[Holeshot71]

Originally Posted by chiphead

How do you tell the difference between a rich stumble and a lean stumble? When the motor is warming up, I can wack the throttle in park and it'll respond instantly. When the motor is at 180F, it develops a bad off idle stumble. It hesitates and stumbles when I try to wack the throttle the same way. It also hesitates slightly when I take off gently from a stop. I've adjusted the accel pump linkage so I know it's right.

Try closing the choke blade some or put your hand over the top of the carb to richen it up some and see if it gets better. If it gets worse pull a vacuum line off and retry.

Any chance you have a video of this car running? I’m considering the 2802 in a 400 I’m putting together and was wondering how it sounds?

[25stevem]

13 inches of hot idle vacuum sounds low to me for that cam and heads with a chamber volume of under 80 CCs , especially if you have a auto trans and that’s out of gear idle vacuum!
The question that’s never been answered here is the carb rich at idle and blackening up the plugs, or above idle?

[chiphead]

I had gotten the plugs to clean up fairly well. I'm running 16* base timing with a slow curve to 35* at 3500 RPM. Another 10* of vacuum advance. The behavior during accel or cruise seems pretty good. Keep in mind I've hardly gotten into the secondaries. Even though 2.56 gears causes the converter to flash pretty hard, I don't want to lug the motor at WOT. (Yes, a gear swap is pending)

I pulled the carb off tonight and flipped it over. Looks like I had the throttle plate settings messed up. The primaries were not showing any transition slot, I had the primaries closed too far. The secondaries were about to expose their transition slot. So I had it idling almost totally on the secondaries.

I opened the primaries until the transition slot is "square" when on the curb idle screw. Look at the pics, is this how the primary should be set at curb idle? I took these pics after cranking open the curb idle screw about one full turn.

How far should I close the secondaries?

[Tom Vaught]

Originally Posted by chiphead

I opened the primaries until the transition slot is "square" when on the curb idle screw. Look at the pics, is this how the primary should be set at curb idle? I took these pics after cranking open the curb idle screw about one full turn.

How far should I close the secondaries?

If you are willing to drill a small hole on each secondary throttle blade you can almost close the secondary throttle blades. .030" is a good starting size.
Some Holley carb applications already have a small hole already on the rear blades.
But now the idle air flow is based off of the engine combination you are running at the time. If the carb is a "Keeper" then you can really dial the combination in for that engine.

Tom V.

I have several "Keep Forever" Carbs. Either the 750 CFM #8156 carb,
the #4781-2 carb, or the 950 3-Barrel carb.

Tom V.

[chiphead]

This is not a forever carb or engine, I'm just learning holley. Is there a way to lean out the idle without drilling holes in the throttle plates? Can I use larger IABs or go to a metering block with screw-in idle feed restrictions?

[Tom Vaught]

You only drill holes in the blades IF you have a LARGE engine that requires the additional
idle air flow to allow the engine to maintain a typical street idle rpm. (800 to 1000 rpm)
with smaller engines not needing the higher rpm unless that have a large duration camshaft. Most time once you get the Primary Blades set then you can just work with the secondary blades. On radical engines, (Large cid engines), in street cars you might need that hole in the secondary throttle blade.

Trying to correct a lack of idle air flow is NOT THE JOB of IABs or larger IFRs.

Tom V.

[chiphead]

So I reduced the IFRs from .033 to .031. The idle quality cleaned up, the throttle plates are sitting about where they should be. Mixture screws are about 3/4 turn from seated. Float levels are good. It fires right up, the choke works, cruise and heavy throttle is good. 15 inches of vacuum at 900.

I still have a stumble when I get into the throttle. If I'm running 1600 down the road and I wack it, momentary stumble. If I wack it at idle, it takes a breath then takes off. If I cover the primaries with my hand and wack the throttle, no stumble. Is this pump cam or squirter?

[Tom Vaught]

Either is a part of the same enrichment circuit. Could be either or both.
I would try a Swap of the 30cc front pump & parts with the rear 50cc pump & parts.
If it had TWO 30cc pumps on your carb, would try opening up the Pump Squirter holes by .004 (each hole).
If you have one 30cc pump and a 50cc pump I would do the .004 (each hole) on the Primary first and then
if it needed more pump shot, swap the front pump 30cc parts with the rear 50cc pump parts and try it again.

Tom V.

The pump number is based on 10 strokes so a 30cc pump actually sends 3 ccs of fuel per pump shot and the
50cc pump actually sends 5 ccs of fuel per pump shot

[i82much]

i played all these games with a quick fuel 850 double pumper. eventually i went back to the stock calibration. Once I got the basics right -the throttle blades, float level, and idle screws, it runs pretty good all over. Could I lean it out a little? Probably. But it runs great and causes no problems.

[ta man]

Originally Posted by chiphead

So I reduced the IFRs from .033 to .031. The idle quality cleaned up, the throttle plates are sitting about where they should be. Mixture screws are about 3/4 turn from seated. Float levels are good. It fires right up, the choke works, cruise and heavy throttle is good. 15 inches of vacuum at 900.

I still have a stumble when I get into the throttle. If I'm running 1600 down the road and I wack it, momentary stumble. If I wack it at idle, it takes a breath then takes off. If I cover the primaries with my hand and wack the throttle, no stumble. Is this pump cam or squirter?

What number of PV valve? Are the acc pumps adjusted properly?

[OCMDGTO]

Going up a couple sizes on the primary squirter cured the stumble/hesitation on my old 400. I had pretty much the same carb but slightly bigger cam.

[chiphead]

Has a 6.5 power valve. 30cc pumps front and back 31 squirters front and back. I've been carefully adjusting accelerator pump levers. The stumble is right off idle. yesterday I was turning left from a stop and there was an opening so I rolled into it pretty aggressively, it paused and then spun the tires.

Last night I opened the 31 squirter to a 33 with a pin drill and took the 330 pink cam out and put the 218 white Cam in. It seems to give a much better shot as soon as I move the throttle. Just a little throttle movement produced a big squirt right toward the boosters. I'll report back on the next test drive.

[400 Lemans]

Try the number 2 hole on your pump cam. It worked better than number 1 hole. I was also getting that stumble.

[ta man]

I would double check your timing, initial, mechanical and vacuum advance to make sure all is functioning properly. You can't tune out erratic ignition with carb tuning.

[1968GTO421]

My thanks to Chiphead for starting this thread. I'm learning and re-learning alot of good material here.

[25stevem]

Rule of thumb number 1 in having a carb performing right is to see the accelerator pumps move fuel out whenever the throttle is moved.

If you do not have this then you will go insane trying to get the carb working right in its transition from idle to part throttle.

And the bigger the carb and or marginal amounts of idle vacuum will exasperate the stumble even more!

[61-63]

And this is why I really like dual quad AFBs.

[25stevem]

Why, one AFB on a single quad intake would get lonely and refuse to run right?

I don’t follow!

[Tom Vaught]

Originally Posted by 25stevem

Why, one AFB on a single quad intake would get lonely and refuse to run right?

I don’t follow!

Maybe Jon Hargrove can explain it for you.

Each AFB has an Accelerator Pump, therefore, depending on how the linkage between the two carbs is arranged, you can meter a more precise part throttle fuel enrichment before you ever bring the secondary barrels into play.

Once adjusted properly the dual quad AFBs (like the 1964 to 1966 GTO carbs), the engine can have outstanding street manners.

I will not even respond to the initial statement posted above.

Tom V.

[25stevem]

Seems to me that if you can't get one Carb out of a pair used on a motor adjusted right, be it a Holley , Carter, autolite or whatever then your chances Tom are just as slim that you will not get the 2nd one adjusted right either.

This of course assumes it's from lack of the needed knowledge and not defective parts.

You say you would not even respond to what I posted but yet you have Tom.