I am running a new ASP .91 two stroke as well. It calls for a 12-6 prop. Yes,
it sounds small....that is a .60 size prop. Even this 13-7 was too much prop
for it....it detonates real bad. The reason is the engine has too much compression.
It blows plugs too. I had to drop to 5% nitro, and a cold plug to run it without it
banging away.

It blew a new Fox Miracle plug on the first run, and a new Rossi cold plug on
the second run on 15%. I am running a 12-7 on it now, and it is OK on 5% nitro.

Any load at all on the engine will cause it to detonate because it is over compressed.
That's the reason for the small props. It's really just a bored out .60 engine. It is
a short stroke engine.

FBD.