01 Merc 50 2 stroke

by Justin
(Greenwood Lake, NJ)

Hi Everyone, Im working on a 01 Merc 50 2 stroke. It was a neighbors boat which sat for about 7 years. The engine has only around 100 hours on it if that. It was just bought and used for 1 season before it sat. Its having an odd issue I cant figure out. When its cold you can get it up on plane and go full throttle. After about 15 secs running full throttle it will start to surge and each surge reduces the overall rpm. Eventually after a few minutes you will be back down to around 3k rpm. This boat does not have a tach but having worked on many outboards in the past I fell its around 3k. Just enough to lift the bow up a decent amount. Everyday I can repeat this same pattern like clockwork.

I've already rebuilt the carbs, rebuilt fuel pump, new filter, fuel lines, primer bulb, new tank, new plugs. I really dont know what else it could be. Once it reduces the rpm's I can have the throttle all the way and it will not stall. Its just like its being electronically limited.

I've replaced all 3 coil packs, RPM limiter module (also left it disconnected to test, no difference), & voltage rectifier.
It seems to me there is something electronic cutting it out. If a sensor is bad would this go into limp mode? Not sure if these engines have a limp mode. Or does this sound like a bad coil pack? Anything else to check? This engine only has 100 hours on it if that. Playing with the trim will sometimes make it go back to full throttle for a few seconds then cut back the rpms again.

Thanks,
Justin