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The biggest differences are:

The FCR is a flat slide carburetor. The slide is mounted on rollers for a nice smooth throttle pull - it is one of the best four stroke carbs made. Practically all modern high performance 4 strokes come with this carb from the factory.

The stock carb is a 42mm butterfly/round slide carb with an accelerator pump like what the 400EX is equipped with. Do not confuse this with a CV carb like the Z400, Raptor, and DS650 have.

With the stock 450R carb, both the slide and butterfly are acutated with the throttle cable. With a CV carb the butterfly opens with the throttle cable and the slide is operated by engine vacuum -- CV carbs do not typically have Accelerator pumps either.

The stock 450R carb is a good unit, but since it has a butterfly in the way of the incoming air, it restricts flow & causes turbulence, which is why the 40mm FCR will develop better throttle response and more overall power.
 

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good to know thanks ATC you have been the carb guy on here 4 a bit. lol
 

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I know the 40 is a great carb, but wouldn't the 39 give you more bottom end than the 40. Where the 40 would give you more top end. I was just wondering. My idea is that Honda used a 40 because the crf dirt bike is so light, so they could afford to use the 40, but the TRX is 100lbs heavier, so wouldn't the 39 work even better for the track or short distance runs. It's not very often that some will run at top speed , but more likely to run in second and third! ecspecially if your at the track or the dunes! :huh:
86atc250r, what is your opinion on this subject? :D
 

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Originally posted by Rico450R@Jun 8 2004, 05:17 PM
I know the 40 is a great carb, but wouldn't the 39 give you more bottom end than the 40. Where the 40 would give you more top end. I was just wondering. My idea is that Honda used a 40 because the crf dirt bike is so light, so they could afford to use the 40, but the TRX is 100lbs heavier, so wouldn't the 39 work even better for the track or short distance runs. It's not very often that some will run at top speed , but more likely to run in second and third! ecspecially if your at the track or the dunes! :huh:
86atc250r, what is your opinion on this subject? :D
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i tried a 39mm and loss no topend at all.........and why people think that i don't know......a yfz comes with one,and it have verygood topend.

the 39mm makes the bike respond very well in bottom-mid and top............the bike really feels faster with it.


it seems like if someone on here say (with a 39 you will lose topend)...everyone takes that and runs with it..............and no one has probably even tried it on a dyno to really see what would happen.

thats not a good way to give info

maybe we should not make comments on something unless we've had some kind of fact behind your statements..............this may help keep bad or misleading info off the org.... :)
 
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