I also wanted to add a short story about my carb. I bought it about 2 months ago at a big speed shop in Sacramento, and I was talking to two of the parts guys telling them I was thinking of getting a 650 dp, and they both kept telling me that id be giving up hp if I didn't go with a 750 dp. I figured for my build, and how often I drive on the street, a 650 dp would be plenty, but these guys had both out on the counter and were pushing for me to get the 750 pretty hard. I decided on the 650, and when I got home and put it on I was moving the box and noticed on the side it said 750 dp. They must have taken the 650 back to inventory and sold me the 750 by accident, I hope. Anyway I figured since it was on I'd try to make it work.

So that's my way oversized carb purchase story lol I was thinking of calling them and explaining everything, and maybe they'd just swap me, but that seems far fetched

 

Are the plugs oil foulee or fuel fouled?

 

Fuel, they are black with soot all over them

 

Sounds all carb/tuning related, 6" of vacuum is very low.

Is the carb straight out of the box and bolted on with no other changes, is it new?

BTW the 750 was the right choice for your engine.

 

Yep brand new, all I changed was jets and power valve. It had 76/80 jets and I put in 71/76, however I'm thinking about bumping up the primary to 73 or 74 because there's a transition "pop" now from a lean condition. And power valve was a 6.5 but with only 6-7"Hg of vacuum I thought a 4.5 power valve would be better.

I attributed the low vacuum to the large amount of overlap on that cam, but I could be wrong.

The biggest issue is the thing is pig rich at idle, I mean REAL rich. This is my first Holley so I'm still learning about all the moving parts.

 

exact carb number... LIST R-_____ ?A

do you have a PCV valve hooked up??? so you have a little bypass air allowing you to close the primary throttles so you are NOT overexposing the idle transition slots.



at idle.. all the fuel should be coming thru the round idle feed holes..

the idle feed holes have the taper of the idle mixture screws to limit how much fuel is being pulled in by manifold vacuum..

some carbs have a hidden idle air bypass screw under the air cleaner stud. others you just open the secondary idle speed screw up thru the bottom of the base plate with a home made double ended J shaped screw driver..

lets look at where the idle fuel and the idle transition fuel come from
linked to open full page instead of way too big

http://i.imgur.com/hKLC09o.jpg

at idle the fuel is pulled past the tapered idle mixture needle.. this limits the amount of fuel at idle..

above idle the fuel is pulled from the idle feed hole but also from the idle transition slot..

there is no fixed spec on the idle screw positions.. lean best idle is one way to set it..

going back and forth with the idle mixture.. idle speed.. using a vacuum gauge.. to get to the highest vacuum then turning the idle mixture screws in till the vacuum starts to drop.. get to the exact spot where it starts to drop.. do the other side.. out to get to the highest vacuum then in till it starts to drop.. back to the point where it starts to drop.. readjust the idle speed.. if you moved it.. go back and do the idle mixture screws again.. once you get everything perfect.. turn the idle mixture screws in just slightly so the manifold vacuum drops about 1/2" of vacuum for grand total of both screws..

this is the normal way to do it..

 

The carb is a 4779-10 manual choke mechanical secondary.


Here's a pic of what I think is the PCV valve. This plugs into the passenger side valve cover. The opposite valve cover just has a breather in it.

 

Yup!
That's a pcv, but why does it look like the hose goes into the air cleaner?

 

After reading the above response and looking at the pics, and also watching a YouTube video on tuning Holleys with long duration cams, I understand what's going on with the carb a lot better now. If I had to guess, id say the transition slots are showing way too much, pulling fuel at idle from the wrong place.

I can turn a mixture screw all the way in to bottomed out, and the idle goes up but the engine runs fine still.

It sure would be nice to have adjustable air bleeds like the HP carbs

 

Adjustable air bleeds would not be the fix for this issue at all.

Clarify for use where the PCV valve hose is connected to??
Should be the large port on the rear of the carb baseplate. If it isn't then that is most of your issue, without the PCV valve bleeding air in at idle, you've got the throttle plates cranked open way to far just to get the carb flowing the necessary air. Since all that air is coming in through the carb, it is pulling a ton of fuel along with it.

 

It seems like you sure are having a hard time with this engine. Did you find out what was causing the polylocks to back off? Did you get it fixed? On this carburetor is the transfer slots adjusted to look like a square on the front and rear. The primary and secondary both have these slots. Once you get them adjusted start the engine. At what rpm is it idling? With it in Park hook up the vacuum gauge and take a reading. Divide the reading by 2 this is the size power valve you need. Looking at the instructions on a 750 4779 they should have came with 71 jet in the primary and 80 jet in the secondary. Does your carburetor have a power valve in the primary and secondary metering block? Use the distributor to bump the idle up to 850 rpm in park. Then turn the idle mixture screws in the primary bowl all the way in, what happens? If it didn't die turn all four screws in to see if it dies. Another thing get a can of Gumout carb cleaner and use it to spray around the perimeter of the intake manifold. Spray it good on the front and rear china walls. Spray around the base plate of the carburetor too. If you hear the idle increase you have a vacuum leak. Back to the idle mixture screws, if your engine didn't die you have a problem with the carburetor. Inexperienced as you are it would be best to take it back to where you got it. Tell them the problem and either fix the carburetor or give you another one. Do you have a fuel pressure gauge on the line going to the carburetor if you don't get one. You should have a line pressure of 6-7 psi. If over this it could be flooding out.

 

It's been one thing after another actually, it's my first build plus I'm not carb savvy. I did get the polylock situation fixed, I bought a set of Ultra Gold roller rockers and new polylocks and put those on. Started it up and let it idle til it was warm and drove it a mile or so and the rocker arm came loose again. I pulled it all apart and found a pushrod that was wearing down, from the lash being to tight I'm sure. Anyway, replaced the pushrod and put it all together and it's seems fine now.

I've never adjusted the transfer slots, I just bolted the carb on and tried to get it as best I could. If I'm reading right there are two ways to get more air in so that the mixture screws are actually useful. One is to open the secondary a bit, and the other is drill small holes in the throttle blades. It seems like drilling holes would be the most effective because it simply allows you to close the blades exposing less of the transfer slots.

 

I agree with the others on here and it to me speaks its the tune in the carb. I have ran across this issues before about two years ago and no matter what I did I kept fouling out my plugs and it was running very rich. Mine was a quick fuel brand holley 650 double pumper and could not understand why as everything I did would not help and I ended up getting an off idle backfire of a momentary lean condition but yet my idle in park was still super rich. I looked at my distributor and I just right before I got the carb I had installed a new rotor cap and control module on my hei and I thought I had it lined back up to where it should be and I got my timing light out and I was off ten degrees even though my mark I made with a marker was almost perfect. I gave my initial more timing from 6 btdc back up to 16 btdc and instantly it made a world of difference. I closed my butterfly's down and had the primary opened up so it was square at .020 to .030 and also had the rear to be almost open as much as the primary but did not open it up to have a square transfer slot on the rear cause it sits up about .020 higher.

With timing reset right I readjusted the idle mixture screws and only could get 1/2 way out on all four or it would start to run really rich. If I still opened up the throttle fast I still got a mild back fire so I know the accelerator pump shot needed to be tuned. First things first as the quick fuel carb came with .034 idle feed restrictor in the primary block and the second was slightly bigger but I can not remember what it was. I then went down one size at a time and eventually I got down to .028 for the primary and secondary for my idle feed size and then readjusted my idle mixture screws and now I got to about 3/4 of a turn out on all four screws and then I was able to get an 1/8 more of a turn on all four.

Now it was sitting nice and lean and no longer fuel fouled my plugs. I then went to tune my accelerator pump circuit and I started on the primary and I had to see if the backfire was coming from the front or rear when opening the throttle quite a bit. My carb out of box had pink on front and rear so I started off with changing my primary pump cam to orange which is about the middle of the pump cam assortment and it would give me a bigger shot but have less throttle duration.

That helped but it still had the slight back fire but not as bad. I then changed my shooter size from .028 up to .031 and it really made it come alive that much more. I would give it some throttle but not to the point the rear is opening up too much. The primary was pretty much set good. I ended up changing out the rear pink cam for an orange cam as well and then the shooter up to .031 and what a big difference it made. I could now smash the throttle to wide open like it was nothing or I could drive my truck around town and never had any problems after that. Your 750 is not too big for your engine specs by any means especially since you run a mechanical flat tappet cam with pretty decent duration and lift. I run about 12 inches of vacuum in park and when dropped into gear it goes down to about 9 inches so I used the old divide by two method and installed a 4.5. It has ran pretty much trouble free for about two years now. Also on the air bleeds I tried different sizes on my carb and honestly it did not make a heck of a big difference and I just left them as they came out of the box and in all of my few years of tuning a holley style carb I never have had to mess with any air bleeds and you have to go either up 4 sizes or down 4 sizes just to get a change in anything but the easier one is changing the idle feed restrictors up or down .01 at a time as they are very sensitive and just that little tiny bit of change it can make all the difference in the world for your idle mixture screws and your transition circuit

 

Have you verified float level is correct? That's usually #1 suspect of idling pig rich. Next are transfer slots.

If floats are correct, next is to verify the primaries are not opened into the transfer slots. If they are, and this carb doesn't have adjustable idle feed restrictions, you will need to close the primary throttle plates to get them below the slots and add bypass air. Sometimes you can add air by opening the secondary throttle plates with the stop screw that's underneath the base of the carb. You only need to turn that screw 1/8-1/4 turn to open the secondaries just a bit. The idle speed should increase, and then you can close down the primaries and clean up the idle.

Engine should die when you close the idle mixture screws.

 

I'm not sure how to verify float levels, but the fuel in the sight glass is just under halfway.

No idle feed restrictors in this carb unfortunately. I watched a video about drilling and tapping to put restrictors in but that seems like a complicated job for a beginner. Will drilling holes in the blades work as good as opening the secondaries?

 

The PCV valve needs manifold vacuum to function. Without vacuum the valve is left open and there is no ventilation. You will need to retune the idle speed and mixture after connecting the PCV valve to the proper source of vacuum.

 

Before making any mods to the carb get your PCV system working right. It will add air to the idle circuit.

 

Thank you guys for all the help with this, I'm learning ALOT from the pictures and explanations. I couldn't have even told you what idle feed and transfer slots were before this.

It seems I have options once I verify how much of the transfer slots are showing. Open the secondaries a bit for more air, drill holes for more air, or drill and tap for idle feed restrictors.

The PCV being plugged into the air cleaner I'm sure was pulling to much fuel through, but with the air cleaner off and no PCV at all it was still fouling plugs from being so rich. Can it be rich simply because the PCV isn't hooked to the back of the carb?

 

Yes, your PCV system is a controlled vacuum leak that pulls air through the crankcase and into the intake manifold. Once you have it connected properly and the idle speed and mixture adjusted it will run leaner.

 

X2 ^^^^ This is what you need to do.

With the PVC hooked to the vacuum port on the rear of the carb, then the metering orifice in the PCV valve is a controlled air leak.....partially acting just like the fancy Idle Air bypass in the new Holley HP carbs and the Demon carbs(like Waynep712 posted the photo of earlier), and actually very similar to the Idle Air Control Valve that all electronic fuel injected cars have.

Just as soon as you get the PCV connected correctly and you fire the engine up, it is going to immediately idle at a much higher speed because of that air now being supplied through the PCV system....you'll have to close the throttle blades to slow the idle, and then you'll need to go back through the adjustment of your metering screws on all 4 corners, at least 1/2 a turn each leaner (inward) or more and probably end up with them between 3/4 and 1-1/8 turns out each.

Drilling the blades is a very last resort.....for much bigger cams than what you are running, the kind of mods done for a cam/engine combo that only pulls 3" of vacuum at idle.

 

Holy mackerel what a difference. I plugged the PCV in to the big port on rear of the carb, and fired it up. It idled way high so I turned the idle speed down to about 750 and started adjusting mixture screws. Those were out 2 turns, and now they are at 1 turn which idled it up even more. So then I turned the idle speed down again and it idles right at 800 rpm. So after it was all said and done I turned the idle speed down about 2 full turns which is a lot. It's funny because the cam sounds much more thumpy now.

A couple things happened.

My fuel pressure gauge is way more steady now, it used to bounce from 3psi to 9psi all the time. It's pretty much stuck at 6-7 psi now.

Also, my electric fans are kicking on while its idling.

It's still a little rich smelling at idle, but the smoke is gone

 

TURN IT DOWN SLOWER and readjust the idle mixture screws again..

if it won't turn down any slower as there is no adjustment..

take the carb off.. flip it over.. you will see a flat head set screw that is the secondary idle speed screw.. turn it in 1/2 turn... put the carb back on.. you will probably have to turn the primary throttle idle speed screw in slightly then readjust the idle mixture screws again. you are getting close..

sometimes i have to adjust the secondary idle speed screw 3 or 4 times.. each time 1/4 turn open or a 1/4 turn closed.. in order to get the primary throttle adjustment at just the perfect spot..

 

Drilling the throttle plates is the quickest way to ruin a carburetor. It is not needed unless you have a race engine and that you don't have. Get the instructions out for the carburetor it will give you tuning instructions. It will show you how to adjust your floats. The float adjustment is the nut and screw on top of the bowls.

 

With the fuel level right at the middle of the sight plugs he doesn't need to adjust them.
You should adjust your idler speed and then adjust the mixture screws to get the highest vacuum reading. Then do the same thing until the idle speed is where you need it and the mixture screws are adjusted to the highest vacuum. Each time you adjust the mixture the idle will rise so you reset the speed and then readjust the mixture. The secondary throttle plates should be as close to closed as possible. Then adjust your accelerator pump linkage so it squirts fuel with the first movement of the throttle, making sure that at full stroke (wide open throttle) you have clearance on the pressure screw.

 

Well here's a small kink in my driving plans. Someone mentioned earlier about my polylock issue that turned out to be a pushrod wearing down and loosening lash. Well I got my carb dialed and was letting it idle for a bit and more rocker arms started to loosen up, so I checked and I have a couple more pushrods wearing down. I think this is from me adjusting my lash to tight on this new solid cam. It's a 282s but I had comp grind it on a 106 LSA. The cam card says adjust lash to .020" but I was fiddlin with it and tried .016", maybe not a good idea

 

When you are adjusting the 4 idle mixture screws you have the vacuum gauge hooked up to the carburetor right? Adjust each screw until the gauge reads the highest reading. I would set my curb idle at 850 rpm with the cam you have. Don't forget to increase the idle open the secondary throttle plates instead of the primary. Did you check your timing again after you installed the PVC valve correctly? I would set the initial timing at 18 degrees. Then set the distributor up with a black advance stop bushing and use one (heavy) silver spring and one (light) silver spring so the all in timing will be in by 3000 rpm. Chart C. Be careful tightening the nut for the bushing. I broke mine off once when changing bushings and I was careful using my Craftsman ignition wrench set. It cost me $75 for a new shaft.

 

Yep I'm using a vacuum gauge but still learning to perfect that skill. The mixture screws weren't really doing anything at all to change vacuum before I hooked up the PCV valve, which I'm sure has changed now. I haven't removed the carb yet but I think I'll open those secondaries like you're saying.

Timing is set exactly how you described you'd do it actually, that's how it's been with this cam and seems to run well. This was my first time tuning a distributor and it was actually really fun. I wondered though if I should try some different springs because my stall (B&M Holeshot 3000) flashes at around 2600-2800 rpm, and it would be beneficial to have timing all in when the converter has flashed right? Just an idea.

 

Lashing the cam tighter to .016" won't hurt a thing, you could have gone down to almost .008" and still been able to run it....too tight just would have actually held a valve off the seat and caused a miss, it would not force it to eat the pushrod tips.

Whose pushrods/what brands??
Sounds like a cheap set if they are chromoly, or a lesser set not meant to be run with the spring pressures you have.

 

Are the pushrods wearing the rockers or area the rockers wearing the pushrods?
I was thinking that the pushrods were wearing into the rockers - the rockers were being destroyed...

 

The pushrods were wearing but the rockers look perfect still. When I took the pushrods OUT to remove the 280H cam, the ends showed a little wear on rocker side, but I was also running stamped steel rocker arms.

 

They were Comp CS high energy 7812-16's. Says they are hardened steel but they were reused from my cam swap when I took the Comp 280H out and put in the 282s. Maybe shouldn't have reused them?

About lash, I can go all the way to .008" without leaving the valves open?

 

About lash, I can go all the way to .008" without leaving the valves open?

You could make test passes that tight, at a strip, just as part of testing but you wouldn't want to run that tight for an extended amount of time.

Point I was trying to make was that setting them at .016" didn't hurt a thing.

General rule is you can go about .010" tighter, but only about .006" looser than the recommended lash spec on the cam card, while testing to find out what is the best for your combination.

Sounds like your pushrod situation started with the previous rocker arms wearing a certain pattern onto the tips, then using them on new rockers places different wear pressure points on them so now they are failing.

 

https://m.summitracing.com/parts/cca-7812-16

 

I'd recommend a set of hardened push rods - the ones you have are not. Something along these lines : Howards Cams 4130 Swedged End Pushrods, 5/16” Dia., .080” Wall, 7.950” Length - Competition Products

Right now is the time to make sure you have the correct pushrod length as well. I would not assume that the stock pushrod is the correct length.

 

So I had a new set of the same pushrods on hand and I threw those in there so I could continue this tuning endeavor, adjusted the lash cold to .017" with a pretty tight drag on the feeler gauge.

I also changed the power valve from a 4.5 to a smaller 3.5 because my vacuum is at 6-7"Hg.

I let it warm up and started down the driveway and as soon as it had a load on it, it was having a small misfire. Could this be because the lash is too tight and the valves aren't closing or would you guess it's the power valve? Should I just set the sucker at .020" and forget it? I really like the idea of tightening the lash, but I'm not quite certain that's not causing a problem.

 

I would follow the instructions of the cam manufacturer and set them at .020. Although I doubt that is causing your misfire. Another thing I wouldn't cheap out on a set of pushrods. I would get a set like these Crane. They are the heavy wall .080 and made from chromoly. The tips are formed instead of a ball welded on. https://www.summitracing.com/parts/crn-95636-16/overview/. Did you have a problem with a misfire before you made the changes or you hadn't drove it yet? You know it isn't the timing or the advance curve. You could try 2 light blue springs. But i wouldn't change to a light silver and a light blue, it might come in too soon. Try turning the idle mixture screws in a 1/4 to a 1/2 turn just to see how it affects the misfire. If it doesn't help put them back like you had them. You can try changing the primary pump cam to a orange one. https://www.holley.com/products/fue.../products/fuel_systems/carburetor_components/accelerator_pump_parts/parts/20-12. I usually purchase one of these Holley Trick Kits because they have everything you need to tune a carburetor. You will need pump cams, Accelerator Pump Discharge Nozzles, spare accelerator pumps, and the .110 needle and seats. this is why it's best to buy a trick kit. Right now you need an orange pump cam. Find out what size accelerator pump discharge nozzles you have now. The heavier the vehicle you will want to decrease the number of the accelerator Pump Discharge Nozzle. If you have a .031 try a.028 or .025.

 

Well I set lash at .020" cold and there was still a slight misfire under load. Went home and set it to .020" hot and then it runs fine.

I don't understand why the lash can't be adjusted a little tighter with this cam?