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hi guys,
I have been a long time reader of the noreast website, and let me tell you it is the best source of info around. I find that i have problem that i have no idea where to start. I have a pair of 2000 johnson 115's oceanrunners, and one of the has developed a unique problem, when running mid range to high end one of them looks like it has a hic cup, the rpm's jump about +100, it happens about about once a minute. the increase in rmp doesn't bother me as so much as the big shake it makes. I tried swapping props thinking it was the hub slipping. any input would be helpful
 

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Just to clarify the engine jumps 100 rpm not 1000 rpm? You say it happens about once a minute, is it consistent or sporadic? You need to try and figure out why its happening (obviously) and one of those ways is to determine when it happens and how often. My first quick guess would be some sort of glitch in the ignition system. After that I'd say that something isn't synchronized correctly with the two motors. I don't know much about how two motors get set up on a boat but I'm guessing you have two throttles. Maybe look there. I've also been told that if both motors are not trimmed exactly the same you may have running issues. I don't think it is your prop slipping. You need to time the problem and let us know exactly how often it happens and what you're doing at the time. Get crazy and use a stopwatch or something. When you reach that mid range does it happen right away or does it take some time? Does it happen after the motors are hot and running a while or does it also do it when they are somewhat cold?

At any rate take the time to also check compression and spark see if that tells you anything.
 

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First things first..

Not a diagnosis offered here but first thing I would do is check the plugs and make sure none is 'clean' that would indicate water getting into the combustion chamber (seeping head gasket). This is something that would lead to a dangerous condition for a 'boom'.

If your mind winds up at ease from all plugs looking proper and the same, next I would be thinking carb, slight obstruction maybe in mid range circuit or bad gasket letting air get pulled in behind throttle plates?

Could even be something like a spark plug boot about to fall off. I had one of those once.

Other ideas will follow from others, this is all that comes to mind right now.
 
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