I have a GMS .47 engine that I've had good and bad luck with. When it was new, it broke in quickly and ran great for the whole season. While it sat in my garage over the winter, however, the cylinder head bolts worked loose and I didn't notice it until I was at the field trying to start it and I noticed fuel leaking out of the head. I couldn't get the old gasket to reseal properly, so I ordered a new head gasket set and cylinder head to replace the old ones that wouldn't seal, and I had some trouble getting it running again.

I decided to replace the glow plug with an O.S. #8, and I got the engine runnining reliably again. I decided to run a couple of tanks through it on the ground just to make sure everything was going smoothly again. Just about the time I was convinced that my GMS .47 was good as new, the drive shaft broke while I was running in the engine on the ground. I sent it in to Hobby Services along with my receipt from Tower Hobbies, and they shipped me a brand new replacement.

Some of this may be pertinent to your problems, some of it may not be pertinent:

1) My engine doesn't have a remote needle valve. If yours does, try bypassing the remote valve.

2) My cylinder head bolts worked loose. Make sure your cylinder head and backplate bolts are completely tight. Check for fuel leaks at these spots.

3) If you start out with a top-notch glow plug like an O.S. #8, it rules out one more variable while you're troubleshooting. Spend $6 and buy one.

4) I'm normally a stickler when it comes to thoroughly checking your tank and plumbing, sounds like you've done that already.

When a GMS engine is running well, it's a thing of beauty. Hopefully you can get yours sorted out and enjoy flying it soon.