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Sportster Motorcycle Motor - Top End Discuss Sportster Motorcycle Top End issues. Rockerboxes, Valves, Cylinders, Pistons, Rings, Lift Rods, etc...

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  #1  
Old 22nd October 2006
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Lightbulb Cylinder Compression and Leakage Tests

Cylinder Compression And Leakage Tests

A compression test should be done as part of any 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 any 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. Run the engine to get it up to operating temperature
2. Disconnect and remove both spark plugs
3. Screw the compression tester into the spark plug hole for either of the cylinders
4. Lock the throttle open, and ensure the choke plate is also open
5. Crank the engine through several rotations until there is no further increase in reading at the tester guage
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 dammage 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 teardown: 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. The piston must be positioned such that both the intake and exhaust valves are closed. To accomplish this, set the cylinder to be tested precisely at top dead center [TDC] of the compression stroke, as follows:

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 here to have either a shop assistant or a wide "wingspan".

Keep a thumb pressed on the spark plug hole while rotating the back wheel. On the compression stroke you will definitely feel the air pressure working against your thumb.

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.

2. Turn on the compressor, connect the gauge to the air pressure, and adjust the gauge
3. Thread the adapter into the spark plug hole and into the guage
4. The gauge now shows the amount of leakage
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


Notes

The piston must be positioned so that both valves are closed

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.

There are many variables here [engine temperature, precise piston position, whatever] so the test results will not be consistent from one test to the next.

Last edited by IronMick; 23rd October 2006 at 05:45..
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  #2  
Old 10th September 2008
veight veight is offline
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what is the normal compresion reading lb? and do you know why my bike would run hotter in the front cylinder ? also won't start after 1/2hr off running sounds to me valve timming off when you try to restart it hot . then when you wait for it to cool down it starts fine . veight
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  #3  
Old 10th September 2008
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Mine is running hotter on front than rear also, trying to figure out why. Have a post going called "exhaust heat" looking for answers.
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Old 5th October 2009
veight veight is offline
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Default poor compression

[QUOTE=clacker24;2205374]Would a bad gasket cause missing and the difference of a compression test and missing? Here is a little background, I did a check of my compression and found that the front cylinder is running around 85 and the rear is running 110 or so. My bike is also missing and by checking the plugs it looks like the front cylinder is the culprit. I have a bad head gasket due to the oil on top of the front cylinder. What I’m looking for is confirmation that a bad gasket would cause all this.
Thanks
Clacker24
1985 XLX-61 1000cc[/QUOTE..... Yes .... I am not sure what your correct compression should be. but my book says 150-160 for stock motors .I would say that 110 and 85 is too low and would cause your motor to run poorly..... and if you have oil around the head gasket and low compression you need to take your heads off and check them out.If your lucky you might just have to replace the head gaskets .
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  #5  
Old 11th February 2010
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1GodJC 1GodJC is offline
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I have a 2007 xl1200c fuel injection sporty. When the bike is warmed up, it idles low. I'm not sure if it is low compression or not. I have drilled out my stock pipe. I also had to replace my air cleaner cover. I had got it used, and didnt know the back plate was for my year, so I had to cut out some of the plastic to modify it, to fit. Since then I have noticed a slow but gradual change in the way it idles at a warm temp.
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Old 8th June 2010
mowen mowen is offline
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I tested my 73 ironhead with a 1000 motor and the readings were 75 and 65 front. Are these low for what is stated above???
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  #7  
Old 8th June 2010
laneappliance laneappliance is offline
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my sporty had cranking compression of 165 but that is modded pretty well
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  #8  
Old 16th July 2010
aoehero aoehero is offline
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[QUOTE=1GodJC;2436913]I have a 2007 xl1200c fuel injection sporty. When the bike is warmed up, it idles low. I'm not sure if it is low compression or not. I have drilled out my stock pipe. I also had to replace my air cleaner cover. I had got it used, and didnt know the back plate was for my year, so I had to cut out some of the plastic to modify it, to fit. Since then I have noticed a slow but gradual change in the way it idles at a warm temp.[/QUOT

Hey JC have yuo read the post on nightrider and spin off site on excessive heat and variation in ides depending on O2 sensor?
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