I've always been having carb issues. Switching from carb to carb. Here is my current issue.

65 valiant 225 with factory electronic black box ignition conversion

Freshly rebuild holley 1920. Vacuum is fairly steady, very slight wiggle. Reading 20hg. Idle is set at 750 rpm. Timing set at ~10 degrees. (I did the whole timing/advance thing to see where I can set it)

The issue is rpm will randomly kick down real low, vacuum drops a few points. If I blip the gas it will go back to 750rpm. Sometimes it will fix it self with out touching a thing. It will happen while sitting at idle in park. Or while driving and taking my foot off the gas. It's random cannot find a pattern when it happens.

I remember having a similar issue with 2 different carters. Then one day it stopped doing it. Now that I switched carbs for a different reason issue came back.

I will also add when using a timing light, it seems to be a bit irratic. Could this be an ignition issue?

Timing dancing around a few degrees will cause idle rpm to run up and down.

What could be causes of unstable timing at idle?

Worn timing chain.

Weak or broken governor springs in distributor.

Sticky erratic movement of governor weights that only occasionally fully retract at idle.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Motor is rebuilt. Has well under 10k miles. Closer to 5k. Distibutor was gone through too

Try to make it a little clearer. The timing jumping few degrees doesn't effect rpm very much the tach shows a fairly steady 750 rpm.

I have not checked with tach how far the idle drops, or see if timing changes when it happens I have to get a bit more time to play with that.

Driving to work today it's first real drive since the new carb. Almost every time I let off the gas idle would be dropped down really low. I'm wondering if the lower idle is the true idle and something funky in distributor is holding it in place sometimes? Again might have to wait till weekend to get a chance to play with it some more.

Here is a wild guess. Closed throttle decel,high vacuum. Hose to pcv valve colapses or pcv valve malfunction,shuts off air flow to carb,causes idle speed drop. Gradualy returns to normal.Just for short term test ,plug pcv port and see if car idles more stable.