1975 Johnson 70hp 2 Stroke Carbs / Timing Trouble

[shakerboater]

Been running this motor on a 24' pontoon, pulling towables alot at 3/4 throttle, sometimes opening up to WOT (no tach - just listening to sound and limited by cable/linkage settings). Running "fine" past two summers (Cleveland, Ohio - 3-4 month season) with no maintenance. This is the third summer using this motor. It was used very little, for 8 years, before I got it. I have been careful to use fresh gas. One day, running as usual, then, after sitting at dock for an hour, restarted as usual (easy), revved and idled in neutral fine. Under load, it won't accelerate, doesn't sound right. Still runs fine in neutral revving and idling. Does this sound like timing? I'll replace plugs (way overdue - 3 or more years old) and see how it runs, but should I proceed to pulling flywheel to inspect contactors? I'll follow manual, but there's no substitute for someone's similar experience. I did see a good amount of gas leaking out of top two carbs. I'll go ahead and rebuild carbs, as well.

[jtexas]

classic symptom of clogged hi-speed jets. carb rebuild and link & sync should do it. Do you winterize?

Might want to go ahead & do a compression test first.

[shakerboater]

Thank you, Jtexas. I have not winterized. I know better, but...
The compression test makes sense (I don't know what the maintenance history is on this motor). I'll be working on it ths weekend.