MTD 61 series, Shop Manual

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61, 65 and 70 Series Vertical Shaft Engines
40
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 pre-
vent the fuel from getting to the carburetor.
The tank is vented through the cap. See Figure 4.6.
To test the cap vent:
1. Drain the fuel out of the fuel tank into an approved
container.
2. Attach a hand-pumped vacuum/pressure tester to
the fuel barb on the fuel tank. See Figure 4.7.
3. Attempt to pull a vacuum on the fuel tank with the
gas cap in place.
4. Attempt to pressurize the fuel tank with the gas cap
in place.
NOTE: 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
operating conditions.
• Usually presents as a “Runs and quits” sce-
nario.
Figure 4.6
Tank
Cap liner / gasket
Filter
Cap
Sleeve (mtg. hole) Hose barb
Figure 4.7
Hand-pump vacuum
/ pressure tester
connected to fuel line