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1987 Dodge D150 High Idle and Dieseling

Truck is the high altitude/California emissions model, 318.

Carburetor has been replaced with a 4 barrel Holley 4160 I believe, although it might be a 4150, intake manifold was replaced to fit the new carb, nothing fancy though.

Truck was tuned and up and running fine when I was driving it weekly.

I recently had a prolonged illness and the truck sat up for two weeks straight. When I got back in it, it was idling extremely high, probably 1000rpm or so higher than normal, but also dieseling very badly at shut off.

I hooked up my vacuum gauge and timing light. Timing with the spark advance disconnected is about 10 degrees (I believe 8 is factory spec but it's an old vehicle), and the vacuum gauge showed steady vacuum at about 17Hg, down to about 6 or so Hg when I hit the throttle. It is not however jumping higher after it sinks to 6 or so.

I believe when I originally set it up it used to sit around 15Hg, jump down to 5Hg and then back up to 20Hg or so before returning to normal when the throttle was activated.

The throttle plate is sitting correctly, the plate for the secondaries is closed, and the choke plate appears to be moving correctly as well.

What else could cause a constant, very high idle as well as very prolonged dieseling at shut off?

I will attempt to re-tune the carb this evening but I'm worried I'm just compensating for a larger problem. I'm thinking of removing all of the unnecessary emissions/vacuum stuff and rebuilding the carburetor if I can't find the specific cause of the recent high idle/dieseling.