See Figures 1 and 2
The distributor vacuum control valve is a temperature sensitive valve which screws into the water jacket of the engine. Three vacuum lines are attached to the vacuum control valve: one which runs from the carburetor to the control valve, one which runs from the control valve to the distributor vacuum advance (outer) chamber, and one which runs from the intake manifold to the distributor vacuum control valve.
During normal engine operation, vacuum from the carburetor passes through the top nipple on the distributor control valve, through the valve body to the second nipple on the valve, and out the second nipple on the valve to the distributor vacuum advance chamber. When the engine is idling however, carburetor vacuum is very low, so that there is little, if any vacuum in the passageways described above.
If the engine should begin to overheat while idling, a check ball inside the distributor vacuum control which normally blocks off the third nipple of the valve (intake manifold vacuum) moves upward to block off the first nipple (carburetor vacuum). This applies intake manifold vacuum (third nipple) to the distributor vacuum advance chamber (second nipple). Since intake manifold vacuum is very high while the engine is idling, ignition timing is advanced by the application of intake manifold vacuum to the distributor vacuum advance chamber. This raises the engine idle speed and helps to cool the engine.
- Block the wheels so the vehicle does not move.
- Check the routing and connection of all vacuum hoses.
- Attach a tachometer to the engine.
- Bring the engine up to the normal operating temperature. The engine must not be overheated.
- Note the engine rpm, with the transmission is in NEUTRAL, and the throttle in the curb idle position.
- Disconnect the vacuum hose from the intake manifold at the temperature sensing valve. Plug or clamp the hose.
- Note the idle rpm with the hose disconnected. If there is no change in rpm, the valve is good. If there is a drop of 100 or more rpm, the valve should be replaced. Replace the vacuum line.
- Check to make sure that the all season cooling mixture meets specifications, and that the correct radiator cap is in place and functioning.
- Block the radiator air flow to induce a higher-than-normal temperature condition.
- Continue to operate until the engine temperature or heat indicator shows above normal.
If the engine speed by this time has increased 100 or more rpm, the temperature sensing valve is satisfactory. If not, it should be replaced.