Engine Back Fire and Lead Mixture Question

I have read in numerous places on the forum that engine backfire can be caused by running too lean. This is exactly the opposite of what I would have expected. Since a back fire is excess fuel burning in the exhaust pipe shouldn't this be produced by running too rich? Can someone explain to me why running lean not rich can cause back firing?


To have a fire, you need 3 things: Fuel, air, and an ignition source. Normally, when you close the throttle on a carbed bike, the idle circuit is still operating, so regardless of rich/lean conditions, there is fuel in the pipe, and the pipe itself is still hot. Add air from an air leak in the pipe, and it goes bang. Likewise, with a lean mixture, there's more air than normal in the pipe, so it contributes. I should point out that a good deal of it is the nature of the particular engine, and a perfectly jetted YZ450 WILL pop "some" on decel if everything is right.

Makes sense now. Thanks.

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