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407777 0188 B1

On a service call, started out, this thing ran like crap. Lot's of back firing through the exhaust. After about 10 minutes of it running & mowing, it seemed to clear up. It was still backfiring, but in spurts, it would completely clear up for several seconds. Maybe even a couple of minutes.

So I thought maybe it was just some junk in the carb that made it's way through. A couple of weeks later, customer calls and says it still running bad and that it wouldn't even stay running. So I picked it up and brought it home. (like a dummy, I didn't even try to crank it there)

At home, I crank it, and it's still got a little back fire issue. But nothing major, like it was the first time I cranked it. I throw on a set of new plugs (RC12YC). Of course that didn't change anything. But it eliminated the possibility of a bad plug. I ran it for about 10 minutes and the plugs were spotless.

So I guess the question is what could make it run like crap one minute. I don't want to do nothing to it, return it to the customer, only for to start doing it again.
Like I said the only real symptoms now is the slight exhaust backfire. I remover the air filter so I could see down in there, and didn't see any carburetor back fire.

Disconnect the shut off wire on both coils and see how it runs. The coil shut off wire has tiny diodes that will go bad and create cross firing issues with the coils.

They make several coil harnesses for these so be careful to match the correct one to the date code.