Stumble at Idle When Gas Pedal is Stabbed

[davis95]

This is something that I've been trying to resolve for some time now with only little luck. Carb idles and runs fine, but when I do a quick tap and release on the gas from idle it'll either stumble or kill the engine. If I nail it and hold it down for more than a second or two it responds normally. This happens mainly when in gear with an occassional stumble when I tap it with it in park. If I increase the idle to around 950 it doesn't happen nearly as much, but it seems to be idling a little high when in park with it set like this. I disconnected the accelerator pump and it still runs decent without it with the exception of a stumble if I accelerate quickly. I checked the nozzles for a good spray when I press the pump plunger down and it sprays normally. With it idling I can quickly press the pump plunger only and get the same response that I get when I do the quick tap on the gas. Also with the pump disconnected it still responds the same way as far as the stumble and dying upon quick-tapping the pedal. Timing is set at 18 and 36 w/15 on the vacuum advance and holds steady when I watch it with the light. The advance is connected to the port on the driver's side corner just above the throttle blades. I set up the idle screws and apt with a vacuum gauge and it runs as smooth as it probably can. What else do I need to be looking at?

[omaha]

It sure seems like the accel pump. Other things to check; cranking pressure, (should have at least 130-160+)  vacuum leaks,( can cause havoc with idle issues) vacuum level at idle. (depends on what cam is in it) [this also sets the idle rpm kindof]  you need to get a good accel pump (from cliff). Maybe it likes to idle at 950 but the jump in rpm's whem put in park seems like either a vacuum leak or low cranking pressure.
Is this motor really tight (not broke in yet)?  Just my .02 worth, I'm sure others will chime in with thoughts.
   (i am not the best tuning guru there is. Usually it takes a bit of experimenting for my car to get where I like it also and it's not there yet if that makes you feel any better)

[davis95]

I did some more tweaking and was able to get another 1/2 lb of vacuum by backing the mixture screws out another 3/4 turn each and it took care of the surging problem. It must have been flooding out when I'd stab the gas at idle due to getting a lot of fuel and not enough air to burn it. I also made a nice score today when I picked up (traded some old Camaro headers for) another large cfm 79 APT unit. It's filthy and still has the factory plugs over the mixture screws. I popped the top and found it to have 72 mains with 52M rods and DP secondarys. This'll probably be the one I build for my 600 HP big block.

[omaha]

good deal.  Good score on the carb! Still has the plugs on the idle screws! Wow, that is rare.

[von]

I worked on one (7029200) that had accel pump nozzles at around .034. It had a very healthy pump shot stream. But with everything set exactly to Recipe #1, new shaft bushings, etc., it had a bit of a flat spot off idle. I'm wondering if someone in the past drilled out those nozzles and the pump shot is TOO big, creating a temporary too rich condition when the accelerator is pushed. Is there any way to downsize the nozzles?

[Cliff Ruggles]

It's not easy, but they can be made smaller.  I make a very small punch that will go thru the larger hole and reach the actual restriction.  Gently tap the punch to "peen" the hole down in size, then re-size as needed....Cliff

[von]

Ingenius! Is it possible too large a pump shot would cause a stumble or flat spot?

[Cliff Ruggles]

Yes, too much fuel if not needed can produce a hesitation much like not enough fuel....Cliff

[von]

Thank you. BTW thanks for the tips last week on installing main air bleed and idle down channel restrictors. Undoubtedly saved me some trouble.