Bowl Vent Valve

Remove bowl vent vapor hose from carburetor. Check open condition of valve by connecting to a manual vacuum pump. It should not be possible to draw more than 0.5 in. Hg if valve is open (as when engine is off).
If high resistance or plugged system is found, check for plugged or restricted hose. Hose may be cleared with compressed air. If hose is clear, remove canister filter. If restriction persists, replace canister.

Fig 1: Vacuum Hose Routing for Single-Canister General Motors Evaporative Control System
Courtesy of GENERAL MOTORS COMPANY

Canister must be lowest point in system.
To check valve closed position, run engine at idle. Manifold vacuum will be applied to valve through control line. Bowl vent line should exhibit a plugged condition.
If valve is not closed, remove control vacuum line, and check for vacuum. If no vacuum, check for hose restriction or leak. Replace hose if required. If vacuum is present, replace canister assembly.