Make sure the motor is idled down.
I gotta idle my 4.3 for a good 15-20 seconds before I shut it off or it'll diesel. Not a big deal for me since I have a sterndrive, but it could be annoying in a jet.
I took it in for warranty work last summer and the dealer said they all do it to some degree. Their "fix" was to turn the idle down a hair, and it didn't help much.
If the timing and fuel are good, you might take a look at the idle mixture or see if the carb needs a spacer.