Chapter 4: The Fuel System and Governor
30
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 suck air through the vent, the vacuum
would prevent the fuel from getting to the carburetor.
The tank is vented through the cap. See Figure 4.3.
To test the cap vent
1. A hand-pumped vacuum / pressure tester may
be connected to the fuel barb (after draining).
See Figure 4.4.
2. The tank should not hold any pressure nor any
vacuum.
• Replace the cap if either pressure or vacuum
builds using the hand-pump tester.
• 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.
Figure 4.2
Figure 4.3
Tank
Cap liner / gasket
Filter
Cap
Sleeve (mtg. hole) Hose bar
Figure 4.4
Hand-pump vacuum
/ pressure tester
connected to fuel line
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