FWIW
My 64 Chevelle 283 has a stick shift carb rather than one for an automatic.
The vacuum port on the stick carb is ported but full vacuum on the auto carb.
So me running with no vacuum at idle was enough to make the engine overheat for no apparent reason.
I replumbed the vacuum to the distributor so it was full vacuum at idle and no more overheating.
Disclaimers:
I don't know what years Chevy spec'd different carbs for stick shift engines and automatics, but I think early 60's, and later on emissions probably influenced how this was done.
No, I can't say for sure why no vacuum to distributor works on stick shift cars. I'm guessing the Powerglide is enough of a load on engine at idle to make a difference.
Lotsa people tell me the PG is a load ; )
