CV Carburetors and Altitude

A CV carburetor compensates for changes in air density in the areas that the slide controls (i.e. all but the idle circuit and part of the progression circuit). CV stands for constant velocity, which , once the slide is off the stop, is maintained by raising or lowering the slide relative to the mass of air that at is passing under it, resultant from the combination of throttle butterfly angle, rpm, and air density. In other words, for a given intersection of throttle angle and rpm, the slide is lowered when the air density decreases, and raised when it increases. Since the jet needle is attached to the slide, this causes the fuel flow to be adjusted in proportion. 2K ft is worth ~1% CO, or approximately one jet size, so, for example, at 6K ft, the non-compensated portion of circuits richens by ~3% CO, or ~3 jet sizes (some of this will be negated on the overlap when the fuel screw is adjusted to suit). The practical limit to how much the mixture can be allowed to richen is when it causes a drivability issue, i.e. a rich misfire (in the range between the correct mixture and rich misfire there is a progressive loss in power that at small openings may not be all that noticeable). Given that the carburetor is in good condition i.e. has good rubber, is not worn, not clogged, and is otherwise set up and jetted correctly for sea level, this would occur at somewhere between 13K ft. (~4000 m) and 21K ft. (~6400 m) without trimming the fuel screw. A properly tuned CV-carbureted motorcycle will therefore go from sea level to at least 13K ft (~4000 m) with no more than a fuel screw adjustment. This does not mean that there could be no improvements made to the progression circuit at altitude - there certainly could. If the motorcycle is always ridden at high altitude, it may be desirable to install a smaller pilot jet. Whether this makes sense can be verified by the position the fuel screw ends up in relative to at sea level when adjusted via the method outlined in the article Idle Mixture and Speed Setting for CV Carburetors.