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Old 05-25-2011, 11:04 AM
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Default strange jetting problem on CR85

     

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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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Old 05-25-2011, 11:58 AM
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First thought was the bike may be starting to get tired (rebuild required), second thought was float adjustment (you did that), then clean the carb (you did that).

Sounds crazy but how about another plug?

Thanks
Scott
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Old 05-25-2011, 02:44 PM
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Installed new plug, same issue. The top end is fresh, btw.

Still puzzled why it happens only in the bumps and not on flat ground...
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Old 05-25-2011, 03:01 PM
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Fuel filter? The bouncing around might dislodge some dirt and spread it around restricting flow, once settled down the sediment falls to the lower portion of the filter and allows greater flow.
Needle jet loose? If its not seated or properly retained, it will bounce around in the main jet and disturb the flow. There is a clip and plate arrangement that the throttle slide spring presses against the holds the needle jet firmly in place. If its missing or the slide or needle are worn, it will cause some problems.
Also, if I am not mistaken, there is a "jet holder" in the bowl that I think is there to keep the main jet's flow stable in at turbulent bowl. It reduces the volume around the main jet and acts as a baffle. If its missing, the jet may be picking up air as the float bowl is jostled.

Just my two cents
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