If oil on the plug that mean's that plug wasnt firing.Do the check test and let me know along with the other's the result's.I never use sea foam but it might be a good thing,I dont know anything about that stuff.If your gas and oil mixture is correct you should have no problem with gumming issue's.A worn or broken ring on the piston can cause this issue also as it will not allow enough pressure to push the gas into the chamber.Do the test with the plug first and let us know.