First time I did it, 7 years ago I used Seafoam for not knowing any better. It still works but Powertune is actually better. With seafoam or PT I warmed up the engine for about 2~3 mins on muffs. Turn it off and then removed all spark plugs. Tilt it all the way up. Sprayed seafoam or PT into each hole. I usually use a full can for all 4 cylinders. Let it soak overnight. The following day, tilt it down, replaced all spark plugs, removed the air covers on the carbs and readied another can of seafoam or PT, this time with a red straw attachment so as to shoot the SF or PT into the carb's throat.
Warm up the engine. With one hand holding the carb tie bar I rev'ed it up to about 1500 rpm, spray in some sea foam (1 sec blast) starting with the top carb and at the same time quickly race the engine to about 2500~3000 rpm for about 2~3 secs. I placed a large metal drip pan by the exhaust just to see how much soot is coming out of the exhaust. It will smoke heavily especially if you have heavy carbon build up. Always give a few seconds between sprays while alternating between carbs.
I switched between top and lower carbs to ensure none of the cylinders ran dry of lube. Remember neither powertune nor seafoam has lubricant in it. An improper use can have devastating results. Always start low with the PT or SF spray. Continue doing the above process until soot from the exhaust is just barely or has significantly reduced. Usually this last about 10~15 minutes depending on how heavy is the carbon build up.
After the decarb process you will need to clean your plugs or even better replaced them. Make sure your cooling water is adequate. Last time I decarb was last year.
