Cub Cadet MTD 70 Series - Manual (Page 50)

61/65/70/75 Series Horizontal Shaft Engines
44
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.
NOTE: Fuel tanks with a roll over valve vent: See
Figure 4.6.
• Have unvented fuel caps
• The fuel caps are tethered to the tank.
• They vent through the charcoal canister.
NOTE: The fuel tanks that do no have a roll over
valve are vented through the cap. See Fig-
ure 4.7.
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 operating condi-
tions.
• Usually presents as a “Runs and quits” scenario
Figure 4.6
Roll over valve
Figure 4.7
Vent