re: Seafoaming my Johnson - advice?

Here you go OP.

Two strokes build up a lot of carbon internally and need to be de-carbonized by doing a "shock treatment" with a detergent like Seafoam.
I like to do the shock treatment once a year on my C115 - in late winter when I am going thru my list of "spring make-ready".
If you use a detergent like Yamaha Ring Free or SeaFoam all the time in your fuel tank, then the carbon buildup is not as bad. Still, a lot of 2-stroke owners will de-carbonize once a year - you will get various opinions on this.
I "shock-treat" my C115 by running the following mixture thru it. This will smoke up the area pretty good, and leave a lot of carbon on the drive way:
16 oz can Seafoam
3/4 gal gas
3 oz oil (2 if oil injected)
Disconnect the fuel line at the input to the bulb and attach a three foot piece of fuel line to the bulb. Mix the above in a small container -
I use an empty 5 quart oil jug. Set the container on the deck of your boat and insert the fuel line and secure to make sure the end stays on the bottom of the container. Pump up bulb and start outboard with earmuffs and water hose, of course -run a little above idle for 15 minutes and turn off. Let sit for 15 minutes. Run again for 15 minutes. Let sit 15 minutes. Repeat until all mixture is consumed. Reattach main tank hose to bulb and run regular fuel/mixture thru motor for 10 minutes - you want to get the concentrated detergent out of the fuel system.
Make sure you do this were you willing to kill off the grass. Never do it in the driveway. A lot of crap will come out.