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I did a cold compression check on my 01 KDX200.


I held the throttle wide open, pressed the kill switch and kicked about 15 times. The reading topped out at 175ft lbs.

But I noticed that when I stopped kicking the pressure on the gauge began slowly dropping.

When I put the tester hose into the spark plug hole I tightened it by hand as tight as I could get it.

I tried a second compression tester and it also dropped pressure.

Does this drop indicate a problem with the engine? Also does 175 seems too high?

The pressure dropping is an issue with the compression gauge likely the valve core in the hose

Since the piston rings have gaps as well as clearance in the piston's ring grooves, I would not expect them to hold combustion chamber pressure indefinitely. IIWM, I wouldn't worry about a slow leak down.

Carbon buildup in the combustion chamber will decrease the volume and raise the compression pressure. Have you removed the head to take a look?

+1 on checking the valve in your compression gauge.

Don't confuse a compression test with a leakdown test. If you haven't plugged the intake and exhaust ports of course the pressure is going to leak right back out.

Julien D wrote:Don't confuse a compression test with a leakdown test. If you haven't plugged the intake and exhaust ports of course the pressure is going to leak right back out.

+1 If you are doing a simple compression you will see the pressure drop off, that's why the service manual has you check the "Max" value recorded. 175 psi is a bit on the high side, but as noted could be due to carbon build up in the cylinder.

bufftester wrote:

Julien D wrote:Don't confuse a compression test with a leakdown test. If you haven't plugged the intake and exhaust ports of course the pressure is going to leak right back out.

+1 If you are doing a simple compression you will see the pressure drop off, that's why the service manual has you check the "Max" value recorded. 175 psi is a bit on the high side, but as noted could be due to carbon build up in the cylinder.

Thank you bufftester and everyone else who responded.

So to check for carbon build up, do I have to pull the head and cylinder?

As a first step, I would pull the pipe and have a look up the exhaust port to see how much carbon has built up there.

Also, you could take a look at the KIPS valves to see if they're clean or not.

If you could get your hands on a fibrescope, you could poke it through the spark plug hole and up the exhaust to see how the head and piston crown look.

Last would be to pull the head.

Hope this is helpful.

pumpguy wrote:As a first step, I would pull the pipe and have a look up the exhaust port to see how much carbon has built up there.

Also, you could take a look at the KIPS valves to see if they're clean or not.

If you could get your hands on a fibrescope, you could poke it through the spark plug hole and up the exhaust to see how the head and piston crown look.

Last would be to pull the head.

Hope this is helpful.

Thanks pumpguy !