I run my Cub without muffler and just bought a new one for my Super H.
I'm trying to remember what I learned in Hotrod 101, but if I remember correctly, if you change the backpressure on the exhaust side, your mixture will be leaned out (why I can't recall). I don't know what carb adjustments you have on the F20, but if you can adjust the carb, use the same adjustment procedures you use to adjust the carb at full throttle and you won't have any worries about burned valves, etc.
