I have always had the best luck with K&B 1L plugs in GMS engines. I used to be a dealer for them until Tower got them and dropped the prices too low.

Another thing to remember about the larger ringed engines is that they take a LONG time to break in. You can't expect full performance until you have a couple of gallons through them. Also, 11K with a 12X8 APC isn't all that bad. A YS 91FZ will barely do that and it isn't considered a poor performer!