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FCR MX 39 or 40 for XR600

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I'm considering putting a FCR MX on my 600.  What if any difference would there be between the 39 and 40?  At this time I know squat about FCR's.  Any input would be appreciated. 

Thanks, Charlie

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I'm considering putting a FCR MX on my 600.  What if any difference would there be between the 39 and 40?  At this time I know squat about FCR's.  Any input would be appreciated. 

Thanks, Charlie

There is a 1mm difference in the bore... they will both work just fine.

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Unless you have pretty significant changes in cam, porting and displacement you will not notice a difference.

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Are you saying that unless there are significant changes in cam, porting and timing I won't be able to tell much difference between the stock carb and the FCR MX ?   or .......... I wont be able to tell the difference between the 39 and 40 FCR?

Thanks

Charlie

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What he said. I doubt you will note a difference between the two FCRs, but you WILL notice an improvement over the stock carb. The throttle response with an FCR is instant unlike the "twist and wait" stock carb.

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Are you saying that unless there are significant changes in cam, porting and timing I won't be able to tell much difference between the stock carb and the FCR MX ?   or .......... I wont be able to tell the difference between the 39 and 40 FCR?

Thanks

Charlie

Bore size only. I've used 38 to 41mm Mikunis and can't really tell any difference power wise. There is a difference as to how much soggy throttle you have wide open unless you have a big cam etcetera. As in the 38 hits peak power and you're WFO. The 41 hits peak power but you can still turn the throttle more but only change the sound not power. The big XRs don't have much over rev stock, once you hit the peak they're pretty much done.

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I appreciate the responses.  I'm just trying to figure out my options.  Two questions ...... (1) Would there be any advantage of an FCR MX over a flat slide Mikuni?  I have read on TT that there is some difficulty in getting a FCR to fit in the allowed space.  (2) Would a flat slide Mikuni have the same issues or would it be more space friendly?   I'm not going to be racing this bike but would like to improve the throttle response over the stock carb.  I don't plan on doing any engine mods.  For me the stock setup is plenty powerful enough.  Im 59 and just wanting to enjoy riding as long as I can.  I'll make it to Loretta Lynn's this fall.  This will probably be my last dirt bike and I'm going to set it up for me, not to impress anyone else.

Thanks, Charlie

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