Cylinder Balance Test; Check Compression; Cylinder Leak Down Test Procedure; Cylinder Leakdown Test - Generac Power Systems GTH760 Service Manual

Figure 1-11. Checking For Fouled Plugs

CYLINDER BALANCE TEST:

If the engine is hard starting, runs rough, misses or lacks
power, perform a cylinder balance test to determine whether
both cylinders are operating to their full potential.
Tools Required:
1. Two Ignition Testers (Generac P/N 0C5969)
Attach an ignition tester between the spark plug lead and
each spark plug, Figure 1-10.
Start and run engine at top no load speed and note spark at
ignition testers. If the spark is equal at both ignition testers,
the problem is not ignition related. A spark miss will be readily
apparent. Now note RPM of engine. Ground out one cylinder
by contacting ignition tester and a good ground on engine,
Figure 1-11. Note RPM loss. Then ground out the other spark
plug and note the RPM loss. If the difference between the two
cylinders does not exceed 75 RPM, the amount of work the
two cylinders are doing should be considered equal.
If the RPM loss is greater than 75 RPM this indicates that the
grounded cylinder with the least RPM loss is the weakest of the
two cylinders. Look to that cylinder for a problem.
Example:
Engine RPM - Both Cylinders = 3400 RPM
Engine RPM - #1 Cylinder Grounded = 3300 RPM
Engine RPM - #2 Cylinder Grounded = 3100 RPM
Conclusion: #1 cylinder is weakest of the two cylinders.
The cylinder balance test will also detect a cylinder that is
not functioning. When grounding out one cylinder there will
be no RPM loss. When the other cylinder is grounded out the
engine will stop.
SeCTIoN 1: GeNerAl INForMATIoN

CHECK COMPRESSION:

It has been determined through testing that a simple and
accurate indication of compression can be made as follows:
Remove both spark plugs and insert a compression gauge
into either cylinder (one cylinder at a time). Open the throttle
to Wide Open Throttle (WOT) position. Turn engine over with
engine starter until there is no further increase in pressure.
Record this reading. Repeat procedure on other cylinder and
record that reading. The difference between both cylinders
should not exceed 25%. More than 25% indicates loss of com-
pression in the cylinder with lower pressure. See example.
EXAMPLE:
Cyl. #1
Eng. #1 165 PSI
Eng. #2 175 PSI
If compression is poor, look for:
• Insufficient valve clearance
• Loose cylinder head bolts
• Blown head gasket
• Burned valves, valve seats
and/or loose valve seats
Model
GTH/GTV-760
GTH/GTV-990

CYlINder leAkdowN TeST

A cylinder leak down test will indicate the condition of the
cylinder. It will assist in troubleshooting the engine's condition
such as leaking valves or rings.
On the tester there will be two gauges, one will be the
incoming air pressure and the other will measure the percent
of cylinder leakage (see Figure 1-12).

CYLINDER LEAK DOWN TEST PROCEDURE:

1. Piston must be at TDC.
2. Install the tester into the spark plug hole.
3. The crankshaft/flywheel must be safely locked down to
prevent turning.
4. Pressurize the cylinder to 90 psi.
5. Observe the leakage of the cylinder and where the air is
coming from.
NOTE: If leaking into intake or exhaust port, check lash,
valve face, and seat condition.
1-5
Cyl. #2 Diff.
160 PSI 5 PSI
155 PSI 20 PSI
• Warped cylinder head
• Warped valve stems
• Worn bore and/or rings
• Broken connecting rods
Normal Compression
130-160 psi
160-190 psi
1