Here's a copy of Dunk's method using Seafoam:
This works for Carbed, EFI, Ficht, HPDI, Optimax and even 4 strokes...
First you need a separate small fuel tank. One of those 3 gal red Tempos works great or an empty gal mike jug will also work, but might be a bit messier..
I use Seafoam over the OEM stuff like OMC Engine Tuner or Merc Power Tune because in the last few years they changed the formula and you have to let them sit up 12 hours. Who's got time for that?? Seafoam you can buy from NAPA, CarQuest or other auto stores. Seafoam works in 15 mins.
You'll need 3/4 gal of gasoline and one 16oz can of seafoam for each engine. Don't forget to add 3oz of oil if you are premixing in a carbed engine. Use about 3 ft piece of fuel hose off the little tank. You connect this tank to your engine by pulling off the main tank fuel hose off the intake side of your water separating fuel filter and plug the hose off the small tank on to that fitting. Or you can separate the fuel line on the tank side primer ball, so you can still use your primer. If you have an engine that has fuel plug then you need a fuel plug on the little tanks hose.
Start the engine, let it warm up and start pulling the mix into the engine. You may have to increase the idle to keep it running once she get loaded with the Seafoam. Run the engine 15 mins in the dock or just cruising around under 2500. Then shut it down and let it sit for 15 mins. Restart the engine, the smoke you see is the carbon burning off. Do the whole thing again and let her sit again for 15 mins. If she smokes after the second time do it again, but I've never seen one still smoke after three doses. The gallon mix should be enough to do this 3 times. You don't need any wide open throttle, you don't need to change the plugs. If it's cleaning the combustion chambers it's also cleaning the plugs, but every 50-60hrs is good time to change plugs in most engines.