The APC 12.25x3.75 is too much prop for a stock 46FX. Check the revs. The 46FX really wants to be spinning at least 12.5k rpm (it's even better at 13-15k rpm) WOT. But the stock exhaust on the 46FX is really restrictive. Put a tuned pipe (i.e. MCP), and the 46FX power increases dramatically. In doing some preliminary test, I've been able to spin the same APC 12.25x3.75 1500 rpm faster (12k vs 13.5k rpm) through the use of the Morris/Rossi pipe combo. The engine spins up (transitions) much faster as well.