I've used SeaFoam for my outboard and found it quite useful. For a shock treatment, you typically add it directly to the fuel tank—usually about one ounce per gallon, as specified on the can, but do check the instructions on your specific product for the correct ratio. It’s best used when your engine is running rough or you suspect there's build-up in the carburetor or fuel injectors. Many users, including myself, notice improvements in idling and throttle response shortly after treatment.
However, I'd suggest not using it too frequently. Routine maintenance should involve fuel stabilizers and other engine care practices. Over-cleaning might not be necessary and could lead to issues if done improperly. As always, run your engine in a well-ventilated area to avoid fumes, and consider consulting your owner's manual or a professional if you're unsure.