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Discussion Starter #1
Hi, I have a 100% stock 2002 400ex, I still have the stock exaust and no plan or money to get an aftermarket slip on. The other day I took the tip of the muffler, and I really liked the way it sounded, it was a lot deeper then stock, and sounded great under a load. My question is, i rode it around quite a bit and it runs great, just as good as it ever did, but I know that taking the tip off makes them run more lean and I've also heard that 400s run a little on the lean side from the factory, so do I have to rejet if it runs good, or can I just adjust the carb, or do I even have to do anything. I also had the thought that cold weather will make it run more lean, and so once it gets to be winter here in oregon, will it start to run bad? Any advice is appreciated. Thanks in advance.
 

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Go up two sizes on the main jet and ride - removing the tip only leans the mixture on the very top end when wound flat out (WFO)
 

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No - you don't have too - if you were worried about it being a tad lean on top end, then jetting up just puts your mind at ease - you would probably not notice any difference in speed or performance
 

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Discussion Starter #5
Ok, thank you, the one thing I wasent sure about was even if it ran perfect, could it still be slowly hurting the motor from being too lean, but that answer's my question. Thank you
 

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Discussion Starter #6
So I just took the tip out after having it on for a while, and it ran great, it didnt hesitate or spudder, I had it wfo and it ran smooth, but when I let off the gas it backfired, I thought backfiring was from running to rich, and it should be running to lean if anything right? What would cause it to backfire?
 

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Backfire is caused by a lean condition - some carbs have a thing called an air cut valve on them - the purpose of the air cut valve is "to shut off air to the idle circuit during deceleration to enrich the idle mixture to prevent backfiring in the exhaust". Any changes to the exhaust (even a hole rusted through the muffler or a leaking exhaust gasket) will lean the mixture slightly not affecting idle, but the reduced back pressure will lean the exhaust gases and induce a backfire during deceleration. It does not hurt anything, but is annoying. There's two ways to reduce or eliminate the backfire; the first is turn the idle up a little, the second is to back the fuel screw out about 1/2 turn. If that doesn't do the trick, then it's carb recalibration time or ignore it.
 
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