Working on my wife's car. 86 mustang with a very mild 347 SBF and a 750HO carb from AED on pump gas. This is a high 8 second 1/8th mile car that my wife is getting her feet wet in the street class with so nothing special going on here. The engine starts, idles and takes throttle fine except when trying to footbrake the car at the track (what it was built for).

2000rpm launch, stab the throttle, the car initially accelerates for a split second, then burbles and stumbles, then takes off again. I've gone WAAY up on the squirters (31/35 to 44/42) which has helped, but not resolved the issue. I have also gone up and down on the idle air bleed (hi speed is not adjustable) only making it worse.

AED insists there is something else wrong with the engine build or that there is a converter issue. Engine is brand new, 3600rpm TCI converter, 3.73 gear etc.

Have tried a couple different timing curves in the distributor (18/36 and 26/36) with zero improvement. Have also tried leaving at different speeds with no change.

Have adjusted idle speed and verified transfer slots are not overly exposed on both primary and secondary. Have run with a slightly rich idle mixture as well as slightly lean. No change. Also went smaller on the squirters which made it way worse. Accelerator pump linkages are adjust properly.

Sure seems like there is a lean hole in the carb calibration but again, AED insists it can't be the carb and I need to look elsewhere.

I'm running out of places to look so looking for some ideas.

I'm trying to find another carb to test which will be one of the next steps as well as running the distributor with locked out timing advance.

This message has been edited. Last edited by: CSRacing,

Maybe a different accelerator pump cam with a longer duration? With more fuel going in with the bigger squirters helping, I'm thinking a longer duration shot could also help? No carb guru here, but I was reminded recently of accelerator pump cam differences by a friend.

One possible help would be to try locked-out timing. Another thought, a vacuum secondary carburetor. [Maybe someone has a loaner you can experiment with.] This problem can be solved, so stay after it. It is also possible that you have too much squirter. With the combination you described, it will be easy to feed it too much fuel at low speeds. Another suggestion: put the carb back as it came out of the box, double-check the float level, and try a one-size smaller squirter. The power valve is another tuning tool you can experiment with. I think a fuel curve slightly toward the lean side is a good experiment. And also experiment with the idle mixture screws, they can influence the tip in performance.

I went back and checked your post and realized you have already tried most of my suppositions.

Sounds like finding another carb to try is now reasonable.

Question: Has it always had this problem?

I'll go in the other direction here and just ask if fuel is coming out of the bowl vents? Seen that cause a stumble after initial shot of fuel and when car starts to move it sloshes a bunch of fuel down open butterflies.

The above post about fuel sloshing from vents is a valid idea. Make sure the internal vent extensions are in place. Even add some rubber tubing to the external vents as an experiment.

Make sure it has a progressive linkage on it. If it's 1-1 it uses up the pump shot too fast. Along the same line of thought that William had on the cam shape. With the progressive linkage the secondary pump shot starts after the primary. Just another option to look at.

Borrow another carburetor!

it is lean try 50cc pumps and progressive linkage

Lock the timing first, put the original squirters back on it, then try it.

Be sure that the pump arms are adjusted correctly.

The accelerator pump shot duration is controlled by the orifice size.

The pump squirts the same volume every stroke.

30cc pump delivers 3cc per stroke.

Bigger squirter, SHORTER duration.

The pump cam changes the throttle angle at which the pump stroke happens.

If it launches and then stumbles I'd be inclined to look at floats to high, no jet extensions, fuel out of vent tubes.
Locked timing would be a good thing. Trying another KNOWN GOOD carb is a good idea. Have you tried launching at a higher RPM?

Could also be too much pump cam causing it to go overly rich after the initial movement. This could get slightly better with a bigger squirter because the bigger squirter reduces the duration of the squirt.

quote:

Originally posted by Goob:
Lock the timing first, put the original squirters back on it, then try it.

Be sure that the pump arms are adjusted correctly.

The accelerator pump shot duration is controlled by the orifice size.

The pump squirts the same volume every stroke.

30cc pump delivers 3cc per stroke.

Bigger squirter, SHORTER duration.

The pump cam changes the throttle angle at which the pump stroke happens.

I don’t think that’s exactly right. Not all pump cams empty the diaphragm. Just look at the pump cam graphs. I think a pink cam only uses half of the available volume.

Regardless of whether it's the full volume or not, it will be the same every time.

I've had similar issues, was a 331" SBF in a Maverick, ran maybe 7.80 1/8. Tried a Victor Jr intake which it liked at the top end (MPH came up quite a bit). BUT I could never get it to leave correctly. Stuck the rpm air gap back on and it came alive again. One of the guys at the drag strip asked if I had checked for vacuum leaks. Of course I did, using a can of carb cleaner. On an engine that idles rough anyway, it's hard to do that. He let me borrow a smoke machine and I put the jr back on, smoke tested it and found that the intake was not sitting down on the cyl heads correctly. I had to let the machine shop cut the faces a little and that fixed the leaks. Went back to the track to try it and it left and ran perfect, was worth about a tenth and 4 mph on that setup.

I did not read the whole original post but have you tried jet extensions in the rear bowl?

Blow out all your air bleeds real good and I would open up your idle mixture more and see what that does. Holley books say the idle mixture does nothing over about 1600 RPM but my experience tells me it has some effect up to 3,000 or more.

Get ultrasonic cleaner and tear it all down and cook it for about 5 minutes in simple green. You will be amazed at what comes out of it.

Then inspect and reassemble. Not sure about your carb but APD is very particular with the way you go about adjusting idle.

Agree that anytime mine did not come up on chip clean I would back out idle screws and it would clean it up. Your problem is different but a small change like that might help.

First step is to make sure everything is prestine clean and not plugged. And replace floats, needle/seats.

quote:

Originally posted by ski_dwn_it:
Get ultrasonic cleaner and tear it all down and cook it for about 5 minutes in simple green. You will be amazed at what comes out of it.

Then inspect and reassemble. Not sure about your carb but APD is very particular with the way you go about adjusting idle.

Agree that anytime mine did not come up on chip clean I would back out idle screws and it would clean it up. Your problem is different but a small change like that might help.

First step is to make sure everything is prestine clean and not plugged. And replace floats, needle/seats.

The carb is brand new with maybe 10 passes down the track. Engine is brand new, fuel system is all brand new including filters. Carb has been apart and is perfectly clean inside.

quote:

Originally posted by Eman:
If it launches and then stumbles I'd be inclined to look at floats to high, no jet extensions, fuel out of vent tubes.
Locked timing would be a good thing. Trying another KNOWN GOOD carb is a good idea. Have you tried launching at a higher RPM?

Locked timing made no difference. Floats are exactly where AED is telling me to set them.

Borrowed carb installed last night but haven't tested it yet.

Borrowed carb did the same thing. Put the original carb back on and picked a lower foot brake launch RPM and it seems much better.

I guess that creates a new question.. What would cause a studder trying to foot brake launch at 2000rpm that isn't there at lower RPM? I initially though accelerator pump, but I tried a 50cc pump with a black cam and it made no difference..