The graphs shown above are describing horsepower and Air/Fuel mixture of a motor versus engine speed. The power scale is on the left, and the Air/Fuel mixture scale is on the right. An ideal Air/Fuel mixture for best power is near 13:1. A lean mixture is 15:1 to 18:1 and is higher on the scale (not enough fuel). A rich mixture is lower on the scale, less than 12:1 (too much fuel).
LOW RPM BOG-
See the high peaks on the left edge of the Air/Fuel Ratio graphs, peaking between 15:1 to 18:1 Air/Fuel ratio. This motor is very close to bogging from a lean-shortage of fuel, as shown by the peaks with air/fuel mixture resulting from quickly snapping open the throttle. At full throttle the fuel must overcome gravity to be lifted up thru the main jet to reach the carburetor venturi and intake port. Even though this is a 450cc motor, the air velocity thru the carburetor is barely enough to draw fuel from the float bowl at 5000rpm. This carburetor has an accelerator pump to squirt fuel, but it's lasting only a fraction of a second. The accelerator pump is designed to force fuel into the carb when there isn't enough vacuum to pull adequate fuel from the main jet, and helps avoid a lean-bog in the lower rpm range. The reason this carb was tuned to use a minimal pump squirt, was to avoid having overly rich jetting when riders repeatedly snap the throttle open when parked, and while riding at lower rpms in rough conditions.
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Main Jet and Air/Fuel Transition with RPM-
Notice how the air/fuel lines gradually slope downwards from 6000rpm to 11,000rpm. The main jet circuit is gradually going from a lean 15:1 ratio, to a good 13:1 ratio with no change in throttle (entirely full throttle). The downwards slope is because the jetting becomes richer as the RPM's and air-speed increases.
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Why does jetting become richer at higher RPM???
-When the motor spins faster, the pressure gradually becomes lower in the intake port and fuel is drawn from the main jet at faster and faster rates. Because air is compressible, the lower pressure air becomes less dense at the same time the fuel is flowing faster. ***The flow of air-mass and fuel aren't rising in the same proportions*** The faster flowing, but DECREASING DENSITY air, and increasing fuel flow cause high rpm to be richer than low rpm and downward slope in the graph.
--The richening trend is common for nearly all racing carburetors, and causes big motors to use smaller main jets than small motors when running the same size carburetor. Small motors don't reach air-flow rates near where the big displacement motors operate, so run lean unless a bigger main jet is installed. When this same carb is used on a motor with half the displacement, the main jet size needs to be increased, and the accelerator pump stroke adjusted to squirt more fuel than on this motor. Bogging can still be a problem for some riders who favor running at lower rpms, and for these people running a smaller carburetor is a better solution.
In order to use a bigger carb with smaller displacement motors, the motor needs to be fully hot, and the rpms must stay higher, to avoid bogging at full throttle.