I have a built 351m. It has a bad stumble right off idle, but runs good after the stumble. Its got me so confused, It always has did this sense I built it. Ive changed the plugs and wires, the MSD ignition helped it some, its not as bad. But I dont know what else to do. Ive thought about several things it could be, either a small vacuum leak or a sticky choke. What do you guys think?

79_custom

 

Bad accelerator pump. Bad vacuum advance.

 

Usually it is either a lean or rich condition. The accelerator pump and Scroob said either being bad or not giving a large enough shot of fuel when you first step on it or the opposite, that is it is getting too much fuel and it stumbles till the rpm can come up and handle it. What carb are you using?

 

What was done to the engine when it was rebuilt ? Does it backfire out of the carb ?

 

It does't backfire not one bit. It has a Edelbrock 750 performer carb
Bock: .030" over
Cam: .510/.536 lift, hyd lifters
Heads: 3 angle V-job, stanless valves, Highperformance double valve springs, Roller Rockers, shaved .030", and the exhaust ports cleaned up.
Intake: Edelbrock performer 400
Ignition: MSD 6AL Box, Accel Coil, Accel 8mm wires, Plugs are autolite 26's
Headers: Long tube hedman hearders

So do you guys think I may have a bad Vacuum Advance on my distributor?

 

What's the vacuum can tied to?

ash
['If the advance was leaking it should affect something other than just off-idle.']

 

the vacuum can is in the factory place on a 79' pickup, on the driverside inner fender under the brake booster..

 

Im running pretty much the same set up as you but im running autolite 25s. I had to re jet the carb mine was running a little to lean for my engine. But you might want to check your idle mixture screws make sure they are clean and hook a vacuume gauge to the engine and adjust the screws till you get the most vacuume out of the engine then readjust your idle. Also if your running a choke on it you can partialy close it to see if it fixes the problem if it does youll need to rejet it to richer jets. E-mail me if you have any further questions

 

When he said "what's the vacuum can tied to" he meant where is the vacuum advance hooked up? It should be hooked to the port on the carb base. If it's hooked to the manifold, that's incorrect.

"'If the advance was leaking it should affect something other than just off-idle."

There is no vacuum at all at idle on the carb port. If it is leaking, the engine will be flat on acceleration due to no advance, and the leak will lean out the A/F mixture. That is certianly something that could cause his problem.

 

Scroob, How will I know forsure that I have a leak? I just can't understand the truck, it will be setting at idle and you can watch the tach, from 700 to 1000 RPM the truck has a really bad flat spot.. But from a 1000 RPM on up the truck runs great, clean crisp throttle responce. Im puzzled, Ive put bigger plug wires on it and went with the autolite 26 plugs and that has helped its responce off idle alot better than it did.. But still it has that flat spot. It will almost die, I got to tap the pedal down when it does this to keep it from dying... but after it gets above 1000 RPM its fine...

 

Sounds more and more like a carb problem.

 

I would try spraying carb cleaner around every vacuum hose & part, to see if the engine picks up. If it doesn't, I'd put a known good carb on it & see what happens.Is the centrifical advance free, does it snap right back? It doesn't sound like a electrical problem.

 

either you have a big vacuume problem or you are running very lean. Running 26s on an engine that doesnt burn oil usualy isnt a good idea. The hotter cylinder heads can make it run leaner. Try going back to 25s and different jets after you've checked for all vac. leaks. Flat spots are usualy caused by running lean

 

Sounds like some of my previous built motors did when they had too much carb....if you are pushin in to much gas it will do that. Does the exhaust smell rich just off idle?

 

The exhaust always smells way too rich.. If you got it parked in a garage and start it, in a matter of seconds your eyes will begin to water.. But it's been like this ever sence ive built the engine. Im just getting tired of it doing this. Im really thinking about swaping carbs. My friend has a Holley, he told me he'd let me barrow it sometime to see if its the carb thats causing my problem, I'll check it for vacuum leaks tomorrow.. Thanks guys