Problem: 383 idle's rough and tries to die after running for 20-30 minutes

[Max Stryker]

Guys I'm at a loss with this one. My 71 383 is built up. 10.5:1 Compression, .545 lift cam, High rise and a reworked 850 Holley. The motor has about 200 miles on it. I've had an insane amount of difficulty with the car for a variety of reasons, but recently this problem has shown up. When I drive the for maybe 20 minutes or so the idle gets rough and the car won't stay running without me giving it some gas. It's tricky to keep the engine going and I'm not sure what the issue is. The carb was "tuned," as well as timed by a resto shop (who managed to mess several things up, so I don't trust their work, but they will remain nameless for now). The car is has some pretty good leaks from the oil pan gasket and rear main that will have to be addressed. The car doesn't seem to like exposure to heat. I've noticed that, on occassion, if I've been running around town in the car, then the motor will continue rotating for a half a second after I turn the key off. I'm telling you guys that because I'm wondering if it's a heat issue and could mean I need to run slightly colder plugs. Cast Iron heads that have had work done on them. Any thoughts would be helpful. Feel free to pm me. I'm in Arkansas.

 

[WP29440SE]

The carb was "tuned," as well as timed by a resto shop (who managed to mess several things up, so I don't trust their work, .

I think you answered your own question.
if you have to give it fuel to keep it running, obviously its a fuel problem. this could be:
excessive fuel pressure/flooding/ floats set too high or,
jacked up carb all together
weak fuel pump
fuel starvation
is this a factory fuel pump or electric? do you have a fuel pressure gauge?
run-on is usually timing that needs attention, or excessive raw fuel still in the intake burning or both
the spark plugs are not the issue here , unless they are getting saturated with gas.

you only need 5-6 psi of fuel pressure for that carb

I would first , try a different known , good running carburetor, and get a fuel pressure gauge
make sure you have between 5-6 psi
I bet it runs completely different

 

[Max Stryker]

I have a Products Engineering Electric Pump and a fuel sump welded into the gas tank. I do have a fuel pressure gauge and it has been sitting at 6 psi consistently. I'll check into the timing issue

 

[WP29440SE]

I have a Products Engineering Electric Pump and a fuel sump welded into the gas tank. I do have a fuel pressure gauge and it has been sitting at 6 psi consistently. I'll check into the timing issue

run it until it runs rough, pull a spark plug, see if it is wet with gas
you can pull one now, if it is black, it is running super rich
it sounds to me like its flooding

 

[Max Stryker]

Update: I had a buddy help me check some things. Noticed that the problem occurred almost right away (before the motor was really hot). Also, once the problem occurred hopped out and noticed that the fuel pressure was up to 13 1/2 psi at idle. I'd say that engine bay heat is an issue, but I don't know what would cause this kind of pressure surge. The surge in pressure would explain excess fuel bogging the motor down or causing some run-on. The question is am I looking at a bad fuel pump, carb, or just some insane vapor.

 

[sleepar]

Bad fuel pump.... the fuel pump should be putting out 5-7 psi and 5 is good for a carb.... above that it's pushing fuel past the needle and seat.

 

[WP29440SE]

Update: I had a buddy help me check some things. Noticed that the problem occurred almost right away (before the motor was really hot). Also, once the problem occurred hopped out and noticed that the fuel pressure was up to 13 1/2 psi at idle. I'd say that engine bay heat is an issue, but I don't know what would cause this kind of pressure surge. The surge in pressure would explain excess fuel bogging the motor down or causing some run-on. The question is am I looking at a bad fuel pump, carb, or just some insane vapor.

are you running a fuel pressure regulator? if not, get one. 5.5 psi would be about perfect. 13 1/2 psi is crazy high, im surprised it runs at all. once you get your fuel pressure under control, I would change the plugs too.

 

[Max Stryker]

Yes, a product engineering regulator.
Seems like a valve issue in the carb. Also, been told the regulator is improperly (it's alongside and underneath the carb bolted to a plate on the valve cover). In addition, it seems that pressure can build up in those lines and drip a little gas from the fittings too the regulator. Rarely, but not good