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.

[Crasherfromwayback]

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.
You can actually assemble a two stroke engine 100% dry, with absolutely NO problems. I've done so several times, and in fact a lot of old school two stroke guru's do like wise. The minute you kick it into life the premixed fuel lubes it just as well as it does when it's running, and for the milli second it moves without creates no wear issues.
If the ring has not been located properly, it can actually get jammed into the groove over the pin and then cause the bike to have little or no compression.
It sounds like the people doing this may well not have had an issue with applying too much force fitting the cyl, seeing as they're learning as they go.
I've seen this on more than one occasion.

[Coyote]

My Dad thinks he knows what the problem was. A bracket holding the head of the barrel was overtightened, so the head wasn't able to be tightened all the way allowing the pressure to escape. We're not sure if this was the problem or not but after taking the head off and putting the bike back together it runs now. And it runs very well

Also this replacement is a Wiseco piston and it felt a fair bit lighter than the 2 original Kawasaki pistons that had been in before and before that. Sounds like it's revving that much higher because of it

[Ixion]

Just be careful with that piston, some of the Wisecos have a habit of cold seizing, because they expend differently to cast pistons.

[Crasherfromwayback]

Mate....I hate to tell you this, but if you've got a std cyl (still electrofusion), Wiesco pistons are not the go.
They SHOULD be ok, but for some reason they don't get on.

[Coyote]

The bike is an '04 KX125, and the piston is designed for it. I shouldn't see why it wouldn't work

What can be done to avoid it seizing whilst cold?

[Ixion]

Just take it easy on the welly until it's good and hot. The piston expands a bit quicker than the cylinder so it gets big quicker (could be a fortune in that if y'could put it into a tablet).

[paturoa]

Sounds like it's revving that much higher because of it

So you'll be doing it again soon! New piston and rings need a little tender care for the first hour. Work you way up to full noise!

[Coyote]

Oh yeah, of course. Didn't ring it all the way, and my Dad always warms the bikes up for a damned long time. The bike came with a motominder so we'll keep an eye on it

[Titanium]

Piston rings in up side down
Pistion installed wrong way round.
No base gasket or badly seated.
no head basket or badly seated.

Wiseco Pistons need double the piston to bore clearence than OEM pistons. - In anything.

We always hone nikasil plated cylinders.

We always use Wiseco Pistons - BUT use rings that are matched for Nikasil or Steel sleeves.

Cheers


Peter

[Titanium]

Mate....I hate to tell you this, but if you've got a std cyl (still electrofusion), Wiesco pistons are not the go.
They SHOULD be ok, but for some reason they don't get on.


It be the rings that are important to match - not the pistons. Wiseco Pistons are fine with nik cylinders, just need tha correct rings to suit.

Cheers


Peter

[Crasherfromwayback]

You're a seller of Wiesco products yeah?
I've used them in many different applications, some results were good, some not so.
I don't believe they're the right thing for electrofusion bores, correct rings or not.
I know nickasil/chrome/electrofusion coated cyl's all have the same goal in mind, but maybe the slight difference in them gives Wiesco pistons grief in Kawasakis.

[Crasherfromwayback]

So you'll be doing it again soon! New piston and rings need a little tender care for the first hour. Work you way up to full noise!
With a 2004 KX 125......new piston and ring fitted..?
Start the bike.
Run it in neutral just blipping the throttle till it gets to normal running temp.
Then shut it down, and have a few beers while you let it cool down.
Repeat, but take it easy on the beers this time.
Now take it out and thrash fuck out of it.
This is the correct running in procedure for anyone that wants a fast two stroke race bike.
Trust me.

[F5 Dave]

On older Kawasakis they were renowned for that bracket causing head sealing problems when the frame flexed. Older & jumped bikes are more of an issue apparently. The Mod according to Eric Gorr’s book (find & buy it) is to enlarge that bracket hole.

[crazyxr250rider]

nothing wrong with wiesco's got one in my bike took awhile to bed in but is good now and has much more power.
using good oil at a healthy mix does help.

[Coyote]

My brother was out testing the KX today. It's running pretty damn sweet. There were no issues starting it up and it sounded great. Another days riding should be enough running in

Also I'm getting much better wheelie-ing the Montesa. I can stay at the balancing point for 30m consistently :D

[RMrobbo]

you did attatch the piston to the conrod with the gudgeon pin right?
Did you replace the old gaskets with new ones?

[RMrobbo]

you did attatch the piston to the conrod with the gudgeon pin right?
Did you replace the old gaskets with new ones?

sorry didnt realise there was more than 1 page to this discussion and that you already had it sorted.
I once rebuilt my 91 kx with a wisco piston and it ran sweet for ages. Until some prick stole it.