Carb stumble

Looking for some help. I have a small block 360 drag race motor. After adding aluminum heads last season, there was a small stumble if you try to jab the throttle from idle. I footbrake race, and when I bring the car into stage I'm at 2500 rpm. It has never stumbled when on track. It only happens from idle. Timing is at 36 degrees. I tried 38 as well, but no change. Primary squirter is 35. I tried both up and down and no change. Power Valves are blocked and jets are at 76. It runs as smooth as can be other than when I jab the throttle at idle. Starting to get frustrating so looking for any ideas/suggestions to try. Thank you in advance!

You might start with the accelerator pump circuit.

https://www.holley.com/blog/post/how_to_adjust_the_accelerator_pump_on_holley_carburetors/

You got a good link on the video follow those directions. One other thing, when you stab the throttle, the spring on the pump linkage compresses so you don't bend the linkage. So in reality the strength of that spring controls a lot of how much and how long of a pump shot you get. If you stack a couple small washers under the spring you will make it stiffer, making sure you get a strong pump shot. Don't get carried away because if you make it too strong you can bend the linkage. But I have fixed several new out of the box carbs that had too weak of a spring this way.

Does the exhaust stink when warmed up at idle? If so you need to lean down the idle mixture to help it not bog.
If it doesn't stink try enrichening the idle mixture screws a 1/4 turn at a time to see if that helps get rid of the stumble
You can make a carb stumble off idle by being to lean or to rich
Do your squirters squirt fuel as soon as you move the throttle shafts any at all? If not adjust the screw so it does
What brand and size is the carb?

It doesn't seem to smell, so maybe I will try enrichening the idle mixture. The squirters work as they should. I've checked that over. It's a Holley....double pumper....750 cfm.....4150 series.

when you say work as the should, do you mean, basically as soon as you breath on the throttle lever you see a shot of fuel from the squirters?

I've revisit the idle mix, fully warmed, lean out to highest idle, then go about 1/16-1/8 turn richer all around, after those two things...i would maybe look at pump cams.

Yes. As soon as it's touch it's squirting. That's the first thing I checked. I think I'll do the idle mix first like you say. Then if it's not fixed, I'll have to keep searching

Does it still have the stock undrilled throttle plates in it?
Is it a four corner idle unit or the front only type? If four corner adjust all of them, if not do what you can
If no extra holes in the throttle plates try the idle mixture screws and make sure the rear throttle shafts are not opened a bunch

I would suggest a wide band so; you can determine exactly what the car is doing and go from there.

Blue cam on 30cc or put 50cc pump on it.

You indicated it's a double pumper. Did you careful check the secondary accelerator pump behavior as well as the primary?

Is the timing locked at 36 or is that the total number?

"Blue cam on 30cc or put 50cc pump on it"

^^^^^this

We never had any hesitation issues on either the 750 or later the 850DP carbs we ran on my 340 car. However in an effort to make things even better with what we had we put a 50cc pump on the 850 and noticed a good improvement even on the street when testing, it had the desired effect at the track on the hit f/braking, and that carb was almost 2/10ths quicker in the 1/4 than the 750DP the car came with so a good gain for not a lot of money, just glad I used to think out of the box as everyone else at the time said that was too big of a carb for a small motor.

Not locked. Initial is 21. Total 36. Locking out the distributor is something I’ve considered.

Check the SECONDARY A/P adjustment

same clearance on the secondary AP as the primary AP? I'm at 15 thousandths at WOT

Get a spray bottle and put gas in it. Have a buddy (that you trust) stab the throttle On the brake, to the point where it stumbles. be ready and give it a spray from the bottle. If it's better you know what to do, if it's worse you also know what gives. I know it's redneck, but saves hours of playing around trying this that or the other. Done it on the dyno, and in the car, most times with the trans brake.


Also if it's 3 circuit, try tooth picks in the intermediate air bleed to block them off. If that is worse, just remove the intermediate air bleed, and try again.

Obviously goal being just to know whether you need fuel there, or have too much.

Motor off, slowly open throttle to WOT and observe when the secondary shooter discharges relative to secondary throttle plate opening. Poor relationship can lead to a lean or over-rich spot; increasing primary pump discharge can cover up lean spot, or changing secondary cam or cam position to fix timing can fix lean or rich..

hi

check the vacuum at idle and reinstall the correct the pri power valve .

this will add fuel before main jet and eliminate bog.

You do not set power valve opening by idle vacuum. That is incorrect.

In addition to the fact that you don't set the power valve opening by the idle vac, the power valve is part of the main circuit. So it can't add fuel before the main circuit. Unless, of course it's blown.

I've seen some guys have the idle speed adjusted so high it is idling on the transition and mains circuits instead of only on the idle circuits