Thread: Carb Issues on drag car

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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!

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!

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...

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...

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!

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.

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!

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.

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.

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.

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.

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

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.

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?