Hesitation Issue - need bigger pump cam?

I have a 350 engine, with a comp cam thumpr cam in it, which has lowered my vacuum, I put in a smaller power valve to keep it idleing, it stays running now at a solid 800-900rpms, but now I am having some terrible hesitation issues off idle, I'm guessing accelerator pump cam, or maybe the pump nozzle? I am currently running the stock cam that came with the carb, and the stock 35 nozzle.

The carb is a holley 670cfm aluminum street avenger, the distributor is an msd pro billet.

Category:

Autos & Vehicles

Tags:

• camaro
• holley
• tune
• tuning
• msd
• 350
• v8

[jpenn1192]

u should put a high volume fuel pump on it...

[hectre13]

@jpenn1192 it has a high volume fuel pump on it.. it was a bundle it came with the carb.. it is also a holley.

[aintmisbehavinn]

Did you ever figure this out? I've got the same problem

[hectre13]

@aintmisbehavinn not yet actually, its been sitting for a while because i am so frustrated with it.

[79fordblake]

Does it ever back fire out of the carb or does it just stumble? Nozzle may be to big.

[hectre13]

@79fordblake nope never has backfired.

[blue06spectra]

I'm having this problem and I know I blew my power valve recently. I have a 750 vacuum secondary on a stock 1971 460 Ford engine. When I'm driving it sometimes it will backfire and lose all power like its going to die. I hit the gas and it falls on its face but it will idle just fine. All of a sudden it will stop and ill have full power again. What could be causing these problems? Could it just be the power valve, accelerator pump, or maybe nozzle size?

[RMAHL]

its a timming issue

[Silver240z]

If it shoots smoke from the tail pipe when the hesitation occurs go down a size in pump nozzle until its corrected. if no smoke comes out of tail pipe go up a squirt nozzle until its corrected. I agree with Jabba974 a 670 cfm is a little big for the motor. good luck.

[jabba974]

@jabba974 And you might want to look at the advance kit in that MSD dist..it comes with a pretty heavy spring in it out of the box... good luck

[jabba974]

First thing I would look at is timing...bigger cam needs more timing...then adjust the accelerator pump if need be... I wouldnt mess with the jets, a 670 cfm carb is pretty big on a 350ci...also make sure your idle screws are adjusted correctly... Working in a performance parts store for 15 years, i have only seen the need for a bigger carb a few times..

[MrRagingbull480]

My 4160 vac sec does the same its driving me nuts!.I thought it was ignition so got a new 65,000 volt dist advanced my timing and re adjusted my air and fuel mixture screws. Doing this actually helped a little. Its not as bad but it still stumbles and runs like shit when I hit 4 grand only on the hwy. From a dead stop punching it there is the slight hesitation but after that it gets up to 55 grand really quick. I'm gonna do some more adjustments with my timing and carb and hope this goes away.

[ACHEESEDANISH00]

@hectre13 i would go double pumper. whats the specs on the thumper cam?

[stangredemption]

Oh ok well 50cc or better yet a pump cam on the primary lol i had the same isuse, check the vid on my page.. That was when I was first tuning it

[calupjensen]

ok i have 670 street avenger and has 65 jets went up all the way to 79 and not much change . and at 4 grand all the way threw when hits 4 grand runs shitty .fine off start but when 65s in has stumble when punch it do i need 750 because plugs show it lean?

[calupjensen]

ok so i have 65 jets in front went up 14 sizes dont do much diffrent and pugs show it runs lean and at 4 grand all the way threw all them jets runs shitty when hits 4 grand .take it back down and have my dead spot at like 2 grand ? do i need buger carb like 670 to 750?