Paging HuyundaiTech ; (pick up on line one please !)

None. Modern fuels do an excellent job of keeping the fuel system clean. Fuel additives are like oil additives, they're a waste of money. The one exception in Drygas and similar products, but now that most fuels contain 10% ethanol, water-removing agents are no longer necessary, either.

The only thing I've actually witnessed make any difference is the procedure GM recommends:

Take a pressurizeable container (specifically designed to add chemicals to the fuel rail) and add about 25% (rough number -- working from memory) GM top engine cleaner to 75% gasoline. Then attach the feed line to the fuel rail, disconnect the fuel pump, set pressure high enough to run vehicle and low enough to prevent return to fuel tank. Then start and run vehicle on mixture.

'The only thing I've actually witnessed make any difference is the procedure GM recommends: Take a pressurizeable container (specifically designed to add chemicals to the fuel rail) and add about 25% (rough number -- working from memory) GM top engine cleaner to 75% gasoline. Then attach the feed line to the fuel rail, disconnect the fuel pump, set pressure high enough to run vehicle and low enough to prevent return to fuel tank. Then start and run vehicle on mixture.'

REPLY: Thanks for that info. What do you think about slowly pouring some GM Top Engine Cleaner into a Carburetors air bleed (the two vertical air tubes on a 4 barrel ) , while the motor is running at fast idle , to clean the passages/bowls within the carb ? Thanks.

Pouring it directly (slowly) into the carb is normally how it's done. Unfortunately, I'm far too new school to know much about carbs, so I cannot comment on how effective the air bleeds might be.

'Pouring it directly (slowly) into the carb is normally how it's done. Unfortunately, I'm far too new school to know much about carbs, so I cannot comment on how effective the air bleeds might be.'

REPLY: Pouring it down the carb venturi's is ok to get the intake and combustion chamber cleaned out, but, it doesnt go thru the tiny passages of the Carb . Thats why i thought pouring it down the air vents of the Carb into the bowls would be useful in cleaning the Carb's circuits while its operating. Ill investigate this f urther. Thanks for your replies.

The top engine cleaner works on the thermal shock principle. Carb cleaner or something similar would likely be a better choice for cleaning out the carb itself.