I have 1968 327/300HP w/AT. A new cam was installed (mild upgrade from stock) and it has a Mallory electronic distributor (no vacuum advance). It has a non-original Q-jet installed in the early '70s after a disastrous experiment with a Holley. The past year or so I have had a problem with when the car has to idle in gear at a long traffic light. The idle continues to drop from 650 to near 500 and is very erratic. I've also noted that the engine has to crank a little longer on the first start up after a few days of non-use. But after that it starts immediately. I've been told that old Q-jets can leak from the bowl overnight and that is probably the cause of the cranking issue. Otherwise the engine runs great - smooth at all speeds and plenty of power.

Several people mentioned that the idle problem was probably a vacuum leak from the wipers, headlights, etc. So I disconnected the line to the vacuum canister and plugged it at the manifold. Drove for about 35 miles and still had the same rough idle problem. Then I did a vacuum test with the following results (as measured at the manifold port):

PB - Off, PCV - Off, Vac - Off 20in in neutral and 16 in in drive

PB - On, PCV - Off, Vac - Off 19 in in neutral and 16 in in drive

PB - On, PCV - On, Vac - Off 17 in in neutral and 14 in in drive

The needle is relatively steady at all test points.

I read that 19-20 in are normal, good readings. Any comments on what these readings might suggest?

 

you fail to mention what idle speed you had when running these tests, if 500 rpm it's too low to get a good reading. Did you try adjusting the idle mixture screws for best idle? In gear your car calls for a 600 rpm idle, so in neutral/park it should be higher about 650 to 700.

 

Vacuum leaks increase idle typically.

My guess to you problem is hot to boiling fuel in the carb bowl, supported by poor idle problem and your empty fuel bowl during start up.

Do you have a 1/4” carb gasket/spacer in place to insulate the carb from the heat of the intake manifold?

 

Idle was set at 650 in P/N and it drops to 600 in D, but at a long light it will drop to 500 and I need to use the accelerator to keep it at 600-650.

 

Yes, I have both the "paper" gasket and the metal plate spacer. I was planning to install a new paper gasket and replace the paper fuel filter in the carb to see if that helps.

 

trying to understand your issue,
old qjet? throttle plate bushing wear causes sticking.
long idle? brings in
plug heat range and idle mix setting
and possible float setting and or leaky float needle and seat

edit
are you running mani vac to dist?
does dist have vac can for advance?
possible timing change if vac can is set to close to vac number and is dropping adv causing idle change

 

I think your biggest problem is you have no vacuum advance.