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  #1  
Old 27th February 2007
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Lightbulb Cylinder Compression And Leakage Tests For Your Ironhead

Cylinder Compression And Leakage Tests

A compression test should be done as part of each tuneup. Record the results and follow any changes over time.

A cylinder leakage test should be done any time you are considering taking down the top end. This test gives you useful information regarding what you should look for when you have it apart.

Each of these requires a tester which you can buy at an auto parts store. A compression tester could be found for about $40.00 more or less, and a clyinder leakage tester for somewhat more than that amount. With the cylinder leakage tester you will also need a small air compressor. Should be able to find a small sausage style for under $100.00.

Use these tools once and they have paid for themselves, compared to paying $50.00 to $100.00 per hour at a shop. Buy a slightly larger air compressor [look for about 5 CFM @ 90 PSI] and you will be able to use it with air tools.


Compression Test

1. Ride the bike to get the engine up to operating temperature
2. Disconnect and remove both spark plugs; insert them into the leads and place them on the cylinder heads for grounding
3. Screw the compression tester into the spark plug hole for either of the cylinders
4. Hold the throttle open, and ensure the choke plate is also open [choke knob pushed in]
5. Crank the engine through several rotations until there is no further increase in reading at the tester gauge
6. Record the final reading
7. Repeat for the other cylinder

The difference between the two readings should not be more than 10 psi. A low reading on one cylinder indicates possible valve or ring damage to that cylinder. To determine which, pour about 1/2 oz of oil into that cylinder through the spark plug hole and repeat the test. If the reading returns to normal then valves are good, rings may be defective. If compression does not return to normal then valves may require service.


Cylinder Leakage Test

This test will tell you what to look for at the top end tear down: worn exhaust valves, worn intake valves, leaky head gaskets, or worn rings.

This test is usually done immediately following a compression test. So the engine is at or close to normal operating temperature, and the spark plugs have been removed.

1. Remove the pushrods to ensure that the valves are closed.

2. Set the cylinder to be tested at or near top dead center [TDC] as this is the wear area for the rings ...

a] rear wheel off the ground, shift to 2nd gear, rotate wheel until it clicks, repeat until you get to 4th gear. Now by rotating the rear wheel you are rotating the engine. Almost impossible with the plugs in; very difficult in lower gears. For the following it helps to have either a shop assistant or a wide "wingspan".

b] you can see in thru the spark plug hole, especially with a flash light. You can also try inserting something soft like a pencil and watch it rise and fall as you rotate the engine with the back wheel; but be careful it does not break or get stuck! You will have to hang on to the pencil with left hand while rotating the rear wheel with the right.

3. Turn on the compressor, connect the gauge to the air pressure, and adjust the gauge
4. Thread the adapter into the spark plug hole and attach to the gauge
5. The gauge now shows the amount of leakage
6. Listen at each exhaust pipe. Excessive hissing here indicates the respective exhaust valve is not sealing
7. Listen at the A/C intake. Excessive hissing here indicates intake valve[s] not sealing
8. Listen at the timing plug hole. Excessive hissing here indicates the rings are not sealing
9. Listen around the cylinder/head connection. Excessive hissing here indicates the respective head gasket is not sealing


Notes

A mechanics stethoscope [with the probe removed for this test] makes a good listening device, as does a length of oil line hose.

Greater leakage results in a sound of lower pitch.

There will always be some leakage, especially past the rings.
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Last edited by IronMick; 13th June 2013 at 03:55.. Reason: sp
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Old 8th May 2007
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Do you mind if I add something here IronMick?

During the leakdown test, when you apply the air pressure to the cylinder the piston will be driven downward thereby turning the rear wheel. So secure the wheel by applying brakes or some other method or it will just turn over the engine and open the exhaust valve.

In aircraft engines, that turn a much lower RPM than motorcycle engines the usual culprit in a poor leakdown test will be a bit of carbon between the exhaust valve face and the valve seat. Often a jug will be pulled when not required do to this effect.

A quick check is to take a bronze hammer and smack the valve stem under the rocker while pressure is applied to the cylinder. Frequently this will dislodge the crud and the valve will seat giving a better test. I personally don't hold the standard compression check in very high regard
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Old 9th May 2007
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Thanx skyboltdan.
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Old 9th May 2007
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Thanks Mick I'll be doing mine soon and this looks just like what I had planned.
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Old 11th May 2007
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I just bought a compression tester, Is there a number on the gauge I should expect to see when checking the compression or am I just looking for the difference between the two cylinders
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Old 12th May 2007
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Quote:
Originally Posted by iron69 View Post
I just bought a compression tester, Is there a number on the gauge I should expect to see when checking the compression or am I just looking for the difference between the two cylinders
If you look in your FM it will tell you that standard is 120 psi. I usually get somewhere between 130 and 160. The number is not as important as the difference between the two - it should be 10 or less they say.
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Old 20th July 2007
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Hiya Mick,

I hope you don't mind if I add another idea... I do a kinda tight bastard's leakage testing since donkey's days.

What you need is an old spark plug. Knock off the ceramic bit, drill a hole into the threaded part and weld a piece of steel tubing to the remainig base. Connect your compressor hose (and (gently) apply some air pressure.

Make sure both valves are closed. To prevent piston/crank movement, stick a small screwdriver into the driving chain (close to rear sprocket), with 1st gear engaged.

From then on, refer to Mick's hints above:


5. Listen at each exhaust pipe. Excessive hissing here indicates the respective exhaust valve is not sealing
6. Listen at the A/C intake. Excessive hissing here indicates intake valve[s] not sealing
7. Listen at the timing plug hole. Excessive hissing here indicates the rings are not sealing
8. Listen around the cylinder/head connection. Excessive hissing here indicates the respective head gasket is not sealing


Cheers,
Ray
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Old 21st July 2007
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Thanx Ray rt101 for this. These old school techniques keep the IronHead forum alive. Cool!
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Old 9th December 2008
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If I may add something;
When using the differential pressure gauge set, the first gauge shows the regulated pressure, which is usually set at 80 PSI, then the air passes thru an orifice, usually .060 (?) then past the second gauge to the cylinder via the spark plug adapter. This pressure would ideally be the same 80 PSI if there is no cylinder, valve or head gasket leakage.
You should take this pressure reading when the engine is healthy, then when you take it later as the engine life is used up, you would notice a lower pressure on the second gauge, indicating an increase in leakage. In a small aircraft engine, it should not be below 60 PSI. For a Sportster, the required orifice size might be different, and the lowest allowable reading on the second gauge is whatever you decide it is? When you notice the engine performance dropping off, and take a compression leak-down reading, the new low reading will verify your problem, along with listening for the leak at the exhaust pipe, air filter, crankcase or head gasket.
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Old 9th December 2008
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Quote:
Originally Posted by SHIPCHIEF View Post
... When using the differential pressure gauge set ...
I have not known about these until now. Interesting. The ones that turned up on my Google search seem to be for liquid and other chemical applications. But apparently there are some that can be used for gaseous? Are these commonly used by aircraft mechanics? Is there a technical advantage? It seems that the cost would be quite high for a home mechanic?
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