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Hello All,
I recently got my recently purchased 1998 Trophy 900 on the road. I noticed that there was some vibration and "shuddering" between 2k-4k when accelerating. When I synchronized the carbs, I noticed that the center carb didn't flow the same as the outer carbs. At idle I was able to get the carbs synchronized, but as soon as I open the
throttle, the middle carb lags behind the outer carbs until about 4k rpm, at which point the carbs are more or less equivalent. I would estimate the difference to be about 20-30% less in the center carb relative to the outer carbs between 2k-4k rpms.

My question is this...Is this normal? I know the center carb on the triples has a leaner main jet...Is that the cause of the lower flow? Or, does the fact that the middle cylinder shares the exhaust of the outer cylinders cause it to flow differently than the outer cylinders? I am thoroughly confused!

In an attempt to discover whether the middle carb slide/diaphragm was the "culprit" I swapped the diaphragms from the middle and right carbs and the middle carb still flowed about 20-30% less than the outer carbs after I swapped the diaphragms. It would seem to me that this differential flow during this rpm band would cause the
cylinders to "fight each other" and produce a "shudder" and
vibration and I am experiencing these symptoms out on the road.

I haven't checked the compression of the motor yet (I am waiting for an adapter for my compression tester...12mm spark plug hole), but I have checked the valve clearances and all of the valves were within the normal specs. Also, my coils have all checked-out to be normal.

Any thoughts would be greatly appreciated.

John
1998 Trophy 900
 

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My experience with most bikes has been much the same, if you balance the carbs at idle they will be slightly out at higher revs. I tend to balance them at about 2000 rpm.

The vibe you are referring to sounds like the one which is common to all the 900 triples. Only we don't call it a vibe around here, people refer to it as "resonance" :-D
 

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Same here. I split the difference and sync at around 2000rpm. But my "shudder" seems less severe, it quits right at 3000rpm. I like to call it the "growl".

j98sprint
 
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