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Low end stutter JUST appeared yesterday!

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Came across a strange (to me) problem. Bike's an 03 CR85 and up until yesterday was running just fine, jetted clean, in varied temps & humidity. Only had to tweak the air screw from day to day but other than that, never an issue. Last night, the bike developed a low end stutter that only appears when on the track and going over jumps, whoops, etc. On the flat terrain jetting area, the bike runs clean and pulls hard with no hesitation. The low end feels like it's getting starved for fuel as it gives you that vaccuum feel and sound when wicking the trottle. Mid range and up still run clean. I cleaned the carb, and just to ensure there was enough fuel, adjusted the floats a smidge to allow a bit higher fuel level in the bowl. Slight improvement but problem still exists. Tweaked airscrew, even went as low as 3/4 turn out (normally 1.5 to 1.75 turns on this bike) but stutter remains. why would this only manifest itself on the track and not in the flat terrain? Oh, I did check gas cap vent, it's o.k. No sand in float bowl either. Plug looks nice and cocoa brown. The fuel we used ran just fine in two other bikes too. Thoughts???

Mark...

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It's this part, right? Took if from the fische

17 JET, NEEDLE

16164-GC4-601

Just so I understand better, first, how in the world can this thing wear out? The bike has very low hours...

And if you could, help me understand how this wear causes the hiccup on the low end and doesn't translate into poor midrange performance?

I'm not challenging your diagnosis, far from it, I'm just ignorant on this... have never seen it before.

Thanks!

Mark

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Out of the blue, a jetting issue. Jetting does not change on its' own. Seals can fail, reedsgo, power valve can be meesed up, engine can be carboned.

You have an 03 with low hours? Under 100? Did you check your powervalve? I assume you clean and do all the regular maintenance by the book, right?

Every time you move the throttle, the needle moves in and out of the needle jet emulsion tube. It only touches one point. That is a wear point, it is the one part other than the float needle that wears out regularly. It tends to rear its ugly head as a slightly rich condition at low throttle openings, out of the blue.

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Out of the blue, a jetting issue. Jetting does not change on its' own. Seals can fail, reedsgo, power valve can be meesed up, engine can be carboned.

You have an 03 with low hours? Under 100? Did you check your powervalve? I assume you clean and do all the regular maintenance by the book, right?

Every time you move the throttle, the needle moves in and out of the needle jet emulsion tube. It only touches one point. That is a wear point, it is the one part other than the float needle that wears out regularly. It tends to rear its ugly head as a slightly rich condition at low throttle openings, out of the blue.

The bike has, at most, 30 hours and I'm seeing a lean condition, not a rich one. The bike doesn't have a powervalve...

I'll look into the emulsion tube option, it's not that expensive.

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