Why does the governor stick open on your 20 hp Briggs and stratton mower engine?

Probably some debris or goop buildup on the bearing or the area where the governor shaft fits into the engine. Try cleaning the area well and spray with lubricant (WD-40, et al) and then "rock" the governor lever to loosen any remaining buildup. Re-spray the area. The governor has very little movement, so don't try to force it.