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I took a compression test today and was SHOCKED at the results. Here I have a 2001 kdx which was bought new in 2003 but the original owner and ridden somewhat lightly. I bought it in May'06 and put about 1,000 miles on it since then. Here's what's strange - I scored 165, and I am on the stock piston&rings! Somehow I expected it to be lower. I also thought 95-145 was within spec. Do I have a bad compression tester or a bike that is FRESH???

Oh OH !


Whatcha gonna do now, to bring it into spec???

I'm clueless.

You're "clueless"? And I thought you were "2001KDX"??
Kidding!

So the compression is fine!...... as it usually is.....in anything except severely worn cylinders.

There are many variables in compression testing (temp/throttle position/kicking technique/oil in the cylinder/etc/etc)
But, it seems clear that *if* the cylinder does need rebuilding at this time, it certainly isn't due to low compression.

SO? .....worry about something else!!

Probably a large amount of carbon build-up on the piston head. This can result in too much compression and blowing the seals on the crank. Take care of the piston and head soon.

skipro3 wrote:Probably a large amount of carbon build-up on the piston head. This can result in too much compression and blowing the seals on the crank. Take care of the piston and head soon.

I did not know if your kidding or not Jerry.

A large amount of carbon on the piston will bump the measured compression in an engine. But no way is it going to blow any seals on the crank. Reason the crank seals are both under negative pressure during the intake cycle of the engine and only see minimal above atmospheric pressure when the engine is on the power stroke.

I'll bet the head is sparkly clean!

Take one heck of a lot of carbon to bump the compression in a low comp engine...to the point where it would be an issue.

Just my 2 cents...

And, I sure hope I haven't annoyed that good ol' monkey....

Now I'm worried every time I ride! Grrr!

Let's see how long it will last! Kidding. I'll look into it.

Surely you have something bigger to worry about than decent compression in the ol' KDX

If not, then I'm jealous!!

You are right. I meant bearings not seals. Excess compression can affect bearing performance. I doubt this would be much of an issue though. However I have seen, personally, an increase in compression due to carbon build-up in the combustion chamber.

If you can find another comp. gauge to compare with, that would at least confirm your findings.

Pull the pipe and look up in there with a flashlight. Pushing on the kickstarter will put the piston in a position to see the head I think. A mirror and flash light might get you a peak from the spark plug hole too. It isn't hard to pull the head though, just a little inconvienant due to the coolant.

and don't worry GS, I'm only mildly annoyed. At myself for mixing up bearings with seals!! Grrr!! (HAW!!)

BTW was that test with throttle wide open or closed? Engine at operating temperature?

I have always kept a spare top-end gasket set just for times like this. I know in a few years I will need it anyways for the scheduled inspection and/or re-ring. It is so easy to tear apart, check, fix, and rebuild. Having the set on hand makes it an easier decision.

For the sake of comparison . . . I've performed three compression tests on my 2005 model. The same drill each time . . . warm the bike by driving it around the block (1.4 miles), use the same off-the-shelf Autozone compression tester, full-open throttle, 10 kicks, repeat once and average results. The results follow:

10-29-2005 with 330 miles -> 162 psi

3-25-2006 with 830 miles -> 165 psi

3-17-2007 with 2400 miles -> 165 psi

My manual indicates a usuable range of 95 to 151 psi. I've simply assumed my gauge was off by 10% but thought the constant readings over time were a good sign nothing serious was amiss.

Ski, my throttle was closed and the bike was fairly cold, had been ridden an hour earlier. I probably should have used different variables.

Tommorow I will do as you say and take a look. I'm scared as to what I'm gonna find!

2001kdx wrote:Ski, my throttle was closed and the bike was fairly cold, had been ridden an hour earlier. I probably should have used different variables.

Tommorow I will do as you say and take a look. I'm scared as to what I'm gonna find!

You better be scared With a warm engine you may bump the compression up a few more psig.

.............not to mention if the throttle is held open during the kickety-kick.

Oh dear!!

just pop the cylinder off when you get a spare hour or two and check out the piston.

Re: ' I've simply assumed my gauge was off by 10% but thought the constant readings over time were a good sign nothing serious was amiss.'

Th'ar 'ya go!

Worry about the number less (Be sensible. A reading of zero is never going to be good!), and use it as a tool as described above.

Well, now I'm worried. Bought a KDX 1998 from a buddy who maintained it to the tee, never did a top end. Also bought a KTM 300 EXC from him (he and his wife were getting on in years) and my son and I have been riding them for a year. Change the oil, etc., religiously.

The KTM fouled a plug yesterday, it was the original plug, my son was riding it. New plug, started right up. Local excellent mechanic said "check the compression" before anything else. Checked it, it with a brand new (this morning) Craftsmen gauge. 195psi. Then checked the KDX, it was 172 on the same gauge. Is the compression too high on the KDX? His wife rode it, never had the head off.

Is it possible the gauge is reading too high? Or is there too much carbon on both? They run like tops -- except the surprise with the KTM fouling a plug. We mix the oil like we're arming the fuse for nuclear bombs -- and always err on the "more oil than less" side, and the pipes shoot a lot of sponge.

Any hints or tips? Love the site, and thanks for the accumulated wisdom and kind attention.

Too much is not "Good", mix the KTM at 50:1 and the KDX at 40:1 using quality oil. If your spooging, you are going to build up carbon on the piston.

Nice 8 year thread resurrection!