I just had my new engine broken in and dynoed. Its a 383 stroked to 434. Its got edelbrock heads, a comp cams xe275hl cam, 9.7:1 compression. I use 91 octane gas. the carb is a 750 DP with 71/78 jets. The timing is set at 12/30 if i remember correctly. i have the idle at 850. On the dyno it made 375hp/401tq at the wheels. Now my issue is this. It runs on after i shut it off. It dies after a few seconds. I cant keep the car running with the idle any lower. Manifold vacuum is a steady 10.5. secondly when i try to do a brake stand or apply the gas with my foot on the brake it pops and sputters out of the carb. a few times puffs of gray smoke have come out. The car is a 4sp and has 3.91's if it makes a difference. Anyone have any ideas on what to try next?

You may have a vacuum leak in the power brake booster. Try unhooking the vacuum hose from the manifold to the booster, plug it, and see if there's any difference.

Ok i will unhook the power brake hose and the vacuum line for the headlights tomorrow. anyone have any other ideas?

when the car runs on it isnt running well by anymeans. It just kind of putters out, and then lets out a big exhale sort of. But my car does have an msd box so that meybe the cause. but it didnt run on before with the msd box and the old engine. but i got a msd distributor for the new engine, while the old one had a mallory unilite distributor.

ok so a few updates. I unhooked the hose for the headlights and pluged that port and it ran a bit better so there is probably a vacuum leak there. also when i unpluged the power brake booster vacuum line some fuel came out of the the large port at the rear of the carb. is that normal? i turned the idle screws in and the car died but it runs best when they are about 3/4 out. the vacuum goes down and it runs rough even at 1 full turn out. the carb is still popping in the same situations though. the running on is intermittent or it has stopped though.

Should not be any fuel in a vacuum port. It sounds like you have a float problem.

so i adjusted the timing today to see if the car ran any better. i started at 12 initial and the car ran fine but still popped and it ran on after i shut it off a few times. i brought the timing down to 8 initial and the car felt a bit down on power. not to much but a little bit down. the car still ran on after i shut it off a few times. it still doesn't run on every time just like a third of the time. i will check the float levels after it cools down a bit. i guess it may be possible that fuel is being pulled in and burned after its shut off if the floats are to high.

Its a Ford link but it might help you. Has to do with engine run on and MSD boxes.
http://www.fordmuscle.com/forums/all-ford-techboard/455242-msd-6a-run-won-t-shut-off-key.html

i will look into the diode trick. i want to take the car to the strip on wednesday so i want to get this stuff figured out as much as possible. when the cars run on because of the msd are they running normally or dieseling

i just checked the float levels and they were both fine? any other ideas?

shut it off in gear. then shift to park.

Some how you got your tune up off. The popping in the carb didnt help either.

I got a 446 440 .030 over .010 in the hole .010 shaved 906 heads the same cam, steel shim head gaskets and a factory 6pk set up.

I can idle my engine down to 500 rpm's with no problems. I let it idle at 650. It's a 4spd . I run 91 octaine in mine with zero problems.

I run autolite 86 plugs. I was running a single point distributer untill 2 days ago. I just put electronic on it. I set the timing by ear, no light. I found the sweet spot and locked it down. You sure your not setting the air fuel mixture while the idle screw is too high? This would cause the problems you are haveing if you are.

It may be the carb itself. But you really shouldnt be having any problems like your having.

Whats the engine temp? Does it do it hot and cold temps?

I guess it didnt do this problem on the dyno?

My carbs open by vacuum. The cam we both have works great with the fuel system I have. The engine runs perfect from idle to 6200. Since I went to electronic ignition, It will spin to 6700 but there isnt a need to do that. Your cam is good to 6200.

Seems that you have a vacuum leak some where. Or you got some dusty dirt in the carb somewhere.

If it's popping, then you may have bad wires or week fouled pluggs. But it's in your tune up somewhere, long as you havent worn out any cam lobes or a bad lifter.