Off idle stumble/pop

[brickyard69]

I have a 383 approximately 475 hp with a Comp Cams 288 roller.

The car has a new Holley 750 hp and a new Flame Thrower HEI.

I have the initial timing set at 15 degrees witha total 35 degrees.

when a rap the throttle from an idleI get a slight stumble and sometimes a backfire through the carb (like a lean pop). I installed larger squirters and cam in my previous 750 and that did not seem to help. I then bought the 750 dp as I thought I neeeded more fuel.

The "pop: is not as bad with more timing. I have been playing around with the vacume advance (adjustable kind) but it does not seem to remedy the proble.

Any suggestions?

[Shaker455]

How's it jetted and did you tune your idle mixtures screws with a vacuum gauge?

How many turns out are they and is it 4-corner idle?

Did you have to move the throttleplates open?

Are there any hole in the throttleplates?

[Tech @ BG]

How does it accelerate if you open the throttle slowly?

How large did you go on the squirter sizes?

[Gordz32]

I have the same problem, And as with yours it gets better with more timing, pretty much none existant really.

[SM377]

I would think its the carb. maybe to much fuel

[Gordz32]

Mine came down to a missing check ball and needle under the accelerator nozzle.

[MrQuick]

Also check your accelerator pump cam lever to diaphram maybe too large.

[PT Goat]

This is a common problem with Pontiacs running tripowers on the larger displacement (421-428-455) engines. By the time you get the engine enough gas at idle, it's too rich. It is often cured by drilling small (.065-.080) holes in the throttle blades to lean the mixture out. But I'd certainly exhaust all other options before getting the drill out.

[David Pozzi]

Make sure your mechanical advance is kicking in early and fully advanced by 2500 to 2800.

[My427stang]

1 - How did you adjust your accel pump?
2 - Is it a solid roller or hyd and in either case, how did you adjust them?
3 - Are the plug wires running together in a bundle?

[ProdigyCustoms]

I will almost bet it is too little fuel or a really bad timing curve. To much fuel cases bog, to little fuel cases boom! Try changing the pump cam setting to bring in the fuel more aggresivly, and go back down on the squirter. If the squirter is too big, you little 30CC pump will not keep up with the bigger squirter.

As for adjusting the accelerator pump. With the throttle wide open, you should be able to compress the pump lever .0015 more then the actuator compresses the pump lever.

Play with lightly bliping the throttle and make sure you are getting a instant strong squirt. If not, do not change any timing or anything else until this is worked out. If the squirt is still weak, try a new pump diaphram (I know it is new, but believe me I just did this). If that does not help you I would buy a pump cam kit (they tell you which is which in the kit).

Now, if you have a strong pump shot, and it pops, then I would look at the timing curve. I know you have 15 at idle and 35 total (I would not be affraid of 18 and 38 BTW) but how does it come in?

If that does not work, you could try a 50CC pump (which is what we did to Michael's BRAND NEW 750HP on his Monte Carlo, fixed everything)

If none of that works, you could spray some carb clean around the possible vacum leak areas 9carb to intake, intake to heads, vacum ports), little shots here and there, and see if the engine picks up.

If all this does not work I would be looking for worn exhaust cam lobe, or a bad intake valve. But you did not want to hear that did you!