SOme more really good info for you. You want to do the by-pass valve test (not the gulp valve test).
Anti-backfire Valve Tests
There are two different types of anti-backfire valve used with air injection systems. A by-pass valve is used in 1972-81 engines, while 1971 engines use a gulp type of anti-backfire valve. Test procedures for both types are given below.
GULP VALVE
Detach the air supply hose which runs between the pump and the gulp valve.
Connect a tachometer and run the engine to 1500-2000 rpm.
Allow the throttle to snap shut. This should produce a loud sucking sound from the gulp valve.
Repeat this operation several times. If no sound is present, the valve is not working or else the vacuum connections are loose.
Check the vacuum connections. If they are secure, replace the gulp valve.
BY-PASS VALVE
Detach the hose, which runs from the by-pass valve to the check valve, at the by-pass valve hose connection.
Connect a tachometer to the engine. With the engine running at normal idle speed check to see that air is flowing from the by-pass valve hose connection.
Speed up the engine so that it is running at 1500-2000 rpm. Allow the throttle to snap shut. The flow of air from the by-pass valve at the check valve hose connection should stop momentarily and air should then flow from the exhaust port on the valve body or the silencer assembly.
Repeat Step 3 several times. If the flow of air is not diverted into the atmosphere from the valve exhaust port or if it fails to stop flowing from the hose connection, check the vacuum lines and connections. If these are tight, the valve is defective and requires replacement.
A leaking diaphragm will cause the air to flow out both the hose connection and the exhaust port at the same time. If this happens, replace the valve.
Check Valve Test
See Figure 14
1975-78
Before starting the test, check all of the hoses and connections for leaks.
Detach the air supply hose from the check valve.
Fig. 14: Testing the check valve
Insert a suitable probe into the check valve and depress the plate. Release it; the plate should return to its original position against the valve seat. If binding is evident, replace the valve.
With the engine running at normal operating temperature, gradually increase its speed to 1500 rpm. Check for exhaust gas leakage. If any is present, replace the valve assembly.
Vibration and flutter of the check valve at idle speed is a normal condition and does not mean that the valve should be replaced.
1979-84
Remove the check valve from the air injection manifold.
Blow into the manifold side (large side) and check that the valve is closed.
Blow into the ASV side (small side) and check that the valve is open.
If the valve is not operating properly it will probably require replacement.
Air Switching Valve (ASV) Tests
Start the engine and allow it to reach normal operating temperature and speed.
At curb idle, the air from the by-pass valve should be discharged through the hose which runs to the ASV.
When the vacuum line to the ASV is disconnected, the air from the by-pass valve should be diverted out through the ASV-to-air cleaner hose. Reconnect the vacuum line.
Disconnect the ASV-to-check valve hose and connect a pressure gauge to it.
Increase the engine speed. The relief valve should open when the pressure gauge registers 2.7-6.5 psi.
If the ASV fails any of the above tests, replace it. Reconnect all hoses.
Anti-backfire Valve Tests
There are two different types of anti-backfire valve used with air injection systems. A by-pass valve is used in 1972-81 engines, while 1971 engines use a gulp type of anti-backfire valve. Test procedures for both types are given below.
GULP VALVE
Detach the air supply hose which runs between the pump and the gulp valve.
Connect a tachometer and run the engine to 1500-2000 rpm.
Allow the throttle to snap shut. This should produce a loud sucking sound from the gulp valve.
Repeat this operation several times. If no sound is present, the valve is not working or else the vacuum connections are loose.
Check the vacuum connections. If they are secure, replace the gulp valve.
BY-PASS VALVE
Detach the hose, which runs from the by-pass valve to the check valve, at the by-pass valve hose connection.
Connect a tachometer to the engine. With the engine running at normal idle speed check to see that air is flowing from the by-pass valve hose connection.
Speed up the engine so that it is running at 1500-2000 rpm. Allow the throttle to snap shut. The flow of air from the by-pass valve at the check valve hose connection should stop momentarily and air should then flow from the exhaust port on the valve body or the silencer assembly.
Repeat Step 3 several times. If the flow of air is not diverted into the atmosphere from the valve exhaust port or if it fails to stop flowing from the hose connection, check the vacuum lines and connections. If these are tight, the valve is defective and requires replacement.
A leaking diaphragm will cause the air to flow out both the hose connection and the exhaust port at the same time. If this happens, replace the valve.
Check Valve Test
See Figure 14
1975-78
Before starting the test, check all of the hoses and connections for leaks.
Detach the air supply hose from the check valve.
Fig. 14: Testing the check valve
Insert a suitable probe into the check valve and depress the plate. Release it; the plate should return to its original position against the valve seat. If binding is evident, replace the valve.
With the engine running at normal operating temperature, gradually increase its speed to 1500 rpm. Check for exhaust gas leakage. If any is present, replace the valve assembly.
Vibration and flutter of the check valve at idle speed is a normal condition and does not mean that the valve should be replaced.
1979-84
Remove the check valve from the air injection manifold.
Blow into the manifold side (large side) and check that the valve is closed.
Blow into the ASV side (small side) and check that the valve is open.
If the valve is not operating properly it will probably require replacement.
Air Switching Valve (ASV) Tests
Start the engine and allow it to reach normal operating temperature and speed.
At curb idle, the air from the by-pass valve should be discharged through the hose which runs to the ASV.
When the vacuum line to the ASV is disconnected, the air from the by-pass valve should be diverted out through the ASV-to-air cleaner hose. Reconnect the vacuum line.
Disconnect the ASV-to-check valve hose and connect a pressure gauge to it.
Increase the engine speed. The relief valve should open when the pressure gauge registers 2.7-6.5 psi.
If the ASV fails any of the above tests, replace it. Reconnect all hoses.
