The official Honda XR500 shop manual offers a handy chart which helps determine the optimal main jet size for altitude and temperature (averages). Refer to the Honda chart below to find C, the correction factor.
If C is 0.95 or below, lower the jet needle one groove and screw the pilot screw out 1/2 turn. Jet needle position and pilot screw adjustments are not needed if C is over 0.95.
To determine the main jet size, multiply the standard main jet size (155) by C.
For example, at a temperature of 28C (82F) and an altitude of 1655 m (5430 ft, which happens to be the altitude of Boulder, Colorado where I live), C is 0.95 and carburetor recommendations will be as follows:
- Main jet, 155 x 0.95 = 147.25 (#148 main jet is the closest, part number 99101-357-148)
- Jet needle, 3 –1 = 2 (2nd groove)
- Pilot screw opening, 2 1/4 – 1/2 = 1 3/4 (1 3/4 turns out)
Note that the above is for a stock engine. In this project we have a few modifications which will likely require a slightly “richer” carburetor setting due to the small increase in displacement and the Supertrapp performance muffler.
Optimal carburation is dependent on several other factors besides altitude and temperature, such as engine temperature, engine tune-up, load, RPM, and throttle position for example. The following chart helps us visually understand which carburetor component is impacting mixture with respect to throttle position. Yes, the chart is in French, now hold your colorful comments for a second, I’ll translate:
- On the horizontal X-axis we have throttle position from idle to fully open.
- On the vertical Y-axis we have the carburetor mixture-impacting components, showing at which throttle position each one is active.
- Orange: needle position
- Green: cylindrical part of the needle
- Yellow: conical part of the needle
- Blue: throttle piston chamfer
- Darker red (left): idle air screw
- Red (right): main jet
- Purple: Pilot screw
This chart can help trouble shoot carburation problems and understand which component should be tuned in situations where the engine is not running properly at some throttle position.
Next step, let’s compare XR and XL stator & rotor.