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Discussion Starter · #1 · (Edited)
Mower: Snapper 7800580 NXT22875E
Engine: Briggs and Stratton 12S907-0141-B1

This is an electric start mower. It's not really possible to manually start it. My dad picked it up from my neighbor who had it out on the front lawn for trash pickup. The neighbor got tired of spending money trying to get fixed.

The gas cap tank was cracked but it could hold gas. My theory is the gas was evaporating through this crack and that left a residue that kept clogging up the carb and it would not start. I've since fixed the gas tank (I hope it will last) and cleaned the carb and was able to start it by connecting it to my car battery. The 1.3 Amp hour 12 volt battery that was in the mower was quite dead and could not hold a charge.

I then got a new battery and am able to start OK off it's own battery if the engine is cold. If I run it for 5 minutes and then shut it down and try to it start again while it's warm it generally won't start. I say generally because I think it may have started once or twice over the many tests I've performed. I can get it to start if I go back to starting it off my car battery but sometimes it does not start right away and it takes a few tries. When it tries to start it warm using it's own battery the engine is barely turning over. The battery was showing 12.9 volts after going through a few attempted warm starts.

I hooked a spark plug tester up when it turning over and the engine was warm and did see a spark.

Ran a test of the voltage going to battery when the engine was running and my multimeter showed -4.0 volts. This is not my area of expertise but this sure seems weird that the voltage would be negative! If I connected the positive multimeter probe to the negative battery wire (that would otherwise be connected to the negative battery terminal) then +4.0 volts was registering.

Does this indicate that something is malfunctioning with the electrical system?

If you look at the Engine Electrical Diagram you'll see an alternator (part 474) and a magneto (part 333). I guess the alternator is needed to keep the battery charged? I did adjust the flywheel gap on the alternator because the alternator seemed to be scraping against the flywheel. I gapped it using a business card and tried to make sure it was not scraping the flywheel anymore. Maybe I messed something up here.

Is the next step to try a new coil and/or a new alternator? Maybe try a new carb?

Thanks for any help you can offer.
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