I am battling the same condition on a B&S 12.5hp 286700 series engine. The throttle butterfly doesn't close all the way. I have cleaned everything but can't seem to see what spring closes the throttle. I can push the governor arm forward and slow it down, but it pops right back open to about 1/2 throttle. Even with the engine shut off, the throttle remain a 1/2 open. Any ideas?