1998 Johnson 15hp top cylinder fouling

I've got a '98 15 hp Johnson, starts great and idles fine, but the top end is a big sluggish and has intermittent bursts of power. I pulled the plugs, and the while the bottom cylinder plug looks nice (clean, tan), the top plug is black with wet oil residue. I swapped the plugs, and again the bottom was clean and top was dirty. So it doesn't appear to be a plug problem. I tried pulling the lead off the top cylinder while running and it was sparking to the block and the engine ran rough, so I do think the cylinder is firing. I'm running NGK plugs. What could cause this? Thanks.

A 2 stroke or 4 stroke.------Must post model #

racerone said:

A 2 stroke or 4 stroke.------Must post model #

Click to expand...

Sorry about that. It's a 2-stroke, model number J15REEA.

As an additional bit of information, the plugs are NGK B8HS-10, which I see is a slightly lower temperature plug than the often recommended BR6HS on this forum. Not sure if that would be a significant factor.

New plugs did not help, in fact seemed to make it almost worse. The fact that I did notice a change in behavior with different plugs suggests to me that it could be a problem with the ignition system, for example not a powerful spark. Is there a good way I can test the quality of my ignition?