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How to do a compression test?

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So, I have heard a few things, and read the manual but I want someone to tell me, explain to me, how to do a compression test on the DR650SE, and what each result will mean. Do A. If this result, ad B, then check again, if same, then replace, if not, nothing wrong...type of explanation.

There are some many of you out there with this knowledge, and above all, experience. I want to learn from that knowledge and experience. And that experience has been proven over the decades, contradict what the theory says. So please, anyone of you up for a nice, concise, step by step write up to explain the compression test and what the results tell you?

I noticed some smoke on my bike when I rev it, but only only just notice the what seemed to have been white smoke. It doesn't smoke when I ride. 70,000km does sound like a time to have to redo the motor or at least the head, OR at least check to see if all it good.

Happy trails.

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I've never performed one on a motorcycle. Different manufacturers of vehicles have different methods. I haven't looked in the manual, but I would follow Suzukis procedure. Be advised it will only be necessary to remove only one of the spark plugs to perform the test. I also recommend using a compression tester that has the screw in fittings. It is difficult to get an accurate reading with the compression testers that have the rubber tip that you hold against the spark plug hole. Be sure to engage the kill switch. You don't want the engine to start while you perform the test as it can damage the gauge. The test will give you a basic indication of the condition of the compression ring of the piston. You can also perform a leak down test which will give you a basic indication of the valve seat condition, and the condition of the surface of the valves that contact the seats. To perform the leak down test you stop the piston at the top of the compression stroke and time how long it takes for the pressure to drop. However, if your bike is using a lot of oil caused by blow by passed the rings, the leak down test will not give you an indication of the condition of the valves. Like I said I haven't had to do a compression test on my bike. I have performed them on some of my previous vehicles and each one had a different procedure. One required all the spark plugs be removed, and one required the spark plugs to be in. You also have to make sure your valves are adjusted to specs to ensure that the valves aren't out of adjustment, and one of them is being held slightly open. That would ruin an otherwise good compression reading!

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Sound ominous. Leak down test.....how easily should I be able to turn my crank (via the crank nut at the one side, behind that plug) with the spark plugs still fitted? I think, if I remember correctly (I think the plugs was still in) that I could turn my crank over quite easily.....which bothers me, since I suspect it will have to be difficult.

Thanks donwann. Much appreciated.

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You haven't said what year model your bike is. However, the '96 and newer 650's have two spark plugs. They are both on the same side of the head, but the inner one is kind of difficult to see. Yea, you should feel some resistence when you spin the engine. However, there is only one compression stoke then the plugs fire just before top dead center driving the piston down and back up for the exhuast stroke followed by the intake stroke back down. Then the next compression stroke back up to top dead center. So, when you spin the engine by hand, one revolution will be difficult while one revolution will be somewhat easier. If all the revolutions are about the same, I would first check the valve adjustment. As the valves wear the valve stems stretch and the seats become flattened preventing a seal on the chamber until they are adjusted back into specs. If the valves are okay, I would then think the rings are suspect.

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never done one on a DR650. One tip is to keep the throttle open while cranking. On the DR650 theres an automatic decompression feature that is active while cranking. This will result in numbers lower then what is traditionally seen on regular four stroke engines. Another tip is to add a few drops of oil into the spark plug hole after running the test a first time. If the numbers are higher after the oil is added the rings are worn.

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That's true about the oil. I don't know if the auto decompression works when you are just trying to spin the bike over by hand though.

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That's true about the oil. I don't know if the auto decompression works when you are just trying to spin the bike over by hand though.

The auto decompressor is a weighted spring loaded mechanism on the cam. It is always active until the engine starts up and attains a specific RPM. This makes checking compression on the DR a moot point. Just a difference in your intake valve clearance settings and even battery condition (engine cranking speed) will affect what pressure readings you'd come up with. :ride:

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