THE FUEL SYSTEM AND GOVERNOR
27
Generally these kits involve mixing a measured amount
of water and gas together and seeing were the bound-
ary 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 allow-
ing air into the fuel tank. As fuel is being used by the
engine, the fuel level in the tank drops. The dropping
fuel level then creates a vacuum in the tank. If the fuel
tank could not draw air through the vent, the vacuum
would prevent the fuel from getting to the carburetor.
The vent is located in the fuel cap. See Figure 4.3.
To test the cap vent
1. Clean off the vent.
2. Blow air into the vent hole. The air should blow
throw the vent with little back pressure.
3. 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.
• 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
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