Have a 2001 Baja with a 496HO. It idles fine, runs great at mid-range rpm (3000-3800), runs great at higher rpm (3800+) occasionally if engine is cool. Once the engine warms up, if you try to maintain an rpm higher than 4000 it starts cutting out. It doesn't seem to backfire, it doesn't drop down in rpm more than a couple hundred rpm and usually doesn't drop at all. If it's already warm it just won't go past 4000 rpm. It varies though, sometimes it won't hold 3800 rpm, other times you can hold it at 4000 rpm. I was having this problem the last time I took it out, only the problem was at a higher rpm (4600). Since then I switched to a higher pitch prop (for other reasons, unrelated to problem) and now the problem is happening at a lower rpm range.
I have changed the fuel filter, used fuel from 3 different stations since the problem started. Have talked to several mechanical boat friends, some are sure it must be bad fuel others are thinking bad plugs. Existing plugs are around 100h old. I do not have a scanner to put on it (a purchase I need to make, I know).
Any ideas on which direction to go here? Any info is much appreciated.