Help with Seeming Unstable Idle

[DreamRyder1963]

My truck has had what seems like a unstable idle since I got it a couple of months ago. The truck almost sounds like it is breathing. It starts at 750 rpm then drop down to 500 rpm and jumps back up to 750 rpm all within a sec or 2. While rpms almost always return, I would like to fix this issue.

Here are the facts on the engine. It is a 350 with a HEI distubitor, Offenhouser 360 degree intake and Holley electric choke carb.

I have done a complete tune up and the only thing I can think of that could be causing is the choke not functioning, since the problem all but goes away once the truck warms up completely. Any other possibilities? Thanks for the help.

[geezer#99]

Could be your timing fluctuating as the weights in the distributor start to advance Due to the high idle speed.
Or it could be your mixture screws need adjusting.
Can you turn the mixture screws in all the way without the motor trying to quit?
What's your timing set at?
Where's your vac advance hooked to? Ported or manifold vac?

[OregonNed]

Sounds identical to mine when I bought it.. the weights in the distributor and springs could not be corrected for proper advance, so we just changed it during the tune up. Runs like a dream now..

[DreamRyder1963]

The vacuum advance is connected to a vacuum port on the carb. I did not think about the timing since it has plenty of power off idle. I will check the timing this afternoon.
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