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Tohatsu M40C Popping/Surging Rhythmically

nymcah

Member
I recently purchased a non-running Tohatsu M40C. I got the motor running this morning, and it seems to run smoothly, but every second or so the engine pops and surges in RPMs a bit, repeatedly and rhythmically until I shut it off!

To get the motor running initially, I diagnosed that it had no spark, and traced the problem to a bad ignition coil. I replaced the coil with the one from an identical parts motor, which made the motor start, but now I’m having this popping/surging problem. Interestingly, the coil from the parts motor is built into the CDI unit, while the coil on the other one was separate (even though motors are identical).

I’d really appreciate any help identifying the problem here! I’ve eliminated the carburetor, by switching it out with the one on the parts motor (the problem didn’t change). Thank you for your help.
 
Thanks Paul. Exciter and Pulser coils have been tested and are within specs. Could you give me some guidance on how to go about verifying the timing?
 
Check timing (made sure line under flywheel was lined up with center line of crankcase) and it was right on. Any other thoughts on what might be going on?
 
I appreciate the help Paul. As I said earlier though, I’ve switched out the entire carb with the carb from the parts motor and the problem remained unchanged. This, I don’t think this is a carb issue.
 
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