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78/83/90 Series Horizontal Shaft Engines

Troubleshooting the EVAP System

There are four failure modes for the EVAP system:

• A leak in the system - will allow dirt ingestion.

• A blockage in the system (between the canister and the fuel tank) - acts a plugged vent.

• A blockage in the system (between the canister and the control panel) - The canister can not purge its

vapors.

• Charcoal canister poisoned (raw fuel in the canister) - prevents the canister from absorbing fuel vapors.

NOTE: The only failure mode that will affect the operation of the engine in a way the operator will notice is a

blockage in the system.

To test the system for a blockage:

1. Remove the fuel cap.

2. Clamp off the hose that runs from the canister to the

control panel.

See Figure 4.12.

3. Attach a vacuum pump to the vent port of the char-

coal canister. See Figure 4.12.

4. Apply a vacuum to the canister.

NOTE: When applying the vacuum, squeeze the

pump slowly. Too much vacuum can close

the roll over valve giving a false reading.

5. If the canister does not hold a vacuum, the canister

and the roll over valve are working properly.

NOTE: If the canister does hold a vacuum, test the

roll over valve to determine if the roll over

valve or the canister is at fault.

Figure 4.12

Figure 4.13

Charcoal canister

MTD 78 Series User Manual

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