Sorry for the book, but here's my story: I picked up a 1969 GT-160 last fall. Great boat. I test drove it once before the winter and it ran good. Had the slightest stumble upon acceleration just off idle, but I didn't think much of it. This Spring it fired right up and ran great the first day out. The small stumble was still there, but I was the only one that even noticed. The next day we brought some friends along and gave some very slow tube rides. The engine definitely did not like that low rpm, high load situation. Started running awful, couldn't give it any throttle without bogging. It eventually stalled and would not start. I figured the slow tube rides had fouled the plugs and upon checking them they were black and oily. I swapped them for fresh plugs and it fired up and we headed for the marina. The stumble off idle was much worse now, but once going still ran great.
The following day I took the boat to a local shop, Jeurissen Motor Sports in Belle Plaine, MN. He had the boat for two weeks, charged me $400 for 3-1/2 hours or labor and parts but didn't fix the problem. He said there was good compression (~135 across all cylinders) and good spark. He adjusted the carb and replaced 2 needle jet seals and the rotor. I took the boat out hoping the carb adjustment would at least get it running as good as it was the first day with the minor stumble, but no, it was the same. I had to baby the throttle to get it over about 2000rpm and then it would run fine. Eventually I just couldn't get it to catch no matter how gentle I was. We idled to the marina, it died about 20 feet from the dock and hasn't started back up since. It would sometimes fire for just a second but immediately die.
I checked the plugs and they look normal. I've checked the fuel and no separation. It's non-oxygenated 91 with amsoil 2 stroke marine oil and seafoam. I picked up a spark tester and I did not see a spark at all although the shop had said it has good spark. Any suggestions for next steps, things to try, etc would be greatly appreciated. Even a recommended shop to take it to. Thanks in advance.