How to correctly measure compression on XR650R

I have just uncorked my XR650R and I thought that this would cure my fast idling - it didnt.

Reading the manual it states that low compression could be a reason for this, as my bike was already stripped down, I thought I would do a compression test.

I connected up the hose (hand tight) to the cylinder (engine cold) and kicked it over a few times.

First with the decompressor level held in I basically got a compression reading of 0.

Second, I kicked it as if I was going to start it which gave me a reading of about 86psi.

When reading the manual it states that with decompressor level held in it should be reading 87psi, and 'normal' operation it should be 160psi.

My question is are my tests valid? Should I warm the engine up and wont the automatic decompressor affect the readings? It seems a bit suspicious that I have 86psi on a kick which is very close to what is recommended with the decompressor held in??

Maybe I should tackle the high idling by adjusting the pilot screw and idle setting before worrying about the compression?

Any help appreciated.


My understanding is that it should probably should be 87 psi with the auto decompressor, not with the decompressor lever applied. With the decompressor lever pulled in, it lifts the valves and you would expect to get 0 psi, right?

Don't you have to kick it over at a certain RPM to avoid the auto decompressor kicking in? Or am I way off? :thumbsup:

Throttle must be wide open.

Yes, the manual says to make sure the choke and the throttle are fully open. Presumably this is to make it easier to pull in air.

It also specifies that the compression should be 160 psi at 400 RPM (if memory serves me correctly). I'm assuming that this is to make sure the auto decompression does not come in to play (hence my comment above). I'm kinda curious to know how to get a correct reading also. Just kick it over a bunch of times pretty fast? (or again, am I way off?)

You are way off :thumbsup:

Definitely adjust your idle speed lower. Then with the bike warm, adjust your pilot screw. You want to set your pilot screw in between the two areas where the bike tries to die, or runs badly when turning the screw in and out. This middle area should be where the engine runs the fastest/smoothest. If the middle area, or sweet spot is less than 1 turn out, go leaner on the pilot jet one notch. If the sweet spot is more than 3 turns out, go one richer on the pilot jet. Then readjust your idle speed.

Thanks for the input - I dont think I have much chance of kicking the bike over at 400rpm. I suspect that my compression is ok at 86psi with the auto compression kicking in.

I think the plan is to do as snagglexr650 recommends - I will try warming it up and adjusting the pilot and idle to get the correct idling speed.

For future reference, how do you correctly measure the compression if I cant kick it at 400rpm?



I personally have no idea. However, I've had XR's of one size or another since '87. My new in '87 XR200 (original owner) was sold in 2002, 15 yrs old and ran better when I sold it than when I bought it. A few of them were bought used and then used even harder by me :thumbsup: Most of them burnt some oil, but they all went like stink, ALL the time. You aren't talking about a 2-stroke MX'er here that needs a new top end every year. Don't worry about the compression until the beast looses all it's ass, or just won't run. In the meantime, keep the valves adjusted, air filter cleaned and keep it full of fresh oil and just ride the damn thing. Just my advice... Ride on!

Thanks for the tips, adjusted the fuel screw and idle - no longer over revs at idle.

Just have to try and dial out the back firing on shutting the throttle off and the slight snatchiness with slight throttle adjustments (could be due to me having tightened up the throttle cables).


Do you have a good shop manual for your bike? Your compression is probably OK, since it is correct with the auto decompressor "activated", i.e. normal operation. 86 psi at kickstart speeds sounds about right. With the manual release activated, 0 psi sounds about right.

The manual for my old 650L explained to increase the clearance on the front right valve to deactivate the automatic compression release, which works by slightly raising the valve when kickstarting.

I have the pdf manual that is linked to via

I use this for any work that I want to do.

Been away this weekend so not had time on 'fine tuning'



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