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02YZ426

Exhaust backfiring on downshift

9 posts in this topic

Latley my 426 has been popping and bangin on the downshift. For some reason, I dont remember if it always did that. Is that normal? or is there something up with my exhaust? I repacked the silencer last season and I wonder if that has something to do with it. I would like to find a way to stop that. Any Ideas?

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Sounds like you have one of three things going on.1 Exhaust leak somewhere 2.You might need to adjust your pilot screw some 3 It could be also that you have some dirt in it the carb and needs a real good cleaning.

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Usually when you have a backfire problem, your bike is running lean. Like Benuk said, you could have an air leak which would increase flow of the exhaust. The exhaust trys to out flow the intake therefore it needs more fuel. You end up with a lean burn giving it backfire problems. when it is cold outside, you have the opposite, the colder more dense air leans out the mixture also making the bike backfire unless you add a larger main jet. So you probably either have a leak at the head pipe, mid pipe joint, or your bike is jetted to lean. Some times if the fuel screw isn't set properly, you will have the same probs. Check the exhaust first because it is the easiest. I would recommend buying a tach/hour meter. You can use the tach to set your fuel screw. turn the fuel screw in with the bike at idle, then back it out until you reach the point just after you have the highest RPM. you can also do this by ear, by listening to the idle and stopping when it gets its highest, but it is hard. If you still have probs, evaluate whether or not you need to go to a larger main jet. Jetting sucks, but it is easier with a 4-stroke. Matt

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On idle screw adjustment:

http://www.thumperfaq.com/jetting.htm#PJ

The main jet has almost absolutely nothing to do with idle mixture. If you should find your fuel screw runs outside the recommended range, it would be the Pilot jet that needs to be changed.

An air leak in the exhaust simply provides one of the 3 required ingredients for combustion. Heat is always present, and some fuel will pass through unburned on decel if the engine can idle at all. The only thing missing is air. Check your midpipe clamp.

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grayracer513, Wouldn't you agree that the main jet affects the entire jetting curcuit though? Just wondering. Matt

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Thanks for the info guys. I was just checking out the exhaust and it looks like the seal at the mid pipe isnt the best. Im going to order a new one with hope thats it. If that doesnt fix it, is there a chance Im not getting enough backpresure from the silencer? Maybe I should re-pack it again. Im not sure what jetting is in it, but I know its stock. Do you jet a four stroke the same as a 2 stroke? should the plug have the same colors? Im very familiar with jetting on the 2s but not the 4s.

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grayracer513, Wouldn't you agree that the main jet affects the entire jetting curcuit though? Just wondering. Matt
Not at an idle, which is the position the throttle is in during decel. In fact, the pilot has more influence up to about 1/8 - 1/4 throttle. Remember, too, that the main and idle circuits are physically independent of one another. The only thing they share in common is the float bowl and the carb throat. While at idle, so little fuel goes by the main jet that you could go up or down 4 sizes or more and not have much effect on the idle. As soon as you start to open the throttle, that starts to change rapidly, though.

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