Twice today I set the idle at 1000 in park, 900 in drive, and after driving the car the idle in park shot up to 1200-1300 when I put it in park.Brought it back down with the idle screw, and when I cranked it back up it dropped down to about 500 at idle in gear...what is going on here?

The car was originally running rich and my Dad adjusted the two mixture screws a half turn clockwise, it was running good. Beat on it a little and when we got back the idle jumped up the first time. Adjusted the idle screw and went counter clockwise a quarter turn to richen it back up a little, had it idleing good,turned it off. Then when I went to leave it dropped to 500 in reverse.Brought it back up and it seemed good.

Drove 35 miles home and when I put it in park the idle shot up to 1200.

Holley 750 vacuum secondary.

I would clamp the PCV hose and PB hose with vice grips with a rag to protect the hose from the serrations & retry it just to elim those vac leak potentials. got another carb you can borrow/sub in for an hour to test? that would confirm/elim the carb but that'd be my guess (carb) expecially with it being P/T & that severe. Is the linkage hanging up anywhere?

I would check the advance curves, mechanical and vacuum (assuming its hooked up correctly). A wandering advance will drive one nuts while blaming the carburetor

vacuum leak

Fast idle cam?

Sticking weights in the distributor.

Pretty much eliminated the possibility of vacuum leaks.
It's surging about 100 rpm.
Played with the mixture and timing.
Couldn't get a good vacuum reading on it as I dont have an adapter to hook my mityvac to the 3/8 full manifold vacuum fittings.

Thinking this mallory distributor is crap like Rick Ehrenberg says,timing is erratic.

Frustrated...it was running strong enough to smoke the tires 150 yards yesterday,then it ran 230° on the way home and the idle jumped up when I got there.I have till Thursday night to figure this out, before a 2 hour trip to Ennis.

Running hot as well? Absolutely sounds like sticking advance in the distributor.

Sheldon

When did it last run fine? What changed? Have you done any work to it?

Itt ran fine yesterday,after a 35mi trip the idle jumped up in my driveway when I got home...didn't touch anything inbetween.

Another common cause of this is a throttle cable or linkage that is adjusted too tight and causing sticking or binding. There should be "SLACK" in the adjustment allowing the carburetor to return to its idle setting, Again just trying to help and wish you were closer

I swapped in a original Mopar distributor and tried timing with a vac guauge. Most I can get is about 10hg and its flickering an bouncing about 2hg.
What should I be seeing on the light at 1000rpm idle in park? The best it ran was showing about 30° on my old Suntech light.

Tightened the idle mixture screws to light bottom,came 1-1/2 turns out and started backing out 1/4 turn at a time on both sides looking for a vacuum change. Nothing noticeable. Having a hard time finding a spot where the idle does not drop in gear.

Feeling pretty lost here.

To top it off the trans, which I just paid a shop to reseal is leaking badly in the rear and either the pan or front seal, plus now my 8-3/4 is dripping too. Guess the car is not going this weekend.

...yeah I know, just not a good day.


...and no offense taken I come here to learn.

For clarity...

Two days ago, it ran perfectly fine. Went for a ride and runs poorly upon return. Tried a second distributor, and it likes 30° initial timing. Idle vacuum is 10" at 1000 rpm.

Correct?

Couple more questions: 1) How do you know or confirm that it is not a vacuum leak? 2) how big is the cam... did it always idle with 10"?

If it ran fine one moment and then not.....something has changed

When was it in for leak repair?, and why was it overheating?
I cannot help but think that there is more to the story that would help diagnose the problem. Maybe a little more history on the car.

I don't want to leave anybody trying to help hanging,but I have to be at work at 2am, I will try to provide as much info as I can after work tomorrow morning. Thanks so much! It's just too much to type on a smartphone tonight.

STEP One, Disconnect and plug the vacuum advance.
Step Two, Check and verify what the timing is doing at or near idle.
IE:
Initial timing is just that and it should immediately return to the same setting when blipping the throttle.
If it floats IE: slowly comes back down the mechanical advance inside the distributor is too loose and will drive you nuts while trying to establish a repeatable idle RPM. WHY??? Because the mechanical advance curve is not snapping back to its starting position.

Hope this makes sense ?? Start with the basic steps 1 & 2 and report back

Sounded like I had a miss. Disconnected the timing light and it smoothed out, fiddled with the idle screws, only drops 100 rpm in gear now. Going by ear,but sounds much better at idle and revving. Shakedown drive tomorrow. For now gonna get it up on stands and chase the leaks. Will report back tomorrow. No good timing light readily available for purchase in time for the weekend, but will get one asap...maybe the MSD is giving my old Suntune light fits?

Disconnect the msd. Wouldnt be the first one of those to go on the fritz. Ive seen them cause strange issues multiple times.

What should my timing show at idle in park?