diesel effect

ok i have a 1980 c-30 with a 366 big block in it sometimes when i shut it off the truck will still keep turning itself over and over like some diesels today it did it for like 30 seconds when i shut it off what could be causing this or has anyone else had this problem

I had a truck do that to me before and it was a bad carb.

i just had the carb rebuilt

if the idle is set too high, it will do that....or some iginitions cause run on, does it have oe style ignition in it or is an aftermarket like MSD..etc.?

its the original

Sounds like timing, if timing is off/advanced too much then the engine runs hot, if the carb is not adjusted right and still loaded with fuel the excessive heat in the cylinders can detonate causing the engine to bang over a couple of times. Sounds like you need a full tune up.