I've got 2 GMS .40s, and have had my share of problems with them. Let me tell you what I found/discovered.
The first engine I got was just a screamer. It ran easily like my son's os .46fx. And it ran great all the time. So, I bought another one for my son for Christmas. We couldn't get it to run good no matter what we tried. Drilled out the nipples, mod the carb, replaced the carb, nothing. The only problem with my good running engine was it wouldn't shut off unless you put your finger over the exhaust. Air leak. So when my engine started running bad, I didn't know what to do. It would run it's usual screaming self, but after about 5 min in the air, it would just quit. Riched it up, tried again. Same. This went on for about 5 weeks. I finally started looking around on the web for some resolution. In the FAQ on OS web, several questions caught my eye. First, air leaks. According to OS, an air leak is no big deal, as it only affects an engine at idle. The amount of air sucked in once throttle is opened is so much more than the air leak, it makes no difference. OK, what else can I find? A question about an engine running great, then suddenly dying after a few minutes of flight. Again, caught my attention. It attributed this to overheating, and leaning out. It said it's either too much backpressure, or not enough. Since my GMS mufflers are totally unbaffled, I made some baffles for the muffler. I first tried just a 3/8" hole in the center, but this seemed to make no difference. I then tried another with a 1/4" hole. Engine ran just fine. Of course I lost some power, but the dying problem went away. Put the same sized baffle in the other engine. It started right up, and ran just fine. Transitions nice, idles nice, and opens right up to full throttle. I need more testing, but think my engine problems are solved. One other note: Before the baffles, the engine was very insensitive to changes on the high speed needle. Now, I can tell when the needle changes.