454 gasser likes to diesel !!!

My older ('86) 454 C3500 High Sierra 3+3 runs great but will diesel badly when hot. When cooler, it idles lower & shuts right off but when hot it seems to idle quite a bit higher & will keep running even with ignition off. Often it ends with a bang or two!
I replaced plugs, wires, & coil. It had an Accel 140005 HEI coil but after a little research, I figured out it was the wrong one & bought the 140003. The former owner had swapped two wires so it was running but the manifolds were red-hot! I got the firing order straightened out, figured out the 'wrong coil' thing & got the manifolds to run cooler but now it back-fires on shut-off (after dieseling for a few seconds). That actually started when I got the wires on correctly, believe it or not.
Any suggestions?

check your timing, and use the rated fuel for the vehicle.... most likely regular unleaded. if that doesent help use super unleaded. just remember diesel doesent use spark it uses compression... the higher the octane rating the less likely the fuel is to combust due to compression, therefore using regular unleaded you have an octane rating from 87 to 89 and may experience a knock or dieseling with tuned up or performance engines

I checked the timing; it's 'supposed' to be 4 Before TDC (I think) and it looked like about 3. Is that right?

I did try 89 on one tank...I don't remember it helping.

Correct the high idle condition when this happens and you will most likely fix the problem. This was common on lots of old style carbed engines. They used to use a method of adjusting idle speed by having an ign operated solenoid set the idle speed, when the key was turned off the solenoid backed down and closed the throttle completely, focus on the idle speed issue.

hmmm -3 shouldnt cause any problems...
if it is carbed what is idle when cold and when warm?

Correct the high idle condition when this happens and you will most likely fix the problem. This was common on lots of old style carbed engines.

Max has a good point here, if it is idling to high it would be just like slamming down on the accelerator and shutting it off, it may keep going.

the sweet spot for a warm idle on a 454 can range i usualy go with around 650 to 750 rpm for warm. cold can usualy sit somewhere between 650 and 1100 rpm

It is a carbed engine.

OK, how do I check the solinoid, and/or adjust the high & low idle settings?

Does it have one on the front side of the throttle linkage?