here is your problem........
Engine and Muffler - OS55 with w/ bison pitts muffler
prop: 13x8
If this is the OS55AX 2c engine , you want to use a 10x7 or 10x8 prop !!!!!
13x8 is WAY too big. It is a 55, not a 95
Read similar posts on the matter.
That will get you at least on the right track.