Because air density changes according to atmosphere
conditions and fuel density does not, it becomes necessary to re-jet the
carburetor according to changing altitude and weather conditions. As the
density or mass of air changes, it becomes necessary to change the mass
of fuel mixed with the air by changing the jets which meter the fuel.
Altitude affect air density the most. All other conditions being
equal, as the elevation nears zero (sea-level), the air density will be great
est and the engine will require the richest carburetor settings that it will
ever use. As elevation increases, the air density becomes less and the
engine will require leaner carburetor settings in order to maintain peak
performance. Generally, an engine which has been tuned correctly for
sea-level will not require jetting changes until about 2500’ elevation.
Temperature also has an effect on air density. As air tempera-
ture nears freezing (32 degrees F), the density of air will become greater.
Conversely, as temperature rises toward 100 degrees F, air density
becomes less. As air temperature decreases and air density becomes
greater, the engine will require richer carburetor settings in order to
maintain peak performance. Generally, an engine which has been tuned
correctly fro room temperature change is about 20 degrees F.
How to Identify Rich or Lean Conditions
Lean Condition - A lean condition is an out of balance air/fuel
mixture where there is not enough fuel in the mixture to deliver peak
performance. The results of lean mixtures can vary from minor drive-abil-
ity problem to overheating and possible severe engine damage. Care
should be taken to identify lean mixtures and correct them as soon as
Typical Lean Mixture Symptoms:
1. Engine acceleration is lat or slow to pick up.
2 It becomes dificult to apply the throttle quickly or the engine
picks up speed when the throttle is rolled off.
3. The engine knocks, pings, or overheats.
4. The engine surges or hunts for a stable R.P.M. while cruising at
5. When the pilot circuit is too lean there will be popping or spitting
in the carburetors as the throttle is opened. Sometimes
there will be popping or backires in the exhaust system on
engine deceleration after the throttle has been closed.
6. Engine performance improves in wanner weather conditions, or
engine runs poorly in cold weather.
7. Engine performance worsen when the air ilter is removed.
Rich Condition - A rich condition is an out of balance air / fuel
mixture where there is too much fuel in the mixture to deliver peak
performance. A rich condition will result in excess carbon deposits
within the combustion chamber and exhaust system, decreasing the
life of the engine and related components. In addition to poor fuel
economy, a rich running motorcycle will pollute excessively and
contribute to environmental problems.
Typical Lean Mixture Symptoms:
1. Engine acceleration is lat, uneven, not crisp.
2 Two stroke engines will sputter or “4 stroke” and skip combustion
3. The throttle needs to be opened continuously to maintain consistent
4. The engine performs poorly when the weather conditions get warmer,
or the engine works better in cold conditions.
5. Excessive smoke from the tail pipe, black smoke from the tail pipe of
6. Poor fuel economy.
7. Engine performance improves when air cleaner is removed.
7. When the pilot circuit is rich, the engine will idle roughly or not
return to idle without blipping the throttle. The exhaust will smell
of excessive fuel and bum the eyes.
9. Black, sooty or fouled spark plugs. Black and sooty exhaust tail
pipes on four - strokes. Greasy and drippy tail pipes on
Troubleshooting & General Procedures