657 engine quits at high speeds – solution

[micro222]

This was quite a head-scratcher for me, but I eventually figured it out. I’m posting this in case someone else runs into the same problem.

The engine would run perfectly until a specific RPM was reached, then it would suddenly quit. I could restart it immediately and resume, but it always happened at the same RPM.

I disconnected the wire from the MPEM to the ignition module and no more problem.

I couldn’t leave the wire disconnected because then the lanyard or stop button wouldn’t shut down the engine. To fix this, I installed a relay between the lanyard/stop wire and the ignition module. It now works perfectly.

 

[etemplet]

So the relay shuts down the engine?

 

[micro222]

Yes. When the relay is off (ie no current flowing through its coil) the relay contacts disable the ignition module by shorting the input, just as the MPEM would. When the lanyard is plugged in the relay turns on, the contacts no longer shorting the ignition. A useful thing to know about the ignition system is that the MPEM is not needed for the engine to run. The magneto powers the the CDI ignition module and the spark plugs fire.