Sbc with Erratic vacuum and dies in gear

[Drkuhar]

I was helping a friend that had a 79 monte carlo sbc that had 2 barrel w/single exhaust upgrade his car. Ran good but quiet nothing special..

He Put used aluminum centerbolt heads on. Aluminum intake, new edelbrock avs carb, headers and new dual exhaust. New plugs and plug wires

I Used a timing light to set 10-12 degrees base timing with dist disconnected . Using a vacuum gauge to set carb needle is jumping rapidly and in the late timing zone. Will idle good at 1000 rpm but not lower and dies when dropped in gear. Has to be something we have done since it was fully functional and driving before his upgrade. I was not there for the head swap just after the fact of it not running right
Any ideas on what to look for. Could he have rockers too tight
Thanks

 

[83StreetMachine]

It most likely has a huge vacuum leak. Check around the carb base and then around the intake where it bolts to the heads.

Do this by spraying a flammable liquid, like brakeclean, or starting fluid around those areas I mentioned. If the engine rpms increase when you spray, that's where your vacuum leak is.

Please be mindful of how and where you spray those chemicals. Get it near the hot exhaust and you'll have a fireball. Always have a fire extinguisher close by.

 

[79 USA 1]

If it was a stock small block in the 79 Monte Carlo with the two barrel carb (5th Character in the VIN is "J") it is likely a 267 (4.4 Litre). Smallest bore size of the Chevrolet small block family at 3.50 inches. Depending on what the heads came off of there is a real good chance the valves are too big for the cylinders and either hit and bent the valves or when adjusted are opening very little due to cylinder interference.
Need to find what the centerbolt heads came from and what the valve size is.

 

[Drkuhar]

I called my buddy and gave him the tips. His 5th digit was U he said. Thanks for some ideas

 

[79 USA 1]

I called my buddy and gave him the tips. His 5th digit was U he said. Thanks for some ideas

"U" in the 5th digit would be a 305...in a 1978 which would have the 2 barrel carb. Is the 6th position in the VIN "8"? Which would be a 1978, "9" would would be 1979.
305 gives a 3.750 bore, little larger then a 267 at 3.50.
From what I've read if the heads are from a 350 they will fit but the combustion chamber may be larger leading to a loss of compression. Most stock 305 heads are 58CC, depending on the aluminum head they could be 64 or 76cc depending on what was ordered.
Just throwing more possibilities out there.