From what I've read here on RC Universe, captinjohn, the GMS 1.20 is a particularly troublesome product. Most folks have only good things to say about the rest of the GMS line, but the 1.20 in particular seems to be difficult to get running smoothly for a lot of modellers. This is similar to the Evolution 1.00NX, where the rest of the engine line has pretty good feedback, but this one particular model seems to be a turd.

With the Super Tigre G2300 available with muffler for nearly the same price, there seems to be little reason to roll the dice on a GMS 1.20, unless you're really looking for a challenge and you love trying to tune engines...