12 posts in this topic
Before you begin, make sure you have a good strong battery and make sure it's charged up. If you have a bad or weak battery, you may want to replace it because if it doesn't crank good and strong, you are likely to get a low, inaccurate reading. Make sure your engine is warmed up to operating temperature(if possible). About 10 minutes of riding should do.
First, take out the spark plug and thread in the adapter for the compression tester. Make sure you have the correct size adapter for your particular ATV. Slide your kill switch to the "off" position. Some ATVs won't crank over with the kill switch in the "off" position, so if yours is like this, then you will need to either unhook your ignition coil or ground the end of the spark plug wire to a good ground. You can use a jumper wire with alligator clips on each end to ground it. Next, make sure the throttle is in the wide open position. You can either hold the throttle lever with your thumb or you may be able to tape it or use a zip tie to fasten it to your handlebars to hold it in the wide open position. If you don't have the throttle in the wide open position, you will probably get too low of a reading. Also, if you are testing a newly rebuilt engine, the engine needs to have been run for, at least, 30 or 40 minutes or you will probably get too low of a reading.
NOTE: Before you begin with the actual test, make sure the threaded adapter is screwed in good and isn't leaking any air out around it.
With the throttle in the wide open position, push the start button and crank the engine over until the hand on the gauge stops moving. Each time the engine turns over the hand should raise a little more until it reaches the maximum compression of the engine. When it stops, that is your compression reading. This usually takes no more than 10 seconds. Try to avoid cranking an engine for more than 10 seconds at a time as this is hard on the starter and the battery. Now, push the relief valve on your compression gauge and that will reset the hand back to zero. It's a good ideal to repeat the test a couple or three times to make sure you get an accurate reading. On kick start models, it will be the same procedure, but obviously you will be kicking it over instead of using a start button. Worn piston rings and cylinder walls will increase the number of strokes it takes to reach the maximum reading. If you're kicking, it could possibly take as many as 10-20 kicks to get the highest reading.
You will need to check your repair manual for your particular model for the correct compression specifications. See note below. Usually, an engine will run OK if it has at least 100 PSI of compression. Most engines will have somewhere between 100-250 and some as high as 300 PSI, depending on the engine. Sometimes they will run with under 100 PSI, but usually not very well. If you get a low reading, you can do a "wet test" to try to help determine the problem.
If your reading is too high, then you probably have carbon built up on your piston and combustion chamber.
NOTE: You may get a low reading on some engines because some engines have a decompressor assembly built into the camshaft. Check the service manual for your quad to see whether or not your quad has a decompressor assembly built into the cam.
If you got a low reading, pour about 1-2 teaspoons of clean motor oil down into the cylinder through the spark plug hole and do the compression test again. If your reading increases, then your rings or cylinder walls are probably worn. If your reading doesn't increase, then it's probably your valves. You could have a bent valve, you may have leaky valve seats, or your valve clearance may not be adjusted properly. Also, low compression can be caused by a blown head gasket.
CAUSES OF LOW COMPRESSION
*Worn piston rings or worn or damaged cylinder walls
*Valve clearance not properly set
*Blown head gasket
CAUSE OF HIGH COMPRESSION (stock engines)
*Carbon buildup in combustion chamber and on piston
NOTE: Compression testing is a good way to keep track or "gauge" the wear in your engine. When you first get your ATV or when you rebuild the engine in your ATV, you can do a compression test and then later on, you can do them periodically. This will help you determine the wear in your engine each time you do a compression test and will guide you in knowing when your engine needs rebuilding.
This is about all I can think of. I hope I didn't leave anything out and I hope this helps everyone with their compression tests.
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