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9000' Elevation Gain/Change. FCR or C/V

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I have a well tuned FCR on my 434WR thanks to an O2 sensor and air/fuel ratio gauge. But I've noticed the FCR is pretty finicky with elevation changes. I have very little experience with the BSR36 C/V carb but I hear they are much more forgiving with the elevation changes. I'm planning the Nevada BDR this fall that has about 9000' of elevation changes and thinking about switching back to the CV for this trip. The CV carb is more compliant with elevation changes, right? Not just my imagination? I really appreciate the quick access for needle/main jet changes on the FCR but would much rather not have to change anything on the trail.

Thanks in advance

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Myth. If the ratio of the air inlet to the jet doesn't change, how does the fuel ratio change? If the carb auto adjusted, we wouldn't need to re-jet when we did the 3X3 and pipe mod.

Just change you jets in your carb if you feel you need to. That's what I did going from sea level to 9k in the Rockies. My buddies didn't change the jets in their WR's , but they did need to use their hot start to start their bikes at the summit.

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the

On 09/07/2017 at 9:46 PM, Bermudacat said:

Myth. If the ratio of the air inlet to the jet doesn't change, how does the fuel ratio change? If the carb auto adjusted, we wouldn't need to re-jet when we did the 3X3 and pipe mod.

Just change you jets in your carb if you feel you need to. That's what I did going from sea level to 9k in the Rockies. My buddies didn't change the jets in their WR's , but they did need to use their hot start to start their bikes at the summit.

the CV is a vacuum operated carb the air flowing through the carb lifts the slide, the less dense the air the slightly less it will lift the slide. it will adjust itself on a very very small level in a similar way to the lectron carb although the lectron works on air density creating an eddy after the needle. the CV carb does not self adjust to the needs of the motor such as a 3x3 and open pipe, if you regular go up in altitude you could try setting up your AFR to run at about 14.1 and see what it richens up to at alititude. depending on what your jetting is set up and which fuel you could try using a fuel that runs leaner e.g if you have set up your jetting to run on 100 percent gasoline trying switch to 91 E10 and that will lean it out almost a full 1 AFR

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The CV carb does deal with elevation (air pressure actually) changes somewhat better than the FCR does because of the vacuum operated slide (thinner air means a weaker vacuum which causes a leaner mix, just what you want as you go up into thinner air.)

What some guys have done is gotten a spare side panel, cut a large hole in it and created a sort of shutter that enables them to vary the size of the opening. As they go up in thinner air, they open the hole. Some have simply remove the airbox side panel when performance got terrible and put it back on as they returned to thicker air.

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20 hours ago, dg400sm said:

the CV is a vacuum operated carb the air flowing through the carb lifts the slide, the less dense the air the slightly less it will lift the slide.

Thanks, I appreciate your explanation of how a CV carburetor works.

At one time I owned a vintage Honda 750F. The carburetors on it were different than four stroke carburetors of today. Unlike the FCR, and unlike the CV carburetor, these carburetors were like four two-stroke carburetors, that actuated together with a linkage. It was a sophisticated linkage, because it operated on an arc pivot, but had to move the slides linearly.

These were a little more tricky to operate, because if you opened the throttle too fast, your air flow would outpace the fuel flow, and your engine would stumble. A few years later, Honda would reveal their CV carburetors, and all the operator had to do was twist the throttle, and let the balance of the CV slide keep them from stalling, much like the fabled BSR on the stock DRZ.

However, whether the carburetor opens by cable, by a sophisticated linkage to gang four linear slides to one radial linkage, by hydraulics, by electric motor, or by vacuum, until you change the jet, the needle or the ratio that the needle moves in respect to the slide, you don't change the mixture.

 In conclusion, if your CV carb seems to handle large increases in elevation well, it was jetted far too lean for the primary altitude you were previously riding at. :)

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Edited by Bermudacat
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Been running a CVK40 set up for sea level - 2K for some time.  Have ridden Angeles Crest La Canada to Wrightwood, summit passes thru Angels 8.  Very soggy throttle response, felt like a gear too high.  Exhaust made the mosquito's woozy.

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