No Compression - Bike Runs Fine ???

[FIVE-HUNGE]

01 KX 500
kicks over like a 125 2 stroke.....but runs fine ???
I brought it to the local practice track 2 weeks ago. Did five or six 10min. moto's with no problems. Brought the bike home put it away for the rest of the week. Sat. morning pulled it out for a wash, air filter service, ect.
Loaded her up Sunday and headed out for the track. Unloaded, suited up, climed on, flipped out the kicker and proceeded with the usual KX500 starting routine. Cycled the kicker a couple of times to get it TDC. Except this time TDC wasn't there ??? Naturally I looked down to make sure spark plug was in and tight, all set .....?? had my buddy cycle the kicker a bunch of times while I looked and listened for any obvious leaks.
But nothing out of the ordinary. " Well give it a few good kicks and see if it tries to start "
Pulled the choke and it fired on 3rd kick. Ran fine all day, no noticeable power loss, did another 4 hours worth of practice moto's with no performance problems, But felt like kicking over a 125 all day and still does.....
Any suggestions before I open the toolbox and play Frankenstein ?

 

[nickyd]

I'd do a compression test - see what it reads. I will state the obvious - but it might be time for a top end - it doesn't take a TON of compression to make a bike run....

 

[RM_guy]

...it doesn't take a TON of compression to make a bike run....

...and you may have been getting such a gradual loss in power that you didn't notice it. A 500 can be missing a lot of power and still be a beast!

 

[Papakeith]

I don't know about theKX but the newer CR500s had a notch cut into the cylinder wall to allow for easier kick starting. Kind of like the decompression levers(without the lever).

From the way you've described it, I would tear it apart and freshen the top end.

 

[marcusgunby]

pistons do break eventually and cause serious damage.

 

[FIVE-HUNGE]

I guess I'll start there, the top end is pretty fresh though. maybe 20 hrs. I ve owned this bike since new and always got at least a full season on a top end.
I thought maybe I wasn't noticing power loss because it can be so gradual, but it still pulls away from the 450's like a top fuel dragster.
Thanks for the input, I'll post again with results.

 

[Nathan Rubin]

Before you try to get measurement equipment do this:
1. take out the plug.
2. try if, if it possible, put your finger tip on the hole
3. ask you buddy to kick.
If the piston will push your finger hard and you fill warming at the skin, it is not the compression. But if you will fill only little pushing without filling temp. change you should search for compression tester.
Do you know the compression spec. of the bike?

 

[cnielse5]

Hey, I have a thought. Use a compression tester and see what it reads!

 

[FIVE-HUNGE]

Did a tear down last night. Top end is fine.
The ignition side crank bearing and seal is no good. I can move the end of the crank where the flywheel mounts up and down - side to side about a 1/16"
when you kick it over there is a good enough pulse of air leaking out to blow out a candle. I'm sure with that leak the top end would have burned up shorlty after.
Maybe time for a crank rebuild, the main bearings only have about 20hrs as well.

 

[nickyd]

good thing you caught it early - time to break out the miller lite and git r done

 

[NO HAND]

...the end of the crank where the flywheel mounts up and down - side to side about a 1/16"...

That freeplay must have very hard on the con-rod bearings. I would also pull the con-rod and see if it has free play up and down on the big end bearing.

 

[crazy4nitro]

watch out after the rebuild,i've known a few people with big bike's, wreck the first time out due to the "forgotten power" coming back.....

 

[FIVE-HUNGE]

Big end of the rod is tight. But after removing the piston from the rod I noticed the wrist pin took a beating, groove city...
I may just replace the crank bearing, seal and top end and run her on the ice for the rest of winter.

 

[motometal]

I wouldn't put it back together without new rod bearings. Not worth it.

I'm still a little baffled, if I read correctly the main bearings were replaced 20 hours ago? At 20 hours of operation on a 500, you shouldn't see significant wear on the top end, let alone the main bearings.

 

[FIVE-HUNGE]

Yeah, your right.....
I have the angel on one shoulder telling me to just rebuild, and you know who on the other shoulder telling me to "just run it "
I'm thinking of going with IMS lower end kit, anybody have anything good or bad to say about them ??? The cylinder has already been done by MaxPower and Weisco pistons have been working just fine. This will be my first crank rebuild though.