What causes "dieseling"

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