THE FUEL SYSTEM AND GOVERNOR
26
Generally these kits involve mixing a measured
amount of water and gas together and seeing were the
boundary layer is.
See Figure 4.2.
The test kit should come with a chart to compare
the boundary layer height to alcohol percentage.
Fuel tank vent
The fuel tank vent performs the important task of
allowing air into the fuel tank. As fuel is being used by
the engine, the fuel level in the tank drops. The drop
-
ping fuel level then creates a vacuum in the tank. If the
fuel tank could not draw air through the vent, the vac
-
uum would prevent the fuel from getting to the carbure-
tor. The vent is located in the fuel cap. See Figure 4.3.
To test the cap vent
1. Remove the fuel cap.
2. Clean off the vent.
3. Blow air into the vent hole. The air should blow
throw the vent with little back pressure.
4. Suck air through the vent hole. Air should freely
enter through the vent.
• Replace the cap if the vent builds pressure or
restricts air movement.
• A cap that maintains pressure will cause the
engine to run rich as the fuel in the tank heats
and expands, forcing it’s way past the float valve
in the carburetor.
• A cap that maintains vacuum will cause the
engine to run lean as the fuel is depleted and no
air comes in to replace it.
• The two conditions may both be present, but the
symptoms vary with fuel, fuel level, and operat-
ing conditions.
• A bad fuel cap vent usually presents as a “Runs
and quits” scenario.
The fuel filter
The fuel filter is located in the fuel tank nipple. It
can be removed and cleaned with a can of carb cleaner
or it can be replaced.
See Figure 4.4.
NOTE: If cleaning a filter, back-flush it by spray-
ing the carb cleaner through the barb end and
out of the screen. Also make sure the fuel tank is
clean.
To replace the fuel filter follow the steps described
in Chapter 1: Introduction.
Figure 4.2
Figure 4.3
Fuel cap
Vent
Figure 4.4
Filter
nse
:
Fuel filter/barb
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