My Evo 46NT (on an LT-40 ARF) did this same thing and it seems like it was caused by flooding. A couple of time it started backwards.

Since its installed in a well proven plane I don't think the flooding was related to fuel tank position.

A simple work around is just don't prime it as much before starting (4 - 5 prop turns with your finger over the carb should do it).

Once flooded, you have to remove the glow plug and spin the engine up with your starter. Watch out for the fuel "shower".

None of my 3 GMS engines (32, 40 & 76) have this problem.

Other than this, the Evo 46NT has been an excellent engine. Very quiet (has an actual baffle in the muffler) and flies the LT-40 nicely.

Dave