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1973 20hp Johnson drops to 1 cyl.

E

edhd13

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I have a 1973 20 HP Omc that's making me crazy. It will start good, idle OK, miss slightly getting up on plane, run good on WOT and 3/4 throttle, at less that 3/4 throttle 1 cylinder will completely cut out right down to idle. If I go back to WOT, it will sputter and fart and then pick up it's other cylinder and run fine. If you leave it at low speed, it will run on 1 lung for a good 15 minutes and then the other cylinder will cut back in and run fine indefinitely. This is the later point ignition with the charge coil under the flywheel and 2 coils on the head. I have changed the points, condenser, all 3 coils and the plugs twice, just in case. I've checked the low tension wires between the points and the external coils extensively for open and grounds, since the problem seems throttle position sensitive and found nothing amiss. The compression is good, 100 PSI both holes. While grabbing for straws, I tore down the carb and cleaned it thoroughly and no issues were found. It makes no difference when I swap the primary leads and the spark plug leads on the coils. When I disconnect the kill switch and short the primary wires to ground 1 at a time at this location , the problem seems to be on the bottom cylinder and doesn't change when I switch coil wires. I'm baffled, any ideas???
 
Check your points real close for being free, CLEAN and set properly. Very critical on this motor.
 
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Check stator plate to ground, or any wires that are moving with the throttle/stator plate movement. Do so with your ohm meter. You should find the problem.
 
Thanks for your responses Boobie and timguy, I am currently replacing all the wires to the mag plate, I will clean and gap the points carefully and see how it goes.
 
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