Just finished the install of a new 350/290 GM crate motor. Started up on the first crank and we thought we were golden however it does not keep idle (goes up and down) and runs like **** (starts to stumble at 3000+). Everything is new on the engine but in trying to figure out whats wrong, we also changed carbs, changed plug wires, changed HEI distributor/coil, reinstalled the intake thinking it could be a vacuum leak, and none of those has helped at all. The timing is set at the specs (10 degrees) with the vaccum advance plugged and we have tried both the ported outlet and the intake outlet on the carb for the vacumm advance with no change in symtoms. We have great oil pressure and all the valves have been set to clatter then 3/4 turns down. Not sure what else there is I can do. Any suggestions?

Reads like mech advance is varying. Might either put in stiff springs or replace springs with wire wrapped around the posts as an experiment, then see what results are given.

Might plug HEI into full time vacuum and adjust from there.

I would look for a vacuum leak, maybe under the carb gasket. I had one and the car would idle horribly, since it throws off everything when you have too much incoming air. Here is a really good site with graphics on how to read a vacuum gauge:

http://www.secondchancegarage.com/public/186.cfm

-Kevin

Hey Wheezer...


Here’s a few suggestions:

Verify the coil is being supplied with 12 volts. If you’ve converted to an HEI from a points-style distributor, your distributor might be powered using the original resistor wire. If that’s the case, your distributor is probably only getting 7-8 volts.
If your car is an automatic, is the vacuum line to the modulator connected?
Verify the plug wires are connected in the proper firing order. That one’s easy to overlook.
Bad or old fuel?
Faulty pcv valve? Try pulling the pcv valve from the valve cover and put your finger over the pcv valve opening. If the idle quality changes, change the pcv valve (they’re cheap).
Hope it helps!

Do you have headers? If you do sometimes those back spark plugs you can accidently crack them installing them, I found that to be my problem with what your describing.

I have that same engine. It has a need for alot of fuel at that rpm. I use full vacume.My initil timming is around 15 total 33 around 3000 rpm. It is a strong mid rpm range engine. Once you get it right you will like that engine!

bingo. Not sure what I did the first time when adjusting the valves but did them again and 3 were too tight. As soon as I loosened them the engine immediately sounded better.

right, isn't 3/4 turn past the noise too much?

think I got mine set to 1/4 past, but 1/2 seemed to be the avg. depending on what rpm and setup the thing will be seeing.