When i shut the car off the engine will sound like fuel is still igniting in it. Not sure what the problem is.

its a 260z w/ the orginal L26 and 70 round top SU's

Sounds like dieseling. Not a carb problem. The carbs are just reacting to what the engine is doing.

Can be from timing being too far advanced, too high compression ratio, carbon build up on the pistons or a combination of these.....

could be carbon build up since the engine is completly stock and has never been rebuilt or as far as i know really worked on at all.

There is a product called Sea Foam that is supposed to do a fair country job of cutting carbon build-up from pistons. You might want to check with your local parts house and give it a whirl.....

Any of you guys have first hand experience with Sea Foam??

try moving to a higher octane fuel as see if that clears it up.

try moving to a higher octane fuel as see if that clears it up.

I will try that as right now i am running regular 87 octane in it

Too high of an idle, or a engine that's running hot will do that.

Or potentially your timing is to far advanced. Check it out!