1990 Yamaha 150 ETLD Issues

[bubba burt]

I bought an Ebbtide 18' with a 1990 Yamaha 150 ETLD, and took it out a month ago to give it its first run for the season, and it planed for a few minutes and when I let off the throttle I could never get it back over 2000 rpms. Had a mechanic clean my carbs, and replace my fuel pumps. I added some Sea Foam to the fuel, and took it out again. It ran just fine for about 15 minutes, but again as I let off the throttle the first time it would never go over 2000 rpms at WOT for the rest of the day. It idles great, and I can get plenty of rpms in neutral. Had the same mechanic come check it again, and when he checked the thermostats there were none He suggested that I get two thermostats and replace the plugs. Does anyone have any ideas what the problem could be? Just looking for another opinion.