a fuel boiling issue which is why it is dying at a stop when it is run hard and is good and heat soaked. IMO you must confirm and correct this issue before you change anything on the carb.

The part throttle richness is common in the "new era" carbs and can be attributable to several factors included the fubar'd e bleed arrangement that has already been mentioned. The t slot width, length and vertical location in the bore are also factors. Also, engines with performance valve timing often produce low vacuum at idle which necessitates a richer idle calibration in order to idle correctly than an stock or near stock engine. When the engine is operated at part throttle the vacuum goes way up and the idle calibration which was made rich to idle correctly is now too rich to provide correct part throttle a/f mixture. This can be tuned to a point with e bleed, IFR and IAB changes but may require the addition of a t slot restrictor.

Finally, the single most important change you should make before addressing any other idle circuit calibration changes is to lower the IFR to the bottom of the main well.

 

Great info^^ and suggestions(especially lowering the IFR's, having them in the upper location has caused many an issue).

 

Some things I'd do first especially with a Holley are to get an insulating spacer at least a quarter inch thick between the manifold and the Holley so the float bowl doesn't see large temperature excursions. The other is a reflective plate under the carb these extend fore and aft to provide a heat shield between the manifold and the bowls again helping to prevent large temperature excursions in the float bowls. These things are available at most performance houses.

The other thing is a remote fuel pressure gauge so you can monitor fuel pump performance from the driver’s seat.

What are the cam, compression ratio and ignition timing? This may need more ignition advance to stabilize the idle and off idle performance.

I would do these things to stabilize and understand the situation before going into jet changes which might not be the Root Cause of the problem.

Bogie

 

Totally agree ^^, get everything else "right" before making changes with the carb. I realize some things won't be able to be totally set until the carb is sorted, but the closer the "other" things are to optimum, the easier/faster and more precise the carb tuning will be. Glad the info above was brought to the OP's attention.

 

"what I found and/or discovered at the website I linked is that this info is somewhat erroneous. I found (through the info provided on this website)that a LARGER MAB will start the fueling process sooner, but will also have the fuel mixture taper off sooner(get leaner as rpm's rise). A smaller MAB will start a little later but will extend the fueling of the mains past what a larger MAB will. Basically from what I have read and/or experienced a larger MAB seems to spit/sputter it's fuel at a lower rpm, causing dips/spikes in the AFR readings, whereas the smaller MAB's will start a bit later in rpm but will do it at a much smoother and extended delivery." -bb79ranch


You know what 79' ranchero? That makes a lot of sense.. my fat MAB's bring the primary circuit in so soon that it overlaps my idle circuit and causes a very rich condition at cruise. As I get up into the 3000 to 3500 rpm cruise range the AFR readings smooth out and hold a pretty decent 14 - 14.5:1 and it stops with the random misfires and seems to run a hell of a lot smoother. I'm going to drop my MAB by .008" to a .034" and see if it has an effect.. I may even try and go straight for a .030" or even a .028" as you have suggested. As far as AFR's on the lean side, the leanest I've had it is just shy of 15:1 when I went to a .027" IFR.. and it hated it lol. It had a lean surge, so I've found it to be much happier in the low 14's. I believe what you've said really backs up what's happening with my fuel curve.. the MAB is bringing it in so soon that it overlaps my idle circuit.. so in order to "fix" this issue I've turned the mixture screws in some to lean it out, but this causes it to idle VERY LEAN when I come to a stop, which in turn causes the car to try and die at a light unless I feather the gas a few times to stabilize it. So as of right now it's either a happy idle/rich cruise or lean idle/happy cruise.


"The part throttle richness is common in the "new era" carbs and can be attributable to several factors included the fubar'd e bleed arrangement that has already been mentioned. The t slot width, length and vertical location in the bore are also factors. Also, engines with performance valve timing often produce low vacuum at idle which necessitates a richer idle calibration in order to idle correctly than an stock or near stock engine. When the engine is operated at part throttle the vacuum goes way up and the idle calibration which was made rich to idle correctly is now too rich to provide correct part throttle a/f mixture. This can be tuned to a point with e bleed, IFR and IAB changes but may require the addition of a t slot restrictor." -TommyK


Hey tommy, thanks for the suggestions! I believe I don't have a heat issue, as I don't run a hood, I have a single plane intake, and a 1/2" wood spacer for my carburetor, and all of my lines are routed away from heat sources! I do believe cam timing has a lot to do with it, as I have a decently lumpy ISKY grind (240/240 @ .050" with a 108 LSA) that pulls 11" in park, 8-9" in drive, and cruises around 15 - 17" on the transfer slots. The whole reason I went to this QFT metering block is because I dickered up my original holley block (messing with IFR's and such, I found it liked .031" as opposed to the .028" it came with and I tried to drill and tap it or 6-32 set screws.. I screwed it up lol)... how hard is it to relocate the IFRs to the bottom of the metering block??


Hey bogie, I don't THINK heat is the issue.. I've compared the temperature of my carburetor after running to my buddy's carb (650 w/o spacer on top of a dual plane underneath a hood) and his gets HOT compared to mine.. so IDK lol. I think my 1/2 spacer seems to do the trick.


As far as cam specs its an ISKY Z-25 solid flat tappet with 278/278 seat to seat and 240/240 @ .050", 108 LSA and around .490"/.490" lift with my 1.6 rockers and .016" hot lash.


Compression is 10.1:1 with 18° initial and 35° total (all in at 3200).

 

i agree with a going lean coming to as stop i would look at fuel supply issue..
what kind of fuel pump are you running.. conventional mechanical fuel pump.. an intank electric pump or an external inline that is attempting to pull fuel up and over the top of the tank.. something that electric fuel pumps are not great at.

with a mechanical fuel pump.. disconnect the fuel line at the carb .. blow into the line up from the pump. you should NOT be able to blow backwards thru the mechanical fuel pump.. if so.. i usually install a 1990 ford ranger fuel filter inline directly before the fuel pump inlet. these are 10 micron fuel filters where most of the others like the clear ones are 70 micron. then i disable the ignition system so no sparks can be created. i stick the end of the fuel line into a clean clear 2 liter soda bottle and do a 15 second free flow fuel pump test. you should get over a pint of fuel in 15 seconds.. each pulse should be the same at the others.. you may want to do it twice.. please hold a magnet to the bottom of the soda bottle when you are done.. does anything stick to the magnetic field.. if it pulses look good so you have no restrictions or air leaks.. you can hook the hose back to the carb.. reconnect the ignition system.. start the engine. i use a Long pair of needle nose pliers to pinch the new hose between the end of the new filter and the fuel pump with one hand.. the other i take the engine to 1500 to 2000 RPMs. when the carb starts to run out of fuel and the engine starts to stumble.. release the pliers.. a full speed rush of filtered fuel will fly thru the pump and the wide open needle and seat.. i normally do this two or three times. again.. i am just relating what i do working on cars daily for the past 35 years.. and i have been fighting this issue since 1986 or so.. this is the only sure fix..

if you have electric fuel pumps.. external to the fuel tank.. install the ranger fuel filter before them.. or one of the Fram HPG1 versions also a 10 micron element.

let me explain.. when the sediment gets in the check valves.. the suction from the tank is reduced.. the pressure held in the line to the carb is also reduced.

when the pump gets hot with the reduced pressure in it.. the fuel can boil and expand as vapor.. this can push the liquid fuel backwards thru the check valve and all the way back to the tank.. the fuel line to the carb will have less pressure and can also have the fuel start to boil and turn to vapor.. again going thru the fuel pump backwards. the reduced fuel pressure at the carb won't push enough fuel thru the needle and seat.. leaning out your motor at higher speeds. or higher temps.

when your A/F ratio goes crazy lean during cruise.. this is where i would start first..

i have some opinions on the idle transition circuits also.. but i am going to hold off on this till you check or describe.. many years ago.. i worked for a guy who ran a local drag strip. i was actually a 6 day a week job.. wed thru monday morning. we raced until just before the sun came up on monday morning usually. he had installed a holley electric fuel pump on his motorhome.. he was experiencing vapor lock. even sitting at signals. it would just die.. i was usually sitting in the passenger seat and i could hear the pump speed up seconds before the engine died.. i tried to get him to install a fuel filter before the pumps. so he put a second fuel pump in parallel. nope.. same issue.. i would hear one pump speed up.. then the other pump speed up and then the engine would die. not fun in a big motor home. he said that fuel filters created too much restriction.. i was there while he was talking to the holley reps on the phone. i ask to talk to them.. i mentioned that i could hear the pumps speeding up before the engine died.. so the bypass valve pistons are being wedged open and dumping the fuel back into the inlet. allowing the pump to freewheel.. so the HPG1 fuel filter was installed before the pumps and it cured the issue.. i just use the ranger filter as i have clogged a few over the years.. but they are available at almost every big box store and at every parts store.. so its just a matter of finding one anywhere and replace it. or wait a few days for an HPG1 element. rangers also have a factory 2 bolt and a hose clamp bracket that can mount the filters neatly..

sorry for the massive reply.. hope it helps..

 

Hey wayne! Nah I love the long write ups.. a little backstory and explanation works wonders when getting someone to understand something.

I have an electric external pump mounted lower than the 10 gallon cell sump at the rear of the vehicle with a holley return style regulator set at 7 psi through 1/2 inch fuel hose and 8an fittings.

The thing is, I had chewed up my fuel pump due to the filter being clogged, so I pulled it apart and rebuilt it and now it sings like a sewing machine.

Anyway, even before I cleaned the red rag lint ball out of my 40 micron serviceable fuel filter (whIch is installed before the 140gph pump) and cleaned the pump motor/magnets and lubed up the bushings, it did the same exact thing! I have two pressure gauges: one right after the pump and one right at the carb.. and both show exactly 7 psi.

The thing is, it doesn't even go lean at high rpm when it's hot!! It'll be happy at 14:1 cruising (above 3000 rpm) and a healthy 12.8:1 at WOT from 4000 to 6500, and this is in a motor running 200 - 210f. Now, when I come to a stop is when it goes lean as hell..

I dont know, I think I'll try smaller MABs and if that doesn't help I'm going to try your suggestion for a 10 micron filter before the pump.

 

Unless I am wrong does it not lean out the mains when the high speed/ main air bleed is bigger and delay its response? That is some interesting info posted above and from what I have read the bigger the air bleeds on the mains then it will delay it on the lower rpm range and make it more on the top end to a certain degree.

I had a quick fuel carb 680 that came with .031 high air speed bleeds and had an issue to where it would for some reason pull fuel when first using my choke and even after opening it up would continue to pull. Sent it to quick fuel and they changed the high air speed bleeds to .034 which makes me think it would make the signal less responsive on the bottom end but more on the top end. I have never seen a 600 to 750 range carb with .040 high speed air bleeds. Hopefully you can figure out what it is as I would love to know your findings.

 

Unless I am wrong does it not lean out the mains when the high speed/ main air bleed is bigger and delay its response? That is some interesting info posted above and from what I have read the bigger the air bleeds on the mains then it will delay it on the lower rpm range and make it more on the top end to a certain degree.

Unfortunately the answer is "it depends".

In general I agree with the behavior of bleed changes you describe. However, the addition or subtraction of air can often have counter-intuitive effects on the start/stop and rate of fuel flow through a circuit. The presence and size of other orifices (kill bleed, tslot etc...), the number and size of emulsion bleeds and their the relationship to the fuel level in the bowl, as well as the particular running characteristics of a specific engine all play a role. As I understand it, when you have a gas (air) and a fluid (gasoline) flowing in a conduit (the passages in a carb) together you have established what is called 2 phase flow. The addition or subtraction of air increases or decreases the viscosity of the fluid. This increase or decrease in viscosity can change the speed of the 2 phase flow. This is why increasing the air bleed size, particularly on the low speed circuit can often result in a richer mixture especially at low throttle openings.

Someone much smarter than me said it a lot better.

http://racingfuelsystems.myfunforum.org/viewtopic.php?p=118#p118

 

Cool thanks for the info as that helps me to even better understand the advance side of tuning a carb.

 

I haven't read this whole thread but it is very interesting as I use the AFR and vac/boost to tune my blown sbc.

One thing that I havent seen noted is that the exhaust sysyem must be sealed very well from the headers/exhaust manifold to well past the O2 port. Even a tiny leak can affect the reading. I use copper gaskets at the heads and collectors.

I use the Innovate Motor Sports unit. This uses a heated O2 sensor. VW Sirroco I think. It has provision for hooking up to a laptop so you can record a test session. You can also get a data recorder addition. I've only used the lap top once feature on mine once. It's too messy without the extra data recorder especially if you make a full throttle run.

Also when I start the car the system goes through a calibration process. Similar to the "wait to start" in diesels. This preheats the O2 sensor and gives a clean calibration.

It works like this; when I turn the key on but don't start the motor, the AFR starts at some value, usually around 14.2 and begins blinking and counting up. When it gets to 22.4 it is ready. This takes about 30 seconds to a minute. Then I start the motor. In my case it takes a good 20-30 minutes of running to get thoroughly warmed up. Yes the temp gage says its at 180 in a few minutes but the entire motor takes much longer. During this time the AFR does move around some. Hot starts at say the gas station it sometimes won't go to 22.4. Sometimes it only goes to 15-16. I think this is due to residual fuel gassing off in the pipe. It seems to not affect the running calibration however. It's the preheat that counts. If the sensor is cold and it gets hit with raw fuel it can burn out or fail. I've replaced mine twice over the years.

Just passing this on as something to consider.

 

Hey bentwings! Lol that's my girlfriend's login name on instagram, she is obsessed with the old corsairs.

Yeah the AEM does something similar on its initial warm up. It'll start out at 14.7 and tick its way back to dead rich and then go full lean (only takes about 15 or 20 seconds) then it's ready to fire up.

For the exhaust leak I may have one because the collector is warped on thag particular header and when I put it together I usually have to seal it with black rtv or the versachem exhaust pipe sealing stuff.. but over time it'll develop a tiny collector leak again.

I'm actually about to redo my whole exhaust system, so that should fix that issue.

I've got a nice long list of things to fix over the holiday season.. radiator, exhaust, new front tires.. etc.

 

Alright so... I'm officially very very confused now.

I made a lot of the recommended changes to the set up (minus the ford ranger fuel filter), but I don't think that's the issue...

I set my quick fuel metering block up to be pretty much a factory holley metering block.
I relocated the IFR to the lower position (.030"), and plugged emulsion holes 1 and 3, and went to a .028" MAB... I even checked float level and set it right at the bottom of the sight hole.

It still does the same thing! So I tried a .032" MAB.... exactly the same. In fact, it's richer. So... I'm wondering if I have a bad metering block gasket that's leaking raw fuel into the power valve cavity or something... or maybe a bad power valve? I checked it by sucking on it but I can't tell if it's holding properly.

It now cruises at 12.5:1and still tries to lean out and die when I come to a stop.

I cleaned my fuel filter and checked pressure which is set at exactly 7 psi when running.

I also checked to see if the boosters are dripping and when idling at 1050RPM... no drips.

I'm thoroughly confused. Maybe I'll change my metering block gaskets and check vacuum or something... maybe I'll even try my factory holley metering block.

 

Go ahead and try your holley metering block as it can't hurt anything. You will notice on the holley metering block it only has two emulsion holes compared to the quick fuel blocks which has three I do believe. Was it hard to relocate your idle feed restrictor to the bottom? I got some taps and stuff and wanted to make my holley brand blocks adjustable with brass allen set screws that I got and have not done that yet.

 

What sizes are the emulsion bleeds? Try the other metering blocks, but check the IFR sizes first, a 4150 750 cfm "should" have .033-.034" IFR orifice sizes. If you go up on the IFR's you can adjust idle mixture screws a little leaner to compensate for the richness the larger IFR's will give, but the increase in IFR size will help your "return to idle" AFR's. Also make sure you have at least .020" but not larger than .040" of the idle transfer slot uncovered. If it is uncovered too little will cause a stall when coming down in rpm from cruise, if too much is uncovered an off-idle bog could be a result and a real rich condition(9-10.5 AFR) when coming off cruise rpm and the throttle is almost closed. The high vacuum when abruptly coming off of cruise and having the throttle blades in the closed position will cause the too-much-uncovered transfer slots to dump fuel. Also make sure you get the primary side working correctly then get the secondaries sorted. Don't give up!!

 

Another thing that crossed my mind here....are you running a manifold vacuum advance?

Also 2x on trying the stock metering block. Just for an afternoon drill, I configured my QFT metering blocks as close as possible to stock to see what happened. As I recall I had a lean surge about 2000 rpm ...barely into the transition circuit. Returning to the stock QFT configuration got it pretty close to right . Then a minor change of IFR got to where I am now. Not exactly correct but very drivable, good but slightly rich dead idle. 12.8. Gently bringing the motor from 800 to 1000 gets the AFR to 13.6 to 14.0 . I don't have any heat riser and blower manifolds are not air gap for sure.

Part of the differences here is that the blower provides very good atomization and good fuel distribution. NA manifolds can have all kinds of fuel problems if they are too big on ports. You may want to try a 1" non metallic spacer, I would use an open rather than 4 hole.

Make sure you have the correct metering block gaskets and the are not blocking any holes.

Pardon a dumb question, but why did you relocate the IFR to the bottom?

Somewhere you noted that you had 7 psi fuel pressure. I've always run 5-5 1/2 on my Holleys. Even my blown car only runs 5 1/4. I run a little lower fuel level due to side mounted carbs and natural rake on the motor. Even so I still can flood it on a fast tight corner.

Byron

 

Eric32- I'm planning on trying the original Holley Metering block (which, 79' ranch, has .032" IFR's according to my wire gauge bits). For now, being the stubborn guy I am, I am trying my best to conquer this QFT metering block. The restrictor was insanely easy to relocate, in fact it was so easy I wasn't even sure I had done it right lol. I used a $5 6-32 3 flute bottoming tap I picked up from Fastenal, and I didn't even have to drill! I just unscrewed the IFR from the top so the orifice is open, and just tapped the IFR to the same exact location as it would be on an old school Holley metering block. I don't even think it took me more than 5 minutes.. just make sure you use the 3/16" brass set screws with the 1/16" allen socket head so it doesnt abstruct the passage located next to it. I followed the instructions provided by speedtigger on LS1tech, which he got from Mark and Yeti off of yellow bullet lol. Anyway here's the link: http://ls1tech.com/forums/carbureted-lsx-forum/1626345-next-level-holley-tuning-serious-ocd-tuners.html

bb79ranch- The emulsion holes SHOULD be .028" as advertised in the QFT metering block instruction sheet that was provided with it when I purchased it, however, I haven't personally measured. Bleeds #2 and #4 have not been messed with, and for #1 and #3 I blocked off with blank 6-32 set screws. The transfer slots were exactly where they should be.. about .020" on the front and just a little south of .040" rear.. funny you mentioned the stall when coming down from RPM, because it does that.. and it also has an off idle bog that shows about 10.5:1 AFR... so I'm thinking something weird is giving extra fuel to the carburetor SOMEHOW.. but from where is not where I'm quite cetrain of. I just cleaned out the whole carburetor and replaced the metering block/bowl gaskets, changed my primary PV, and ensured the secondary PV plug is snugged down.. I also looked for blocked passages/holes on the main body/metering blocks/baseplate. I noticed the secondary idle feed holes on the baseplate beneath the throttle blades seemed to have a lip from the machining process, so i smoothed those down with 240 grit sand paper just for the hell of it.

bentwings- I am in fact running manifold vacuum advance. I have it set up to 50° with the advance full in, and it doesn't retard the advance until manifold vacuum drops below 10 in/hg. The carburetor used to run awesome, even with my single plane intake.. the only thing I changed was I got rid of my PCV and installed the Moroso crankcase evacuation kit on there.. but even before that the carb would try and die when I'd come to a stop and the motor was nice and warm. Even when I first went to the single plane/750 HP/Isky cam set up (changed it all at the same time from my old set up) about 4 months ago, the engine wouldn't try and die at a light, that only came about within the last month or so, and it's gradually getting worse! It went from having a decent 14:1 cruise AFR and only trying to die at a light when REAL warm, to cruising at low 13:1 and trying to die at a light pretty often, to now where it's high 12:1 range and almost every light it wants to die. It's almost acting like either a vacuum leak or like it's getting excess fuel from somewhere else. I'll try and drop pressure to the 6 PSI range and see what happens. In regards to the moving the IFR to the bottom, I've read many posts from the corral, LS1tech, yellow bullet, and speed talk discussing the movement of the IFR from the top to the bottom to mimic the original Holley set up, and many have said it has actually made their carburetors meter cleaner and leaner on the idle circuit.

 

Check the T-slot widths and lengths and post measurements.

 

it'll go dead lean and try and die when I come to a stop at a light (anywhere between 16:1 and 18:1).

this is a fuel supply issue.. or a vapor lock issue..

do you have a fuel pressure gauge to monitor the fuel pressure?? where you can see it ... even if its just a temporary installation..

does the electric pump have to pull fuel up and out the top of the tank. if so.. is there any way to install a low pressure booster pump in the tank to supply your conventional pump.. like an airtex #E3903 that puts out 5 PSI at 35GPM.. probably not enough for a performance engine..

since i am just looking at one issue of the lean out and stall..

if there is no room for that. or a fuel pressure gauge... how about using some kind of digital volt meter with a that will read 15 or 20 amps DC.. breaking into the fuel pump circuit for testing and wiring it right thru the amp meter ports .. to allow you to do some test drives.. if your fuel pump is freewheeling instead of pumping the amp draw will change..

the Fram HPG 1 is a 10 micron element as far as i recall.. again i don't know what you have..

i worry also that you are sucking air when slowing down perhaps uncovering the pickup in the tank..







on a side note.. being a former street racer in los angeles. i heard about early experiments with nitrous.. ball valves to control it.. because of the nature of street racing.. miles were driven.. the fuel tank was a trunk mounted moon. he did not have time to top it off before the race spot..

during the race the almost empty tank left the fuel pump without fuel. just the conventional fuel from the carb.. flames came out the headers. then the fenderwells. and the car rolled to a stop.. the first few inches of primary tube were gone.. and there was aluminum hanging out the exhaust ports. i was not there.. this was relayed to me by another racer. to be sure about fuel pickup locations..

at former job a class A motorhome came in.. 440-3 would overheat and loose power only on uphills.. passed the 10 second cranking fuel fiow test. boss was a former chrysler master tech and WWII PBY flight crewchief. very little escaped him. but we changed a whole lot of things and it still did it.. when the fuel pressure test came up .. flow had checked out at 10 seconds. going up the grade the fuel pressure dropped and the temps shot up..

checking the 30 feet of fuel line.. found the camping world tech who had installed a new generator set had used a multi vacuum tee that said right on it.. NOT For Gasoline.. without cutting the barbs off.. so the pump on long grades was not able to pull enough fuel after the almost 30 feet of 3/8 fuel line was emptied . the engine leaned out.. when we performed the fuel flow test again with that tee in place.. after 10 seconds the fuel flow reduced.


back on the carb.. i would like to know if that model has the hidden under the air cleaner stud air bypass screw... if so.. open that up.. and close your throttle settings farther.. to expose slightly less of your idle transition slots..


during idle.. there should be really not much fuel coming from the idle transition slots as there is more of that slot uncovered above the throttle blades.. all your IDLE fuel control should be on the idle mixture screws.. changing idle air bleeds does very little for that.. unless you have made major size changes.. changing the idle feed restrictions does effect the way the idle mixture screws work.. if the IFR is too small you will back out the idle screws and still be too lean... if the IFR is too large.. you will need to keep the idle mixture screws farther in than you would expect..

off idle.. below 1200 rpm.. you may want to use a mirror and a timing light hooked over the coil wire or one of the spark plug wires while a friend watches your tachometer.. the timing light aiming down the carb primary throat to light up the primary MAIN fuel flow from the booster.. see what RPMS the primary booster starts to flow at.. so work below this RPM to begin to the idle transition circuits


see if your wide band air fuel display says about idle .. idle transition below the primary operation.. i should tell you that the idle transition circuits operate part way into the primary main operation.. so you have both going say below 2400 rpm.. it varies on carb size and engine displacement..


idle feed restrictions limit max fuel flow thru the idle and idle transition circuits.. idle air jets adjust the fuel curve.. how much fuel gets pulled up and over thru the IFR at how much vacuum is exposed to the idle transition slot. so looking at tuning the idle transition circuits with the mains flowing.. does not good..

if you spend a little time on the idle transition circuits.. you may greatly improve the out of the hole effect your car has. it may also effect your fuel economy.. as around town cruising RPMs and lower highway speeds.. you are mostly working on the idle transition circuit and the lower end of the primary main circuit. above 2500 or 2600 the throttle blades are past the idle transition slot.. so there is no pressure change across it and it stops flowing for the most part. at that point you are running on the mains..

sizes of idle feed restrictions, idle air bleeds, primary jets, primary air bleeds, emulsion jets.. all effect different parts of the fuel curve and every motors needs are slightly different. unless you are building identical motors on an assembly line.. that is why there are wide band air fuel displays.. to assist in tuning the different components in the carb to meet you exact fuel curves.. that is why i don't get into sizes.. its a diagnose with testing..

i do seat of the pants testing when i do something like adjust the APT settings in a quadrajet.. a quarter turn of a time upward from a minimal setting that just barely lifts the primary metering rod hanger. and the owner behind the wheel doing 15 foot long part throttle acceleration tests..
it will spit at the beginning.. but when i get close.. you should see the changes.. not just in how the car will burn rubber just off idle.. but the size of the smile on the owners face.. this takes only a few minutes on some.

i hope i have splained this well..
sorry for the wall of info .. related and partially related..
wayne...

 

In my humble opinion if you have .040" slot showing on the secondary side of the carb your secondary butterflies are open too far. The secondary slot is typically located higher in the bore than the primary slot so to get .040" exposed you have them open quite a bit. Ideally there would be no slot exposed. I believe one of the Erics posted pictures of the differences in slot location recently in another thread. Might be worth a search.

 

I thought that was on a VS carb/2 corner idle carb? This one is a 4 corner idle HP model, the transfer slots appear to be located at the same height in the baseplate.

 

I have not checked my double pumper carb but yes on a vacuum secondary carb the rear transfer slot sits about .020 higher in the rear but its been a while since I had my double pumper off and can't remember if the secondary side transfer slot sits higher or not. I think on the double pumper carbs they sit the same height.

 

My 870 Avenger has Pri/Sec T-slot widths of .025", the pri side t-slot length is .209" and the sec side has a T-slot length of .170". I have a 750 or 770 baseplate lying around and could check but they are both a vacuum secondary type of baseplates.

 

On my quick fuel 680 hot rod series vacuum secondary carb is a four corner idle and the rear transfer slot still sits up .020 higher in the rear. I have never messed with a holley hp and know some things are different. From my knowledge they have one version with vacuum connections on the base plate and the other one does not. I would assume the difference between the vacuum secondary and mechanical secondary would be the same on the base plate.

 

Well, I haven't checked the transfer slots to see if they are higher, but I did slap the carb on and get a vacuum gauge to it.

It would appear I'm reading almost 2 inches less on vacuum. I wonder if that has to do with the fact that I tightened up my lash? Do any of y'all know if a tighter valve lash will reduce vacuum due to the lifter riding for a longer duration on the lobe causing a little more overlap? I went from .022" hot to .016" hot to see if it gained any top end grunt, which it did a little.

I think the lower vacuum is causing my power valve to crack open at idle and dump some fuel.. I have a 7.5 power valve installed. With the .022" lash it would pull between 11 and 12 inches in park and about 9 inches in drive.. now with the tighter lash the highest it pulls is 10 inches in park and 8 inches in drive.. when rpm dips lower than 850 it gets into the 7 inch range and I think that might be opening the power valve.

I'm going to loosen the lash and let y'all know what happens.

 

Tightening the lash increases the overlap which will reduce idle vacuum.

If the power valve is stamped less than 85 which means 8.5 inches of Hg is the switch point, the part number will look like 510-125-85, then the valve is probably switched on.

Bogie

 

You're thinking is good except for the fact that even if it is wide open due to low idle vacuum the power valve will not supply fuel to the idle or transfer slot.....so it matters not that the vacuum drops below the valve rating at idle.

This is a common misconception repeated often, that the power valve with too low a rating will cause a rich idle...if you look at a cut-away view of the fuel path in the metering block you can se how this cannot happen. Car mag's seen to vomit up this bad info repeatedly

The only way a power valve can cause rich idle is if the valve diaphragm has a hole blown in it. Then it leaks fuel into the vacuum cavity leading to the manifold.

 

Some new info ericnova72 and I myself have came across this before of what you said but why does holley them selves pass on this information as they are the source of that I do believe? What your saying does make sense. Opinion on why holley would state that? From my research folks did not have a blown power valve and with the divide by 2 method that holley states there overly rich idle went away at least that is part of the story. Never had that problem myself though. Not questioning your comments either so hope I got it worded right so as not to come across that way as no harm intended just wanting to learn more about that. Hope heavycheavy your getting your carb stuff ironed out lol even with an air fuel ratio gauge hooked up it seems it still can be a bit picky to tune it all out.

 

I have to agree with Ericnova that the PV doesn't affect the idle. I can't say as I have proof however I have at least 4 of every PV from the lowest to about 14. I've had most of them installed at one time or another in my Willys. My chase was to prevent high speed lean out rather than idle quality. Frankly I never noticed any idle effect during this.

So proof might be to see what happens when the PV blank is installed. I don't have any and haven't had the need so I don't know this answer.

 

X2 unless his mains are activating on idle. This set up sounds pretty screwed and I don't think that air bleeds were the original problem, I'm sure they are additive to the Root Cause now. So much is hoked up at this point that getting it straightened out will be next to impossible unless the changes are backed out, get back to baseline out of the box performance and start over with the basics before going off on tangents.

Bogie