03 YZ450F misfire (with solution)

I've been playing with the idle / accelerator pump setup on my 03 pretty hard over the last week or two and it developed a strange miss. 10 min. before this it ran better in tight woods than it ever has before - just about have the perfect setup for those random stalls. Left one section of our trails and it's about a mile up a dirt road to the next section we ride. In a steady cruise it popped one time out the exhaust then developed a slight stumble with a throttle roll on at around 45 mph. Figured I had the pump shot a little lean for that situation and rode on. 1/4 mile later I could barely take off - stumbling and popping out the exhaust. Once I got going from 3/4 to full throttle it ran perfect. Stalled it and it would fire right back up and at this time idled fine. Figured I better head back while it was still running. By the time I got back I figured out if I kept pumping the throttle it helped so I'm figuring a lean condition. Still not sure why it would run great 3/4 - full throttle. Also by this time it wouldn't stay running at idle anymore either.

Since I've had the carb off about 10 times recently I started there as it seemed fuel related and possibly owner induced. Took it apart and found no trash, no water or anything loose / missing. Pulled the rubber boot thinking I might have torn it causing a vacuum leak - nothing. Checked fuel flow out of the tank - no problem there. Checked gas cap for plugged vent - nothing there. Since I had everything off anyway I checked valve clearances - all in middle of spec. Cam chain tight and cam timing hadn't jumped. Put it back together with a new plug for the heck of it. Fired right up and idled good. After about a minute it started to surge, miss, pop and shut off. Bottom end was still cool to the touch so didn't think it was a stator problem - not enough time to heat up. Coil would be the next thing I could think of that saw any heat at this point and that turned out to be the problem. Swapped one off my 250 to test and it idled for several minutes perfectly. Haven't ridden it yet but I'm confident that was it. Also for future reference there is about 1/4" difference in length between the 2 with the 250 being the longer of them. Still have no idea why a bad coil seemed to be throttle position related but hopefully this post will help someone one day. Clay

 Still have no idea why a bad coil seemed to be throttle position related but hopefully this post will help someone one day. Clay


The electrical load varies with throttle position.  Compression rises as the throttle opens because more air is allowed in.  The higher the compression in the cylinder, the greater the resistance across the plug gap.

True but it appeared to run fine in a high load situation which should require the highest voltage to fire but had less time for coil saturation. I'm wondering if it might be a tad rich up top which would make the mixture easier to ignite with a weak spark. I haven't tried main jet changes on the dyno with this bike yet but it makes good power (52 with a street tire) so it's not terribly off. Clay.

If you're talking about high RPM s. low, there's more absolute compression at lower RPM because there's more time to fill the cylinder.  Also, basic system voltage and current available are significantly higher at high speeds

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