1971 IH cub cadet 109 Hydrostic garden tractor back firing

My 1971 Cub 109, 10hp Kohler keeps back firing out the exhaust. Sometimes when it hasn't been messed with for a couple days, it will start up and run smooth for a while as long as you don't try to throttle all the way up or engage the blades...at that point it misses a little and starts back firing out the exhaust. I have emptied the gasoline and added new with sea foam, changed fuel filter, replaced fuel line, checked the points (amateur)-sanded them a little and watched them spark, new spark plug, have not checked the spark at the plug. I have heard different guesses... Does any of this make sense to anyone?... What am I missing or haven't gotten to yet? I don't want to keep replacing things such as coil, condensor, etc unless I need to... Anyone have this problem and possible solution? I'm still not sure if it is fuel related, fire related or maybe an unseated valve... the title should say Hydrostatic, btw

1971 IH cub cadet 109 Hydrostic garden tractor back firing