Some people call it snake oil, I guess because they think the smoke is caused by the oil content in the Seafoam.
If you have carbon build up, Seafoam will remove it if used properly.
They way I suggest you use it is to set your warmed up engine at a high idle and suck the seafoam into the engine using a convenient vacuum hose.
When you have about 1/2 the can left let the engine really gobble it up until it stalls, that way everything inside the combustion chamber is soaked.
Let it sit overnight then when you start it the next day there will be a lot of smoke.
Take it for a drive, on the highway if you can so the engine is revving and blowing the carbon out and so that it doesn't stick in the cat.
Seafoam is only of any use if you have carbon build up in the combustion chamber.
Some people say the Techron product is better.