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I just installed a 1990 70 hp Johnson on my 1989 Sport 15. Total rebuild, pistons, rings, bearings, power pack, plugs, rectifier. Rebuit carbs. Stator tested ok. Idles ok, but when I increase engine rpm to move in no wake zone, it misses. Sounds like it is running on 2 instaed of 3 cyls. Consistent miss. Once I open it up to say 4000, I notice , feel the miss going in out, then at higher throttle, it clears up.
My mechanic said it may be the ethanol in gas or someone may have drilled out the low speed idle jets.
Any thoughts?
Thanks
Gary

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Does that engine have reed valves? I've experienced that sort of missing in motorcycles when reeds are broken. As I understand it this will cause a rich mixture and perhaps even fuel pooling in the crankcase. The motor stumbles until the crankcase is blown out by WOT operation. Just my .02 experience from way back when I spent my time breaking and then fixing two strokes. :)

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Also, pull the plugs out (I assume they're new on a fresh rebuild) and compare them. Is your mechanic the one who did the rebuild?

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How long are you in the "no-wake" zone and how long is your motor idleing? Have you checked your spark plugs to see if they're carboned up? Does it only happen after being in a no wake zone for a period of time or always when in that RPM range?

Sounds like to me that the plugs may be fouling some while at idle for long periods of time and then when you open it up, the carbon may burn off. Seems like you might be running rich on your fuel mixture.....that is if you are able to adjust it on your motor.

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This is "guess work" at best since we don't have the motor in front of us but, I suspect a bad wire, plug, or something electrical "jumping" and grounding out, I say re-check "electrical" system, and hook-up's, crack's in wires, etc., if you can, remove the cover and check in the dark for a spark or arc with the muff's on in "N" at different RPM's. Are you running "Heavy" oil mix due to the "Break in" ? That can cause, it too, but that usually causes "Hard starting".

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I have a 1997 70hp johnson that does exactly the same thing. I have adjusted the air screws many times to no avail. I am beginning to believe it is the reed valves, like the above poster said. I have a friend that has an Evinrude 70hp that does the same thing. Very frustrating.

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Thanks for all your input. New plugs are not fouled. Coils sparking. I put lower unit in tank and ran it at verious rpms. Same thing. It just sounds sick until you open it up. Im going to check all wires, carb / timing sync and lastly carb jets on rebuilt carbs.
Thanks Again.
Gary