Racer,

Your GMS is a dyke's ringed engine. At least from the [link=http://www.mecoa.com/gms/specs.htm]source info[/link].
It needs a very rich, four-cycling initial break-in; 10-20 very short runs (10-30 seconds), with complete cooling between them.

This, to anneal the cast iron ring in the steel sleeve. With a dyke's ring, you don't need this break-in to be too long; about 30 minute should be more than enough. After the short cycles, give it 15-20 minutes of continuous four-cycle (two tanks) and it should do it. After this you should check if it can hold the needle. If it does, you are done.

If not, give it 5 more minutes rich and try again.