Generac Power Systems GTH1000, Service Manual

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SECTION 1: GENERAL INFORMATION
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 function-
ing. When grounding out one cylinder there will be no RPM loss. When
the other cylinder is grounded out the engine will stop.
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
compression in the cylinder with lower pressure. See example.
EXAMPLE:
Cyl. #1 Cyl. #2 Diff.
Eng. #1 165 PSI 160 PSI 5 PSI
Eng. #2 175 PSI 155 PSI 20 PSI
If compression is poor, look for:
• Insufficient valve clearance • Warped cylinder head
• Loose cylinder head bolts • Warped valve stems
• Blown head gasket • Worn bore and/or rings
• Burned valves, valve seats
and/or loose valve seats
Model Normal Compression
GTH/GTV-760 130-160 psi
GTH/GTV-990/1000 160-190 psi
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 some testers 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 of the compression stroke.
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.
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