The M-Tron just adds something else to the diagnostic algorithm.
That being said, it’s usually NOT the MT itself that’s the problem. Don’t get me wrong, it can be, and you did the smart thing. When it it the MT, it’s usually the solenoid.
My only advice is to forget it’s an MT. Pull the muffler. Clean the exhaust screen, it could be caked up with carbon. Check the piston condition. If the slug looks good and you feel good compression, your internals are generally ok.
Change the $2 spark plug. After spending countless hours and money on engines over the years, it’s become SOP on engines that aren’t right. Just because you see spark at atmospheric pressure, it doesn’t mean there’s spark in a hot cylinder under compression.
Vacuum test the engine. MT can cover up a lot of easily diagnosed problems and make them more difficult to detect. Pull the carb apart. See if it holds pressure. See if the popoff pressure is acceptable. See in the fuel screen is clogged w shít.
IMHO, I don’t think it’s the nozzle check valve. The function of the valve is to stop the saw from pulling air instead of fuel into the wet side of the carb during idle. It’s generally more of an idle than running issue. It’s possible that the MT is overcoming it, but doubtful.
If it all checks out to the old school testing, then dive into the MT itself. If it’s all good, the last step would be to swap out the coil. The ECM of the MT system is in the coil. Make sure the little wiring harness isn’t broken or shorting out.
