How to Compression Test a Motorcycle

Santa brought me a nice OTC compression tester for Christmas this year, so while I had Sue's bike up on the lift doing the oil change, I thought I'd see what the compression looked like (although there was no reason to believe there was a problem; I simply had the new toy and wanted to try it out).

This is something new to me, and I figured I wasn't the only one who might have never done this before, so I thought I'd walk through the process here. It's a fairly inexpensive diagnostic tool that's easy to use. Meaning, if you can remove a spark plug, you can do this.

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Although you can do this test on a cold bike if you have to (say you're buying a used bike that isn't running), it's recommended that you do it with the bike at operating temperature. So warm up the bike and make sure the bike is in neutral.

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Once the bike is warm, you start by removing the spark plugs.

NOTE: If you're like me, and don't remove the plugs very often, it's a good opportunity to inspect your plugs. You can see by how black these plugs are, that the trike is running really rich (which probably explains the occasional backfiring on deceleration). I hadn't noticed it, but when I mentioned that her bike was running rich, Sue asked if that could be causing the backfire. It probably is.

Sues Plugs.jpg

I'm going to get a fresh set to see if this is an old problem or something relatively new (I don't know when the plugs were changed last).

UPDATE: I took a picture of the gapped new plugs before I installed them so you can compare to the plugs above. The difference is pretty obvious.

New Plugs.jpg

The next thing you need to do is attach the meter. The Harley shop manual says to start with the forward jug, so that's what I did.

front jug first.jpg

This OTC tester has a short whip that attaches to the meter via a quick disconnect which makes it easier to thread into the spark plug holes. Once you've tested the first cylinder, you'll move on to the next. You won't be done until you've done them all—in the case of this '98 EVO, we were going to do both the front and the rear.

With the bike warm, the plugs removed, and the meter attached, you need to open the throttle all the way and crank the bike for a few seconds (if you have an electric start) or kick the kick starter until the needle stops moving if your bike has a kick start.

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On this bike, the shop manual (check your manual to find out where the bike you're testing should be) suggests if the meter reads below 90, there is a problem. This forward jug looks great.

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The rear jug checked out too.

While they don't need to be exactly the same, you want them to be pretty close (within around 10 percent). 

If you're looking at purchasing an unfamiliar used bike (particularly one that hasn't been run for a while), this is one way to evaluate it before you agree upon a price. If the compression is bad and the bike doesn't run, it will give you a clue before you buy it whether or not it might need some top end work. To run, every motor needs air, spark, fuel, and compression—this simple test will tell you whether or not you have compression. In other words, whether or not you have a potential problem with the rings or the valves, and if the price is right you can decide if you want to tackle the challenge of fixing the problem, or you want to walk away from this particular bike.

Also, I would think showing up with a compression tester when looking at a used bike would tell the seller that you know what you're doing and they might be less likely to try to blow smoke up your skirt.

If you've never done this before, Lowbrow Customs has created a nice video that walks you through what you'll need to do to compression test a motorcycle. Todd, the mechanic, is demonstrating on a Triumph parallel twin with a kick start, but the procedure is basically the same for any bike. I like Todd's videos—he's a pretty good teacher and seems to know what he's talking about.