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black smoke is usually too much fuel,,blue smoke-oil,,,white smoke--water
back firing where? exhaust? could be unburned fuel igniting in hot exhaust,same thing can happen if you go down hill fast in a lower gear,shut off engine a few seconds then turn it back on.carbed engines will still draw fuel and the hot exhaust can ignite it. If you have ignition/weak spark/old plugs,under extreme conditions you might replicate that backfire
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