Carbon deposits in the combustion chamber of a gasoline engine can build up causing all kinds of problems - pinging, dieseling, running on, rough running and the worst case engine knocking. This was especially true in engines fifteen or more years ago.
Back in 1977, a co-worker pulled up in his old Chevy truck with the engine absolutely banging. Everyone backed away believing that the motor was about to sling a lot of serious metal. We helped him pull the vehicle to a coal truck shop where a friend worked. He listened to the knocking for a few moments and declared a valve was stuck open due to carbon build-up. In a few minutes and a couple of dollars, the mechanic had the motor purring like kitten...here's what he did!
Before getting involved in this, I will caution that I have only done this on conventionally carburetored engines. Newer engines that have sensors in the valve body and exhaust system may have problems with this, so I don't recommend it for all vehicles. Check your application carefully.
Remove the air cleaner from the carburetor.
Have two small soda bottles handy - one with tap water and the other with ATF.
Start the engine and rev it up to 3000 or more RPM.
With the two bottles in one hand, simultaneously and slowly pour the water and ATF into the carburetor while using the other hand to play with the carb linkage to keep the engine from stalling. Keep the revs up high. Do not let the engine stall. You may have a problem getting it started again.
Pour the entire contents into the motor. There will be a huge cloud of steam and smoke coming from the tail pipe. Keep reving the engine until there is no more smoke. This process will beak away and burn any built up carbon on the valves and combustion chamber.
Repeat if necessary, but have never had to due it twice.
Once the engine is running smoothly, clean the carb with carburetor cleaner of any residual fluid and reassemble the intake.
