Pontiac Repair: 1967 Pontiac 326 poor idle, gm hei distributor, 1967 pontiac tempest

Question
Hi Todd -

I have a 1967 Pontiac Tempest with a 326 2bbl, turbo 350, p/s %26 p/b. all apparently stock. The engine is original and was rebuilt about 10 years ago (I didn't do the rebuild). It runs fine at anything above idle. It idles very rough, like it has a vacuum leak, but not a misfire - just general roughness. It doesn't smoke or make noises, really runs like you'd expect above idle. It doesn't have a miss - the engine accelerates quickly when I hit the throttle and no putt-putt miss sounds out the exhaust. No hesitation, backfires, etc. I sort of doubt it has a performance cam, no headers & 2bbl carb & previous owners weren't the racy type.

I replaced all vacuum lines, spark plugs, wires look fine, rebuilt carb and set timing according to the book. It does have a later model GM HEI distributor, and visually, it seems fine. The car runs well at speed %26 feels nice and torquey - not a powerhouse, but solid. I've balanced out the idle on the carb and it responds ok to adjustment. I've run it with all vacuum lines removed and plugged. It looks like the original manifold and carb (2bbl stock).

No obvious problems I can see. Do you have any idea where to take this a step further to help smooth it out?

Thanks,
Dan  

Answer
Hi Dan. Looks like you coved the bases here. Have you put a vacuum gauge on this when it's at idle. It shou,d be near 20" and steady. If it's bouncing around you ethert have a vacuum leak or a valve issue. You said you checked for a vacuum leak. did you try this. take the breather tube off the air cleaner and pull the pcv valve out of the valve cover and plug that line. spray carb cleaner into the valve cover and see in the idle picks up it could have an internal vacuum intake gasket leaking.. if you find leaks then unplug the vacuum advance off the distributor and see what the engine does. It shouldm't have vacuum to the advance at idle I would think that should be ported vacuum not manifold vacuum I would think.. being that it's not stock I can't tell you how the original distributor was equipped.. To be honest with you this car is 2 years older then I am. have you done a compression test on all 8 cylinders on this? there shouldn't be more then say 10% difference from the most to the least. If all else fails I would then dive into the carb and have that rebuilt.