What causes "dieseling"

Princess Valiant said

So I have a 74 dodge D100

Slant six, 4 speed

I recently rebuilt the carb, 2 barrel. Adjusted the carb per instructions

Then I tuned it to 800 rpm per the emissions sticker, set the timing and everything was running perfect.

Then I turned it off and it "dieseled" pretty good and then went off. I restarted it after that and it started on the first shot as usual.

What did I miss

Thanks

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Lots of those emissions era engines had an idle solenoid. When you turned the key on they were generally not strong enough to open the throttle a bit, but a small tap on the throttle and they would hold. The bolt that pushed on the throttle linkage was used to adjust the idle speed. When you turned the key off, the solenoid snapped back, closing the throttle a bit. If you disconnected the wire to the solenoid, you could set the idle to say 600 RPM. You should be able to get a new solenoid from a parts store. Uses power supplied from key on engine run ignition switch terminal. The other way is to just set the idle to 700 to 750 RPM.