Performance and Technical information on carburetor
jetting for all motorcycles.
Jetting considerations for motor performance factors
How do weather factors like altitude, temperature and humidity effect the jetting of a
carburetor? Here are some simple rules that can help you fine tune the jetting to get the
best performance from your motorcycle.
ALTITUDE
As you increase your altitude the octane requirement decreases 1-2 octane per 3000 feet
elevation. This is because the density of the air is reduced or there is less air
available for your motor to burn. The higher the altitude, the richer your motor will run,
making it necessary to re-jet the motor in order to lean it out. The fuel volume remains
the same and the air volume goes down. If you have a vacuum advance, as the altitude
increases, the motor makes less vacuum and the air fuel ratio becomes richer due to the
decreasing air to fuel volume.
TEMPERATURE
When the temperature goes up, the air density decreases, thus you have less air available
for combustion and your air fuel ratio becomes richer. The same works in reverse. As the
temperature goes down, you end up with more air per cubic foot, and without re-jetting
your carburetor, the engine will run leaner.
AIR DENSITY
As the air density increases, your engine will lean out. As the air density goes down, the
engine runs richer. Like driving up a mountain, at the top, the motor has less power
because you have less air to burn.
HUMIDITY
When the humidity increases, octane requirements ease. The formula is something like...
for every one gram of water increase per one kilogram of dry air the octane decreases by
.25 to .35. WWII aviation engines used water injection and it worked well for a short time
by cooling the cylinder temperature. As temperature goes back the effect goes away.
The bottom line is to make sure to pay very close attention to your jetting on cold,
dry days with low humidity and when at sea level.
