ever hear of ..GM upper end cleaner ?
 

After the engine is warmed up you pour this stuff through the carb slowly and then shut the engine off for at least 20 min. Then start it and all of the carbon build up is suppose to be released from the pistons and cylinders out through the exhaust. Anyone have any experience with this ? Recommended ?

This post is on general discussion aswell look there for some answers :D :D :D

I got some at the GM dealer, I think it's Called "Top end cleaner". I used it on my 1995 Tahoe because I was starting to get some "pinging" under load (with 87 octane) that was never there before. Went on the Tahoe/Suburban chat site and everyone recommended it. Worked great, pinging went away. Truck has 153K on it and I guess I had got some carbon buildup. Never tried it in a boat and not sure if I'd use it on a newer vehicle, but figured it couldn't hurt on my old Tahoe.

read this post under general discussion about using water it works the best !!!!!

We used it at the Chevy dealership I worked at 20 some years ago. It does work. At that time there was a problem on some 4 cylinder engines. Carbon would build up on top of the piston and contact the head. This contact would cause the piston skirt to slap against the cylinder wall causing a slight knock on deceleration. TEC was the fix.