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How does a Lean mixture cause backfiring?

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Ok I though backfiring was unburn fuel being ignited in the hot exhaust as a result of the mixture being too lean.

From reading here I have learning that this is more of a mis-fire and the backfiring is actually a lean mixture condition. But I'm at a lost to figure out why being lean would cause a backfire?

Most of my engine knowledge is based from what I have read here

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At the very least, your engine knowledge is probably better than 95% of the population.

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Do not get a correct mixture when the bike is running under load with the throttle open with a lean condition caused by the engine running at high rpm and the throttle closed when decelerating. Two completely different scenarios.

I'll assume you mean when decelerating as that is what 99.9999% of the people mean. When at high rpm and you shut the throttle, you place the intake in a very high vacuum condition. No fuel can get sucked in, only some air. Way more air than the amount of fuel. So it is super lean and the engine cannot ignite it. However, after multiple engine revolutions and no ignition, the fuel accumulates somewhat in the exhaust. Eventually, enough is there and it ignites due to the hot exhaust. You have a backfire. Some woods bikes, most street bikes have a ACV (Automatic Coast Valve) This adds a little extra fuel during high vacuum situations to enable the engine to consume the fuel in the cylinder. Also, having an efficient muffler 'muffles' many backfire pops that are still present.

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The reason I'm asking is a friends 250R is backfiring while idling. The idle is pretty rough too.

So while trying to research possible causes "I though backfires were always a too rich problem" I discovered that lean mix's can causing backfiring. So I figure the correct adjustment of the fuel mixture screw should help with the problem. I'm just trying to understand why/how a lean mix can cause idle backfiring.

Thanks for the responce Willam1, but I think I have a slightly different situation to your discription as the bike is idling. I'm wonder now if maybe "backfire" isn't the correct word for whats going on. I haven't hear the bike running yet.

Could the same fuel buildup and subsequent backfire discribed occur at idle conditions if the fuel screw was enough out whack or if the idle was turned up enough? Thanks

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Make sure the ignition timing is set correctly and the flywheel key is still intact. Other causes would be a malfunctioning CDI, trashed spark plug. I doubt this is a jetting issue, although it wouldn't hurt to clean the carb anyway.

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Poor idle could simply be a slightly clogged pilot (instal a brand new one) and/or misadjusted fuel screw. It helps when you give us all the info so we do not have to guess.

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

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