REF: Service Procedures 20

Cylinder Compression Test

A compression test should be done as part of each tune-up. Record the results and follow any changes over time. 1)

  • The compression tester has a one-way valve in it's hose, allowing it to see the pressure of the compression stroke
    but not relieve it on the other strokes. Additional strokes that produce higher pressures will increase the
    reading of the gauge.
  • Always look for a compression tester that has it's check valve as near to the chamber as possible. Otherwise, you're
    adding the volume of the hose (up to the check valve) to the chamber volume which causes the compression reading
    to be lower, because the piston is trying to fill that hose space as well as the chamber. 2)
  • A proper compression test should be done with the engine close to normal operating temperature, when possible. 3)

Compression Test vs Leak-Down Test

  • These are both fancy names for a deceptively simple process (installing an air gauge in the spark plug hole and noting the results). 4)
  • A compression test checks the cylinder's maximum pressure ability. A low reading reflects poor sealing of the chamber.
    • Generally if a compression test shows anything suspect, the next step is a leak down test. 5)
  • A leak-down test checks the cylinder's ability to hold pressure (seal) and will reflect where the leak is occuring.

Test Each Cylinder for Compression

  1. Ride the bike to get the engine up to operating temperature. 6)
  2. Disconnect the power to the coil to prevent it from firing.
    • The extra two plugs (on dual plug heads) need to remain in place during this test. 7)
    • If you can't disconnect power to the coil, ground out all the plug wires.
  3. Clean the area around the spark plugs BEFORE you remove them. 8)
    • If you haven't disconnected the coil, you can insert the plugs into the plug wires and press the plugs against the cylinder heads for grounding. 9)
  4. Connect the compression gauge to the first cylinder.
  5. Make sure the throttle plate is fully open (and the choke, if present) before turning over the engine.
    • If using a CV carb, hold the slide up. 10)
    • If the cylinder can't freely pull in air, the compression reading will be low.
    • Do NOT pull the enrichener knob - The enrichener is mis-named a choke ('86-'06) but it is not a choke plate.
  6. Crank the engine continuously through 5-7 compression strokes. (until there is no further increase in reading at the gauge) 11)
  7. Record the readings at the end of the first and last strokes for this cylinder.
  8. If you get a lower than normal reading, add app. 1/2 ounce of oil into the cylinder, repeat the compression test to see if the reading will increase. Record the oil-added readings.
  9. Repeat for the second cylinder.

12) 13)

Interpreting the Readings

  • Compare the readings for both cylinders.
Table of Expected/Acceptable Readings for Evo Engine Models
Model Years 883cc Engines 1200cc Engines
1986-2003 120psi Min 9.0:1 CR Not in Manual, but expect
150psi or greater
9.0:1 CR
1200 Sport Model → Not in Manual, but expect
200psi or greater
10.0:1 CR
2004-2008 120-140psi 8.9:1 CR 200-225psi 9.7:1 CR
2009-later 165-180psi 8.9:1 CR 200-225psi 9.7:1 CR
Notes: The recorded results between the two cylinders should be within 10psi of each other.
PSI expectations are from HD manuals.
  • Also, pay attention to how many pumps it takes to pump up each cylinder. 14) Both cylinders should be relatively comparable.
    • Pumping up too fast means the exhaust isn't opening (pushrod worn too much or loose enough to fall out of place).
    • Pumping up too slow is the usual suspect list below:
Possible Ring problemsCompression low on 1st stroke tending to build up on following strokes without reaching normal spec.
Improves considerably with a re-test with oil
Possible Valve problems
Head Gasket Leak
Compression low on first stroke tending to not build up on following strokes even with a re-test with oil

If you have low test results, go back through the steps above and re-test before moving on to the Leak-Down Test.

Tracking Your Test Results

Compression Test Results from XLFORUM Members

Engine: 2002 883R > 1250R conversion.
Compression Test Results:
Front Cylinder: 270 psi.
Rear Cylinder: 55 psi.
Sudden rattling noise in engine with noise increasing.
White smoke coming from the A/C and pipes.
Oily spark plugs.
Immediate disassembly of the rear cylinder.
Results Upon Disassembly:
The exhaust side of the rear piston was broken and missing.
Possible Cause:
Piston ring gap too tight upon installation. 15)
Full Article:
Pics of the Damage: 16)

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