Thanks, guys, for all your input. You've certainly given me some good information. From your comments, it sounds to me like my problems have to do with tuning; the engine carb settings may be too lean. First of all, I'll replace the plug with a new NGK CM6. Then I'll open the low end needle a bit and see what happens. If it quits after that, I'll richen the high end slightly. It's going to be a few days before I'm able to get out to the flying field to see what happens, and after I find out, you'll hear back from me.