Carb Issues on drag car

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??

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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??

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Vacuum leak

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

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Originally Posted by zaneyboi

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

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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 ?

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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.

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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!

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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.

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Will try this afternoon!

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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.

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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 .

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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 .

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

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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.

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

Just thinking, if you've confirmed no vacuum leak what could cause a dead lean condition when the engine's been running fine, AFR good, pulled up to a stop & idling. Something getting hot and opening up an air path, then closing back when you parked for 5 minutes & not opening up later on the ride home, perhaps when it was cooler? Seems a stretch, and can't think what it would be.... or how to find it!

What about something simple, like a piece of flotsam in the gas tank blocking the sock and causing a loss of fuel pressure & drop in the bowls. Seem to recall someone here (jb?) having a problem that only popped up occasionally where the engine starved until shut down, and they found a piece of a brown paper shopping bag in the tank, and no idea how it got there? The paper would block the tank exit, covering the sock but still letting some fuel through, then when shut down would float away to come back another time.

Grasping at straws here. It's an interesting dilemma!

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Originally Posted by rspears

Just thinking, if you've confirmed no vacuum leak what could cause a dead lean condition when the engine's been running fine, AFR good, pulled up to a stop & idling. Something getting hot and opening up an air path, then closing back when you parked for 5 minutes & not opening up later on the ride home, perhaps when it was cooler? Seems a stretch, and can't think what it would be.... or how to find it!

What about something simple, like a piece of flotsam in the gas tank blocking the sock and causing a loss of fuel pressure & drop in the bowls. Seem to recall someone here (jb?) having a problem that only popped up occasionally where the engine starved until shut down, and they found a piece of a brown paper shopping bag in the tank, and no idea how it got there? The paper would block the tank exit, covering the sock but still letting some fuel through, then when shut down would float away to come back another time.

Grasping at straws here. It's an interesting dilemma!

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Sorry for the late reply I have been away. But I do get a constant 6-7 psi on my fuel pressure gauge. It is a head scratcher for sure...

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Originally Posted by zaneyboi

Sorry for the late reply I have been away. But I do get a constant 6-7 psi on my fuel pressure gauge. It is a head scratcher for sure...

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It's not really a head scratcher, you have a good setup for a track only, idle to wide open throttle engine. On the street, you are going to have hesitation, bogging, and will not be happy if that is your long term plan to drive on the street 90% of the time, and the drag strip 10% of the time. You will not be able to tweak the carb to serve both purposes, and still have good passes on the track.

Bill S.

Automatic or manual transmission, if an automatic what stall speed converter? There really is no way to tune one carb to make it perform equally well on the street and the track, an EFI system will come close to doing both but even then you would need to get your lap top out and do some "tuning" unique to either operation. Also, you didn't mention fuel pump, regulator, or fuel line components, the amount of fuel flowing to the carb is equally as important as the amount of fuel pressure!

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Originally Posted by mrmustang

It's not really a head scratcher, you have a good setup for a track only, idle to wide open throttle engine. On the street, you are going to have hesitation, bogging, and will not be happy if that is your long term plan to drive on the street 90% of the time, and the drag strip 10% of the time. You will not be able to tweak the carb to serve both purposes, and still have good passes on the track.
Bill S.

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Originally Posted by Dave Severson

Automatic or manual transmission, if an automatic what stall speed converter? There really is no way to tune one carb to make it perform equally well on the street and the track, an EFI system will come close to doing both but even then you would need to get your lap top out and do some "tuning" unique to either operation. Also, you didn't mention fuel pump, regulator, or fuel line components, the amount of fuel flowing to the carb is equally as important as the amount of fuel pressure!

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I don't disagree at all that it's going to be all but impossible to "detune" for smooth street operation on a carb optimized for the strip, and the questions Dave poses are good info to know too, but to me that doesn't address what would cause the A/F ratio to go from "good" (I assume that's in the 11.5 to 12.5 range) up to 18.0 while idling at a stop light, then after being shut down for five minutes being "good" again for rest of the trip to his buddy's, and then all the way home later. Something had to either starve the fuel or dump in a bunch of excess air, then correct itself with the shutdown. The Holley Street Avenger on my SBC says fuel pressure should be 5-7psig, so his "...constant 6-7psig" is dead on. No answers, but I don't think we're addressing the problem yet. Just my $0.02, and still scratching my head.....

I agree with Roger. the "jumping" AFR ratio is what we were discussing. My other thought was the gauge or sensor had a malfunction.

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Originally Posted by 34_40

I agree with Roger. the "jumping" AFR ratio is what we were discussing. My other thought was the gauge or sensor had a malfunction.

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Where is your O2 sensor that drives the A/F indicator located in your exhaust? Is it possible that when you stopped at idle you got a "backwash" of ambient air to your sensor, or that you have an exhaust leak that started sucking air as you dropped to idle? The guy who helped with some EFI tuning mentioned to me that the clamp on sensor that he used was good with the revs up, but questionable at idle when exhaust pulsations could puff in & out near the exhaust tip.

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Originally Posted by 34_40

I agree with Roger. the "jumping" AFR ratio is what we were discussing. My other thought was the gauge or sensor had a malfunction.

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That could be the issue but the gauge and sensor are both almost brand new so I am not sure...

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Originally Posted by rspears

Where is your O2 sensor that drives the A/F indicator located in your exhaust? Is it possible that when you stopped at idle you got a "backwash" of ambient air to your sensor, or that you have an exhaust leak that started sucking air as you dropped to idle? The guy who helped with some EFI tuning mentioned to me that the clamp on sensor that he used was good with the revs up, but questionable at idle when exhaust pulsations could puff in & out near the exhaust tip.

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The sensor is right past the collector, then to dumps about a foot pasted the sensor.

Zaneyboi,
Did you figure out your carb issue? Lots of ideas tossed in, just wondering what you may have found?