How do you go about performing a compression test?
What tools are needed?
In the end, what does it tell you?
Curious minds want to know...
Step 1. Aquire compression tester
Step 2. Remove spark plugs
Step 3. Install compression tester into spark plug hole
Step 4. Crank engine for about 5 revolutions using starter button
Step 6. Obtain PSI reading from guage on compression tester
Step 7. Repeat steps 3 to 6 on other cylinder
Why do you suspect you need to check compression? A compression test tells you if your engine has good compression inside the combustion chamber. A better test would be a cylinder leak down test which is able to help isolate a potential problem such as bad rings or valve seats. A compression test will show if you have a bad valve. Im not sure what good compression test numbers are on a Duc but would think they should be around 130 - 150 psi. If you compare the two cylinders and the numbers vary significantly then you have a problem.
What tools are needed?
A compression tester, essentially it's a pressure gauge with a check-valve that screws into the spark plug hole.
How do you go about performing a compression test?
Install the compression tester on one cylinder, hold the throttle wide open, and crank the engine with the starter until the reading stops climbing.
Rinse and repeat.
In the end, what does it tell you?
It gives you a general reading of the sealing quality of the valves, head gasket and rings.
IMO, somewhat limited utility, especially on a 2-cylinder engine.
More useful on an engine of 4 or more cylinders, as you can compare readings across all cylinders, and focus less on the absolute reading.
A leakdown tester is a better tool, but more expensive.
With that, air pressure is applied to the cylinder, and the leak rate is measured with two pressure gauges on each side of a small orifice.
Added benefit is you can determine which of the areas is leaking; intake valve, exhaust valve, rings, or head gasket.
Downside is the added expense, a need for compressed air, and the requirement of holding the piston at top dead center.
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edit: I type slow. :P
Thanks for the info. With three bikes now, that I plan on keeping for some time, I wanted to know if there a was "cost-effective" way to measure engine health. Was talking to a guy, and he indicated a compression test is one good indication of where a motor is at.
I still plan on following regular routine maintenance, but was hoping I could acquire the necessary tools to due some "diagnostic" measurements myself.
+1 "Hold Throttle Wide Open"
I used to put a little engine oil in the cylinder to determine wether any low readings were due to rings or valves.
Do keep in mind a cold engine will have lower compression than a warm engine.
You can make a cheap cylinder leakage tool out of a cored spark plug and an air coupler, just weld the part together. True, you won't be able to measure % leakage. On compression (both valves closed) nothing should be coming out of the intake or exhaust. Any leakage means a leaking valve.
+1 on leak down test its by far easier to tell whats going on with your engine, and with good ears you can actually tell where the leak is at, be it rings (air coming out the oil fill tube), exaust valve (air coming out exaust), intake valve(air coming out the throttle opening), head gasket (water cooled take off your raditor cap and watch for bubbles, air cooled you would be able to hear it or feel it)
There are two general types of compression testers: the kind that screw in and the kind that have a rubber end you just hold in with pressure. The second type is easier and faster but you often need one person to hold the tester and one to crank the engine. Some compression gauges come with both ends.
As already mentioned, hold the throttle wide open when taking a reading. Do each cylinder once dry THEN add a little engine oil and do it wet. You want dry and wet readings for all cylinders. If there's a large discrepancy between dry and wet it's likely you got problems with rings.
Also, if your valves are not adjusted properly the readings can be misleading.
Scott
I still owe you one Munch, bring the bikes over sometime and we'll benchmark each cylinder (compression or leakdown). If anything changes as you check them periodically, you'll know before the problem gets worse.
Marty