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18. Carburetor
Note: X1 carburetor illustrated, X3 & X5 models similar
Overview
-Your Polini is a high performance racing motorcycle. Because of the high level of performance, the carburetor will need
adjusting for various riding conditions. Changes in temperature, elevation and load can all require major changes in jetting. If the jet-
ting is not adjusted properly, problems can occur, ranging from a stumble or hesitation to severe engine damage. There are many fac-
tors which affect the proper jetting. Adding more oil to the fuel results in less fuel to burn and a lean condition. Cold air is denser,
and will tend to make the bike run leaner. Hot weather will make the bike run richer. Changes in barometric pressure can affect jet-
ting. Unfortunately, there are so many possible variables that change constantly the jetting must sometimes be adjusted daily for best
results. To change jetting, you will need a selection of main jets, pilot jets, and needles. A jet selection is available from Action
Racing, as well as optional slides, a fuel bowl with a plug for main jet changes, and various other parts. The idle speed and fuel
metering screw will also have to be adjusted.
This is probably the most difficult aspect of working on your Polini, but in time will be one of the most rewarding. Mastery of jet-
ting will enable your rider to always have the best tuned bike, which might be just enough for an advantage at that next start or high
speed straightaway. Certainly, an ill-running bike can slow a rider considerably. Fortunately, there are a number of tests and proce-
dures you can perform to help dial in the proper jetting. Like any skill, it will improve with practice and dedication. The following
sections will attempt to cover the fundamentals of jetting and theory. However, ultimately you are responsible for the bike running
properly, so use this information as a guide but at your own risk.
Before making adjustments to carburetor, be sure that the air cleaner is clean and properly oiled. If the air cleaner is dirty or too
thick of an oil is used, a very rich condition will occur. Several mechanical problems can occur, which will affect the way the bike
runs. If the carburetion appears to change dramatically for no apparent reason, look to mechanical causes before adjusting jetting. If
the motor revs upon startup and idles at a very high speed you have an air leak. Find and fix the air leak before adjusting jetting!
Note:
When working on carburetor, never use a wire or other sharp instrument to clean jets and passages as damage will result. Wear
eye protection when using compressed air or solvents.
Rich conditions -
Rich conditions can occur if: dirt enters the carburetor, the float can get stuck open, causing the bike to run rich
and fuel to pour out of the vent tubes. Check float level and operation. Shake float gently to make sure no fuel has entered float. If
the bike smokes alot and the plug seems fouled with oil, the transmission seal behind the clutch assembly may be leaking oil into the
crankcase. Also check air passages in carburetor throat. Jetting will be adversely affected if they are clogged.
Lean conditions -
Lean conditions can occur if the engine develops an air leak. Air leaks can occur in the carburetor, in the intake
assembly, and in the engine itself. An air leak will make the idle speed erratic and very high. Common causes include: loose clamp
on carburetor, punctured or damaged intake boot, leaking and/or warped reed block assembly, leaking cylinder base gasket, porosity
or cracks in crankcase or cylinder castings, and a leaking seal on the alternator side of the engine. Spray carburetor cleaner around
suspect areas with engine running. If idle increases you have located an air leak. If any of the jets get clogged with debris the motor
will run lean due to the lack of fuel. Check timing to make sure it is within specifications.
Other things to check include making sure air filter oil is not dripping into the carburetor and partially clogging air passages or
jets. Make sure to allow filter oil to “set” before installing filter as any dripping oil will flow directly into the carburetor. Maxima
FFT air filter oil is recommended to help prevent this problem. Make sure fuel/oil mixture is fresh and well mixed. Check fuel flow
from fuel hose to make sure fuel is flowing freely to carburetor. If fuel flow is sluggish, check hose and valve for blockages. Check
and clean fuel filter as needed. A blocked fuel cap vent hose will also prevent fuel from flowing. Check carburetor vent hoses for
kinking or blockage. Make sure o-ring is not lost or damaged on fuel metering screw assembly.
Jetting variables -
The following list includes most of the factors affecting jetting. Other factors can also affect jetting, from fuel
additives to slide wear. Proper jetting really boils down to the proper amount of fuel (and oil) well mixed with the proper amount of
air. Most changes not relating to the fuel can be simplified to air density, which is simply the amount of air in a given space. Air den-
sity varies with elevation, barometric pressure, and temperature so the jetting requirements will also vary accordingly. A higher air
density will result in a lean condition, a low air density will result in a rich condition. Gages are available to measure air density, and
can be a very useful tool.
Elevation
- can have a considerable affect on jetting. As the elevation increases from sea level, the air thins out, so the mixture will
become increasingly richer. At the bottom of a mineshaft, you might get a little lean.
Barometric pressure
- is similar to changing elevation. More pressure = more air = leaner condition. Lower pressures will tend
to cause a richer condition.
Temperature
- Higher temperatures (low air density) will cause the motor to run richer. Lower temperatures will make the motor
run lean.