I have a question my gt 390 has been rebuilt and I can’t seem to get a good idle as well
An being cold blooded.
Things I have done are
Compression test Bailey low I think but all between 125-135
Psi
Leak down test ball better than 89%
Tried timing changes
Tries a new carb
Vacuum test shows 13-16 in vac and fluctuating between
No obvious vacuum leaks in all the usual places
Check plug gaps and wiring order.
I’m
Kind of out of ideas of what to check next
I had the same problem after my engine rebuild. My car had rusty sediment in the fuel system that kept plugging up the carb. Changing carbs didn’t fix it because the new carb would also plug up in minutes. Replaced fuel system and cleaned out carb again to fix it. You might also look at plug wires in the dark to make sure the sparks aren’t jumping to ground. Your compression numbers sure seem low for a new rebuild.
That vacuum reading is on the low side. Also the spread is wide but that would be consistent with a rough idle. On a temporary basis I would pull all the vacuum accessories off and cap the ports including the transmission vacuum if it’s an automatic. Then see if the vacuum improves.
Another thing about manifold vacuum is that it depends on idle speed. Mine runs 14” at 600 rpm in drive, 16” at 750 rpm in park, and 18” at 1400 rpm on fast idle cam. Should be steady and not fluctuate.