A proper compression test should be performed with the engine at normal operating temperature when possible.
PART NO. | SPECIALTY TOOL |
HD-22332-1 | Cylinder compression gauge |
1. See Airbox, Battery, and Spark plug removal.
2. Connect compression tester to front cylinder per manufacturer’s instructions.
3. Reinstall battery.
4. Make sure transmission is in neutral. With throttle plates in wide open position, crank engine continuously through 5 to 7 full compression strokes.
5. Note gauge readings at the end of the first and last compression strokes. Record test results.
6. Repeat steps 2 through 4 on rear cylinder.
7. Compare test results to the upper and lower limits posted in Table 3-27.
Table 3-27. Cylinder Compression
FRONT | REAR | ||||
BAR | PSI | BAR | PSI | ||
Upper limit | 9.45 | 137 | 17.03 | 247 | |
Mid-range | 8.62 | 125 | 15.51 | 225 | |
Lower limit | 7.79 | 113 | 14.0 | 203 |
Note. The automatic compression release (ACE) has its greatest effect on the front cylinder because of differences in valve timing between the front and rear cylinder.
8. If compression of a cylinder does not fall within its upper and lower limits, see Table 3-28.
Table 3-28. Compression Test Results
TEST RESULTS | DIAGNOSIS |
Compression low on first stroke, tends to build up on the following strokes, but does not reach normal. Improves considerably when oil is added to cylinder. | Ring trouble |
Compression low on first stroke, does not build up much on following strokes. Does not improve considerably with the addition of oil. Verify valve lash. | Valve trouble |
Same results as valve trouble. | Head gasket leak |
9. To verify worn piston rings, inject approximately 1/2 oz. (15 ml) engine oil into each cylinder and repeat the compression tests on both cylinders. Readings that are considerably higher during the second test indicate worn piston rings.
Note. After installing spark plugs, be sure that throttle plate is in the closed position before starting the engine.