Compression???? I need help

[michaelaverett]

Hey guys. Thanks in advance for any info. I've just completed alot of work on my KX 125 and I am at my wits end. :bang: I was going to put a new top end in the bike but when I took it apart the piston and ring looked OK. The exhasut valves also seemed to be working fine. So I cleaned off all of the carbon and put the bike back together. Now, I only register a 25 on compression. My question is this..... How many parts of the bike affect compression? Is it just the piston, ring, and exhaust valves or am I missing something. If there is nothing else involved with compression will a new top end give me compression?

Thanks,
Mike

 

[Pauly250]

What kind of compression did the bike have before you took it apart? It would be very difficult to tell if the piston and ring's are worn just by looking at them. Did you use new gasket's? Did you torque the cylinder and head correctly? You may have broke the ring putting the cylinder back on.

 

[michaelaverett]

Thanks for your reply! The bike had about the same compression before. I had an incident with a puddle that took the bike out of commission(long story). Me and a buddy disassembled the entire engine looking for the problem. We fixed alot of stuff along the way. Anyhow it all looks great now, but I don't have compression. I am afraid that when I pulled off the cylinder and honed it that I may have taken too much off. I recently read that the KX has very delicate plating. I am asking the compression question because I am hoping that the lack of compression is confined to the top end. What do you think?
Thanks,
Mike

 

[motometal]

are you holding the throttle wide open when you kick it over to check compression?

 

[Pauly250]

The only way to tell if it is worn beyond specs for sure is to measure the cylinder diameter...if you sucked alot of water into the engine at high RPM it may have a bent the rod.I would take a closer look at the top end components,did you have the case apart?

 

[Pauly250]

How old was the top end before the incident?

 

[Ryan12833]

spend the $20 on new rings and $40 on a good caliper. Measure your cylinder and if it is within specs all you should need is new rings. And since you have very low compression get a new head gasket.

 

[michaelaverett]

I guess I was trying to get out on the cheap. We took out the top end again and their was no damage. I was not giving full throttle to check the compression. I did not know I had to do this. I will try this again and if compression is still low I have a buddy who can mic the cylinder and the piston to make sure they are still up to factory specs. If the cyclinder is still good then I am going to just bite the bullet and put in a whole new top end. This whole process has been extremely frustrating!

 

[motometal]

let us know how it works out. It takes fairly specific tools and a good operator to really do an accurate job measuring piston and cylinder, especially cylinder. A calipers, although very very handy for working on bikes, is not really the tool here. Most calipers are +- .001" or .0005", not accurate enough.