Vanguard BIG BLOCK M540000, Repair Manual, Page: 29 / 168

• from the carburetor indicates air is leaking past the
intake valve and valve seat.
• from the exhaust system indicates air is leaking past the
exhaust valve and valve seat.
• from the breather tube or oil fill dipstick tube indicates air
is leaking past the piston rings.
Causes of poor compression and cylinder leakage:
• Loose cylinder head bolts
• Damaged cylinder head gasket
• Burned valves, burned valve seats, and/or loose valve
seats
• Insufficient tappet clearance
• Warped cylinder head
• Warped valve stems stem seized in guide
• Valve
• Worn cylinder bore and/or piston rings
• Broken connecting rod
Cylinder Balance Test
If the engine starts hard, runs rough, misses or lacks power,
perform a cylinder balance test. This test will assist in
determining whether both cylinders are operating correctly.
A cylinder balance test will also detect a cylinder that is not
functioning. Grounding the spark plug of a cylinder that is not
functioning will result in no RPM loss. Grounding the spark
plug of the other cylinder, which is functioning, will cause the
engine to stop.
1. Obtain a tachometer and 2 ignition testers. See
Special
Tool List .
2. Attach the ignition tester ( A , Figure 26) between the
spark plug wire ( B ) and spark plug ( C ). Repeat on both
cylinders.
26
3. Start the engine and run at top no-load speed.
a. Note the spark at the ignition tester.
If the spark is equal at both ignition testers, the problem
is not ignition related. A spark miss will be readily
apparent. Repair and retest.
4. Connect the tachometer to the engine. See the
tachometer instructions for proper placement and setup
steps.
5. Note the engine RPM.
6. Ground one cylinder by contacting the ignition tester
resistor ( D ) to a good ground on the engine.
a. Note any RPM loss.
7. Repeat step 6 and 6a for the other cylinder.
If the difference between the 2 cylinders does
not exceed
75 RPM , the amount of work the 2 cylinders are doing is
considered equal.
An RPM loss
greater than 75 RPM indicates that the cylinder
with the lower RPM loss is the weaker of the 2 cylinders. See
example:
Engine RPM - Both Cylinders =
3400 RPM
Engine RPM - Cylinder #1 Grounded = 3300 RPM
Engine RPM - Cylinder #2 Grounded = 3100 RPM
Conclusion: Cylinder #1 is the weaker cylinder.
Problems Affecting 1 Cylinder:
NOTE:
A 2-cylinder engine will run on 1 cylinder as long as
the power required for the application does not exceed the
power produced by one cylinder.
1. Spark plug - fouled, broken or worn. A fouled spark plug
may indicate a fuel system problem.
2. Spark plug wire
3. Head gasket
4. Intake manifold - a leak at either end of the intake
manifold will affect only 1 cyclinder.
5. Valves
6. Piston rings
7. Pistons
8. Cylinder
Problems Affecting Both Cylinders:
1. Fuel system
2. Ignition timing
3. Crankcase vacuum
4. Intake loose or disconnected
30 vanguardpower.com