KX125 - Just replaced piston, now has no compression

[Coyote]

My brother has over the last few days taken the 125 apart to replace the piston and ring. He's followed the manual to the letter, but now after reassembling the bike it has no compression. What are the possible problems causing this?

[paturoa]

have a look at the standard pile of left over bits (just like I have everytime). the big round one, looks sorta like a cup, is called the piston. It needs to be in the motor.

Give us some more clues.........

[Coyote]

Yeah, sorry. I don't really know much myself. All I know is my brother tore down the top end, swapped the piston, rings and gaskets, put the bike back together again and couldn't start it. Him and Dad took the bike apart to try figure out what's wrong but couldn't understand what was going wrong. This is the first time any of us has tried to replace a piston and they only had the manual to go by.

I remember them saying about bending a connecting rod between the powervalves and the thing that controls them. They bent it back to the way it was, but I couldn't see that being a problem anyway.

I think thier plan for tomorrow is to take it apart again and put it back together. If all else fails, take it back to the shop

[paturoa]

Power valves could be the problem if they are stopping air getting to the transfer/inlet ports.

How do u know that there is no compression? finger in the plug hole?

[paturoa]

Power valves could be the problem if they are stopping air getting to the transfer/inlet ports.

No, scratch that in second thought, my KDX stuff is onthe exhaust side. What year KX?

[Coyote]

As you kick it theres no pressure

Powervalves seem to be in good order anyway

[paturoa]

Take out the spark plug and sticj you finder in the hole, then crank using your hand, if you have compression you will know (dont get soem one to kick it!)

[paturoa]

shit I'v ehad too much wine tonite - my tyoing is crap!

[Jamezo]

Hey - related, err, thing.

I dropped the RG this morning (cold tyres + cranked over + giving it heaps + white lines on road = falling over tiem! (before I'd barely got out of my house...)), and ever since, it takes like a 3rd of the effort to kick over! That means the compression is down right?

I've noticed two things, a little top-end loss, and also, when starting cold, a little smoke coming out of somewhere inside (instead of just the exhaust lol).

I presume this means that the header pipe is loose or something? Is that enough to cause an appreciable loss of compression?

[cheese]

I hate to ask, but did you put the rings in? make sure that the psiton is in around teh right way too.

[T.W.R]

Questions:

1) Were the rings fitted rightway up?
2) Was the Barrel given a hone before reassembly?
3) Hope it wasn't dry assembled
4) Was it a standard piston & ring kit, it hasn't been placed into oversized bore

[Crasherfromwayback]

Questions:

1) Were the rings fitted rightway up?
2) Was the Barrel given a hone before reassembly?
3) Hope it wasn't dry assembled
4) Was it a standard piston & ring kit, it hasn't been placed into oversized bore

Assuming it's got the std cyl still.....
DO NOT HONE IT....they are electrofusion coated and should NEVER be honed.
A VERY small amount of two stroke oil may be used on the piston and cyl when putting it all back together.
Unless it's been fitted with an after market cast iron liner, it's not possible that it's over size....no such thing if it's still stock.
Forget the remarks regarding power valves....they have no effect on compression unless they've garked fuck out of the piston and ring ass due to improper assembly.
Did you make sure you had the ring in the right spot as regards to the ring stop pin?
Pm me if you're still having trouble....I've had dozens of two stroke KX's and have worked for two Kawasaki dealerships....so am reasonably familiar with the snotty coloured things!

Pete

[Racey Rider]

Did you put in a home made base gasket?
If it was too thick a gasket, that could lose some compression.

You've differently checked everything else?
You've got spark at the plug?
you've got fuel getting to the plug? (Good fuel, not last years fuel!)

You checked out the barrel properly before installing? Look at the port windows to check they're not broken (ie. missing a bit).

What caused you to change the piston to start with? Was the bike still running before taking it apart?

[T.W.R]

Assuming it's got the std cyl still.....
DO NOT HONE IT....they are electrofusion coated and should NEVER be honed.
A VERY small amount of two stroke oil may be used on the piston and cyl when putting it all back together.
Unless it's been fitted with an after market cast iron liner, it's not possible that it's over size....no such thing if it's still stock.
Did you make sure you had the ring in the right spot as regards to the ring stop pin?

If the bike has any age to it it's more than likely it has had a replacement sleeve.
never be to tight oiling the new piston & cylinder, that the includes gudgeon pin aswell.

If the ring wasn't fitted properly on the piston and riding the locator pin there'd be heaps of compression, as well as one farked ring a scored bore & 2 people who'd have noticed the difficulty assembling it.

[k14]

If the bike has any age to it it's more than likely it has had a replacement sleeve.
never be to tight oiling the new piston & cylinder, that the includes gudgeon pin aswell.

If its a nikasil bore then I don't know what you are talking about. You can't rebore one of them, can get a replate but I doubt that's been done. I'd say its the rings as the most likley problem.