Briggs & Stratton

My little 9 hp keeps backfiring. It happens when I stretch her out, say above 4000, and while she's backing down it consistently backfires. Foot off the pedal, butterfly closed, ignition disarmed, (cuz foots off the pedal), and valves adjusted to .002 intake, and .004 exhaust. Valve settings are per the manual. Any idea why I'm getting the backfire? It sounds like it's in the exhaust and not the carb.


Cartaholic - V.I.P. Sponsor
What kinda muffler ya running? The OE ones get hot spots that'll create backfiring like you're talking about.

This is what it looks like only I've patched the can. What does one do about a hot spot? That doesn't sound fixable unless I were to replace the whole thing, it's all one piece. Is that backfiring bad for anything but the ears? This will all be replaced soon anyway, but I'd like to keep the motor in good shape for a back up or whatever...


Cartaholic - V.I.P.
i'm guessin' it only does it when you back all the way out of the throttle? will it do it if the pedal is down just enough to keep the coil from grounding.
You think that unspent fuel is loading up in the muffler and getting ignited because the coil is not burning that up? If that's the case, then would bypassing the micro switch and making it key start solve the problem?
So I bent the arm that flips the micro switch so I could try the key-start idea to see if it addressed the backfiring problem. Worked like a frickin' charm.

I need to permanently bypass the switch, and there are three wires. So just to be certain, I want to run this by you guys. Here's the switch:



The black wire on top is grounded to the frame. The yellow and other black continue on into the harness. So I'm guessing the last two mentioned, (yellow and black not grounded), get connected together and I'm done?
Thanks guys!