It's beeb a while since I owned a GMS.47, but give this a shot. I generally used K&B 1L plugs, no idle bar. Make sure all your fuel lines are snug, and in good shape. The blue silicon stuff has a bad habit of developing hairline cracks and can cause air leaks. My guess is that the engine is way lean or way rich on the main needle. Run the engine as fast as possible and give the fuel line a quick pinch. If it speeds up, its too rich. If it dies, it's too lean. Adjust the hi speed needle first, then work on the lo-end. In general, I found the GMS to be a very good engine, and wouldn't hesitate to own another.
good luck
Z