99 YZ400 Tuning?

Hey there

I need some help tuning this bike. All stock except for e White Bros E Series exhaust. I can't get the bike to idle right and it back fires quite a bit when I get out of the gas. I cleaned the carb and checked for any air leaks and found nothing. I don't believe that the jetting has ever been changed to compensate for the exhaust but I was told that it does not have to be. It does have a 180 main jet. I bought this bike a few years ago and have ridden it very little. I would say that the bike has less that 25-30 hours on it. It did idle fine when i got it but allways had the back fire issue. Any help you can give me is appreciated.

I would start by checking your valve clearance. Tight valves can cause idling issues.

180 seems big, without looking into it further. Stock was a 175.

Before you get too deep into it, be sure you have the pilot screw set right:




...then check your exhaust for air leaks if the backfiring persists. If the idle "hangs", that is, the idle tends to stay high after blipping the throttle, but then drops off to normal or below, or will not drop off unless it's set so low that it won't idle after it drops back, you have a lean condition caused by an air leak in the intake, or tight intake valves. When looking for air leaks in a YZ400, it's important to check the external hot start plumbing. The hose runs from the head near the carb joint, and back to the valve and air boot.

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