Hey all, i have a 400ci mated to a c6. I just fuillu rebuilt the engine and have driven it 200 miles roughly. It idles high out of gear with vacuum advance hook up and low in gear. Without vacuum advance i can get the idle in and out of gear much closer. When i start to drive off from a stop of when the trans shifts and i have low rpm in gear, the truck wants to stumble out and die. I believe the carb is adjusted fairly decent as i usef an emissions scanner to adjust mixture screws. I believe the timing might be off since it drives perfect once you get going. I have tried to baby the throttle, mash it, and in between but it wants to die when starting from a stop. It sounds like a sneeze coming out of carb. Very occasionally it will diesel if i shut it off in park (little bit higher rpm) if this helps. From what ive read it sounds like it might be too retarded. Thabks.

 

It sounds like a vacuum leak could be the problem. I'd also adjust the idle mixture for highest vacuum reading.

 

vac advance on the correct port?

 

It sounds like you don't have a timing light at this point? While it's not hard to get some results from just winging it, it's still nice to have a light.

What year is your truck? And have you removed all the original smog equipment? If you have, then you may benefit from using ported vacuum for the advance, rather than full manifold vacuum. Is that how you have it connected?

For the high idle, it's always possible that your choke linkage (not the butterfly/plate, but the actual "high-idle linkage" is not adjusted exactly right, or is binding.
But since you say it rides up when you connect the vacuum then it's probably advancing as it should. The more advanced the timing is, the higher your idle will be. The more retarded, the lower your idle goes.
Once you have the timing set, you have to compensate at the carburetor.

With all the smog stuff connected this full manifold vacuum to the advance is not an issue as it's mostly controlled by the thermal switches and such. So while some have great success with full vacuum, I always recommend trying both. If one doesn't work well, try the other.

If you do have a light, what is your base timing set at now?
Oh, and as was said, you may very well have vacuum leaks too. So check for that asap.

Is the engine newly painted? If so, then my favorite method of vacuum leak hunting won't fly. That's a can of carb cleaner basically sprayed all over the place.
Old paint is fine, but new paint does not appreciate solvents being left on it for long!

And speaking of rebuild engines... Were you able to break-in your cam properly for the first half-hour of high rpm? If not, at least part of your trouble could be a cam lobe or ten going flat right before your eyes.
Maybe this is not your issue, but I bring it up anytime someone mentions rebuilding an engine. A new cam and lifters have a very critical initial break-in procedure with ABSOLUTELY ZERO IDLING done in that first 20 to 30 minutes. So it bears mentioning again and again.

Good luck.

Paul

 

1978 f250 built 400 c6 trans with stock torque converter.
I rebuilt my engine with someone who really knows what they are doing, has built many engines over the years. The base timing was set at 16 deg and we broke the cam in correctly.
I have all the smog off the truck since this is supposed to be a performance build so theres a decent sized cam.
Yestarday i used propane and went all along the intake, carb, and all the vacuum lines with no change to rpm. I made a baseline mark on my distributor and very slightly changed timing. If i retarded it the idle change between in and out of gear was much smaller (normal change) but i couldnt drive it because with any gas it would spit out the carb and stall. If i advanced it the idle between in and out of gear was too much (didnt drive it however). I tried all of this with the vacuum advance hooked up and also unhooked to manifold vacuum as ive read this is actually better than ported. With that being said, i originally had it hooked up to ported but it ran like it did and when i switched to manifold it seemed to run a tiny bit better.

I do have a timing light borrow from a friend but i cant read my balancer and have to get underneath it with a paint marker so i can read it better.

Right now i can mash it to the floor and itll stumble (almost like a manual car if you dont give it enough gas and its about to stall) then it picks up and drives decent (although i feel like their should be more power than what is there). If i give it light throttle all it wants to do is die.
Trying to get it so i can actually drive it around to break it in fully anf then we are potentially putting it on my schools dyno for emissions class and to tune my truck better.