BACKFIRING WHEN I LET OFF THROTTLE

I HAVE A WR426 2001, AND HAVE A STOCK PIPE (US) WITH A VORTIP. RUN A 172 MAIN AND 45 PILOT. THE AIRBOX IS OPEN, HOWEVER I GET BACKFIRE WHEN I LET OFF THE THROTTLE (DECLERATE). PULLED THE PLUG AND ITS BROWN TO BLACK. I STARTED WITH A 165 MAIN AND HAVE GONE UP AND PROBLEM ALWAYS EXISTS. TRIED RUNNING WITH NO TIP IN THE PIPE, STILL BACKFIRES????? HAVEN'T MESSED WITH NEEDLE OR FUEL SCREW YET. ANY SUGESTIONS WOULD REALLY BE APPRECIATED.

Changing the main will not affect what happens when the throttle is closed. That is what the idle mix screw is for.

You are too lean on your idle circuit, but a little bit of popping is normal on some less restrictive aftermarket pipes.

BTW, you accidentally left your Caps Lock key on.

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