Air Control Diaphragm And Valve

In warm weather, it may be difficult to find malfunction in hot air control system. In cold weather, vacuum leaks or faulty air control valve will result in engine stalling or hesitation, increased fuel consumption and/or lack of power.
When such problems occur, check the hot air control system before carrying out inspection of the carburetor. Check vacuum hoses for cracks and proper connections. Inspect air control diaphragm (vacuum motor) with engine stopped.
Place a mirror at end of air cleaner snorkel and observe position of air control valve. Position is correct if valve is closed (hot air inlet blocked). If valve is not fully closed, check valve linkage and pivot points for binding.
Disconnect vacuum hose from air control diaphragm, attach another hose and apply vacuum to diaphragm. Valve should rise to open position (hot air inlet uncovered, fresh air inlet blocked). If diaphragm holds vacuum but valve does not open, check valve linkage and pivot points for binding.
If diaphragm does not hold vacuum, replace it. With valve in open position, pinch hose to trap vacuum. Valve should remain open for at least 30 seconds. If valve closes too soon, replace air control diaphragm.