Thread: Carb Issues on drag car

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Hey guys! I’m building a nitrous drag car that I am having issues with. It’s a carb swapped lt1 385 stroker fully built. It’s strictly a drag car that I will take out to cruise here and there. Fully gutted, the whole 9 yards. Now I have an issue that I cannot seem to pinpoint maybe you guys could help. The other day I took the car to drive to my buddy’s house, kept an eye on my AFR gauge and everything was fine, even through traffic. Although halfway there out of nowhere it goes dead lean while I’m at a red light, 18:1. So I pull over to the nearest parking lot and shut it off for maybe 5 minutes. Start her back up and drive it to my buddys house with zero issues, and zero issues on my way back home later that night. I’m assuming vapor lock maybe? But I am doubtful. Also, if it is in park and idling I can rev it up high with zero hesitation but if I am in gear there will be a dead lean stumble and then it will accelerate a second later. Any help on this??

Originally Posted by zaneyboi

Hey guys! I’m building a nitrous drag car that I am having issues with. It’s a carb swapped lt1 385 stroker fully built. It’s strictly a drag car that I will take out to cruise here and there. Fully gutted, the whole 9 yards. Now I have an issue that I cannot seem to pinpoint maybe you guys could help. The other day I took the car to drive to my buddy’s house, kept an eye on my AFR gauge and everything was fine, even through traffic. Although halfway there out of nowhere it goes dead lean while I’m at a red light, 18:1. So I pull over to the nearest parking lot and shut it off for maybe 5 minutes. Start her back up and drive it to my buddys house with zero issues, and zero issues on my way back home later that night. I’m assuming vapor lock maybe? But I am doubtful. Also, if it is in park and idling I can rev it up high with zero hesitation but if I am in gear there will be a dead lean stumble and then it will accelerate a second later. Any help on this??

Vacuum leak

I do not believe so, only have two vacuum lines and checked those first.

Originally Posted by zaneyboi

I do not believe so, only have two vacuum lines and checked those first.

Spray a solution of soapy water at the base of the carb, and around the intake manifold to check for additional leakage.


Bill S.

Interesting that it appears to be present then not.. the no load revving and loaded stumble isn't really revealing but. What are the 2 vacuum lines running? Is 1 of them to a vacuum advance on the distributor? If so - remove and plug it then drive the car around, see how it reacts. Is the other to a PCV valve? As MrMustang mentions use some soapy water and see if it gets drawn into any gaskets or joints.. you can also use WD-40 it can effect rpms when drawn into the engine during operation, it's flammable. Probably the only thing it's good for imho! LOL

What are you using for ignition? Carburation ?

Originally Posted by mrmustang

Spray a solution of soapy water at the base of the carb, and around the intake manifold to check for additional leakage.
Bill S.

My first thought too. If you don't find it with the soapy water, how confident are you with your head to intake seal on the bottom side? Did you check the fit of the intake before assembly, assuming that the block was probably trued up to square the surfaces?

On the stumble, have you tried increasing your accelerator pump stroke a bit?

My apologies, I meant three vacuum lines, one from the PCV to an air oil separator back to the carb. A brake booster vacuum line, and my vacuum advance. I will try riding without the vacuum advance to see how it goes. I will also try some soapy water on the lines this afternoon to see.

And I am 100% confident in my intake seal!

Originally Posted by mrmustang

Spray a solution of soapy water at the base of the carb, and around the intake manifold to check for additional leakage.


Bill S.

Will try this afternoon!

Originally Posted by mrmustang

Spray a solution of soapy water at the base of the carb, and around the intake manifold to check for additional leakage.


Bill S.

Did not see any signs of leakage :/

Just thinking about this, at idle you're pulling significant vacuum, so the soapy water would tend to get sucked in like Mike mentioned, not create bubbles, wouldn't it? Mike mentioned WD40, but another choice is an unlit propane torch moved around the possible leak points. Less mess, more flammable. Just a thought....

Speaking of vacuum.. have you measured it? Have you adjusted the carb using a vacuum gauge? We don't know what cam you have. If it's race only, It may be a dog under 2 or 3 k rpms.

what kind of carb do you have ? .. i am only familiar with fixing stumble issues on Holley carbs .. i`m sure as you were running out of gas that caused the afr lean spike .. you may not have an issue there except solving the fuel starve problem .

Originally Posted by HOSS429

what kind of carb do you have ? .. i am only familiar with fixing stumble issues on Holley carbs .. i`m sure as you were running out of gas that caused the afr lean spike .. you may not have an issue there except solving the fuel starve problem .

I have the 750 brawler race carb. Mechanical secondaries and no choke.

Originally Posted by 34_40

Speaking of vacuum.. have you measured it? Have you adjusted the carb using a vacuum gauge? We don't know what cam you have. If it's race only, It may be a dog under 2 or 3 k rpms.

Comp cam, only spec I know off the top of my head 236 duration, 510 lift, 112 lsa