That is a LOT of propeller load for that engine - 20x8 is about all they really like to swing
try to setup a prop that allows at least 8000 rpm on the ground at a nice easy adjustment a bit rich .
depending on the mufflers used a slightly smaller prop may be best. Ther were a number of heads/mufflers intake setups so I don't know what you actually have
anyway let it rev a bit for best power and good running. The best setup would be alky and oil a wee bit of nitro and a spark ignition -but that is a lot of work-
gasoline conversion - avoid that on that engine
They did offer one at one time -it was not very good .