Is the engine coming on the pipe? It may never reach the optimum rpm's and fighting too much load. Too much load, too much heat.
Is the engine broken in? How much fuel have you run through it?

I agree with Rudeboy, it is possible that you have toasted the engine but the only thing in your favor is that you were not turning 25K where the engine can be destroyed in the blink of an eye. Keep your fingers crossed.

I have run 10X8, 10X9, 10X10, 11X7, and 11X8 on my YS45 but kept the pipe long. I tuned it for 11,500RPM static and never had any troubles with over heating. Tuning is the key. With proper tuning, fuel mixture and prop blast, you should not have any troubles with the 10X8 MA prop.

Kaos1