strange new problem with my engine.

[vetteKID]

I've had my vette for about 3 years now, and I just started modifying it. I always had a problem with bogging under hard acceleration, but its wasn’t that bad. This winter, I put on a 50 shot nitrous system (haven’t used it yet) put on some bullet mufflers with dumps, new wires, coil and cap and rotor, and an edelbrock manifold. I had driven it briefly after every modification and it drove fine. Now I brought it out the past few days, and it drives ok...until I get it above 3000 rpms then for the rest of the time of my driving it, doesn’t matter what rpm, it sputters, hesitates, misses and pops pretty violently and if I don’t give it gas it dies. I went back and checked all of my connections and hoses they seem fine. Is this a vacuum problem?

 

[AKRAY4PLAY]

have you replaced your spark plugs?? i have had problems in the past with plugs that crack the insulator and misfire at higher rpm. nitrous has a bad habit of blowing out spark, you need a smaller plug gap. i would put in some new spark plugs and gap them at .030". if that cures your problem, them you can start opening up the plug gap until it misses again. you want the biggest gap possible without misfire. a coil test wouldn't hurt either. good luck, Brian.

 

[vetteKID]

i just spent about an hour swaping out spark plugs. gapped them at .035, which is what the manual told me to gap it at. the car idles a little better, still a miss somewhere. i took the k&n air cleaner off and ran it open carb around the neighborhood worked ok. put the air cleaner back on and the problem came back. could a 3 X14 inch air cleaner choke a carb out?! i cant explain how this hesitation feels other than its the worst i have ever felt, the rpms are at 3000 the car sounds like its underwater and doesnt move, then when it comes down to idle, it sounds like a have a serious cam and doesnt want to stay on! but then two revs and its back to normal! so confused! need more suggestions! could it be a Q-jet problem?

 

[JohnZ]

The K&N air cleaner lid is probably sitting too low, right on top of the carb inlet, restricting airflow and bowl venting. Does it run OK with the stock air cleaner?

 

[Ken Anderson]

Too much oil in the K & N is like sticking a sock in your carb.

 

[drags1998]

Make sure that the air filter lid is not closing off the bowl vent on the carb!!!:L

 

[vetteKID]

stock air cleaner wont fit under the hood anymore and it still does it after i took off the air cleaner, just takes more rpms now, if i hit 4000 it starts the problem. i just put plugs in too and it still has a strange miss at idle...could my timing be off? maybe my secondaries are off?

 

[silver 80]

Are you 100% positive that the wires are on right?
tom

 

[vette-dude]

What kind of Carb are you running on this car?? Is it backfiring out the carb or popping through the exhaust system?

Randy

 

[vetteKID]

i checked the wires to the hayes manual when i did the plugs, they were right. the cars running a stock Q-jet. when it doesnt have any load on the engine its fine, but the second the clutch is let out the car just bucks and hesitates, i wouldnt say it backfires like a gun shot, just makes a popping noise out of the exhaust after every hesitation.

 

[craig32]

VetteKid, Have you checked your fuel filter? Could also be a fuel delivery problem. What type of intake did you put on? I once tried a 'tunnel ram' on a '57 Chevy, but it only allowed about a quarter of an inch movement on the distributor between the intake and the firewall, so needless to say it came right back off. I've always had beter luck with factory equipment. Have you changed anything inside the engine, like bigger cam, smaller heads? That will affect your timing. Just my opinion.
Craig

 

[vetteKID]

i have an edelbrock performer manifold, put that on way before this problem started. where is my fuel filter? engine internals are stock.

 

[craig32]

Your fuel filter is either a bronze element in the carb right at the inlet, or an in-line filter between the fuel pump and carb - or both. If it is a plastic in-line, make sure the arrow points to the carb.
Craig

 

[oopsme]

breaking up above 3500 RPM

I had the problem you describe, after doing a pressure check on the fuel pressure I found that I was only at 3.5 psi when the spec calls for 7-8 psi. A new fuel pump solved the problem.

 

[Evolution1980]

What strikes me funny is this statement...

...until I get it above 3000 rpms then for the rest of the time of my driving it, doesn’t matter what rpm, it sputters, hesitates, misses and pops pretty violently and if I don’t give it gas it dies.

So this only happens after you hit 3000 rpm, and then stays that way? Until when? When you shut it off? If you shut it off then restart it immediately, is it back to being fine again? It could be vacuum related, but more related to something sticking. To my very uneducated knowledge of the Qjet, I'm thinking your secondaries may be sticking open or your accelerator pump is not functioning properly.
You get the car up to 3000, the secondaries start to open. You come off the gas, the butterflies stay stuck open a bit causing your air/fuel ratio to go to hell. You shut the car down, vacuum releases entirely, and the butterflies close.
I don't know. I could be completely 100% wrong. I'm just throwing things out there. Someone will know. My mechanic is coming over today. I can ask him (if I remember).

 

[mike weyman]

my guess is fuel delivery problem,but it sounds a lot like when i wiped a lobe on the cam, is it getting progressively worse??

if fuel works out o.k. check the cam lobes

 

[74bigblock]

Sounds more like a choke issue to me. I bet your choke is engaging at half throttle and the prob doesnt go away until the temp goes down.

 

[Bob Chadwick]

When I was having the high speed intermittent miss on my car there were a lot of suggestions that due to changes with headers and and a new intake that I might have a fuel starvation issue. The suggestion was that I needed to upgrade the fuel delivery capability of the carb. At this point I got totally confused with Lars, who evidently one of the guru's on carbs, telling me to upgrade the secondary jets and others telling me the rods. I bought that carb book that is so popular and at one point could have told you why one over the other but can't any more.