Troubleshooting 2nd seizure on KX 125

[dale.tho]

Hi guys, hoping to get some valuable advise here, my bike has seized twice recently and I?m trying to troubleshoot why.

The bike is a 2004 Kawasaki KX 125.

The first time it seized we had stopped for a break, when I started the bike it took a few extra kicks, made it around 25 meters of slow riding and it seized. The bike had done 33hr's since the original owner had done both the top and bottom end. The bike hadn?t been ridden hard but at the time the compression was getting low need a top end refresh.

I figured I'd do the top end rebuild myself because I have a semi decent amount of mechanical experience and enjoy working working on the bike, It?s like an adult version of legos

When I took apart the bike I found it hard to determine the cause because there were a few possibilities:

1) A screw had worked lose in the reed valve assembly, so chips of the carbon fiber petal tip had broken off and made its way through the barrel.
2) The piston head had a few pin holes which could have been made by sand, I do clean my filter after each ride and it was in good condition but you never know what might sneak through.
3) The compression was low so maybe the ring had worn too far and collapsed.
4) The bolts that hold the exhaust power valve assembly were lose and the valves and some play, I don?t see how this would have caused the issue but I feel it?s worth mentioning.

So I fixed the issues above, had the barrel re-plated (nikasil), and put in a new OEM top end kit, checked all clearance?s and ran the engine in by the book.

The bike was running 100% and never felt better, had about 1.5 hrs breaking the engine in and another hr riding. Then a bit of a disaster to cause the 2nd seizer or add some confusion to why the bike seized the first time. The road I needed to get through was flooded and deeper than I expected, ended up dropping into a 4x4 rut and water got sucked through the intake.

After that I drained the oil (which had about 25% water!) and remaining fuel in the carb, then left the engine open for a month to dry. Once it was all back together starting the bike was a mission, took about 30 minutes and I eventually got it started by taking out the plug and heating it up.

While warming it up I noticed that if I rev it up, while the revs would drop at a certain point I would feel a nock with my hand on the seat! I had never noticed this before and have a feeling this could be the cause of the 1st seizer and the next one to come, I rode the bike for a bit but it was occasionally bogging down with the throttle open. I figured I did a bad job at draining out the carbs the first time (instead of opening them to drain them I turned them upside down and drained what was in them till I thought they were empty) so I opened it up properly and cleaned it out, there was a small milky clump under the main jet.

Bike started better the next day and rode it for 5 mins to test, all seemed ok with no bogging but while I was idling to come to a stop it seized for the 2nd time.

When I compare the piston damage from both seizures they are almost identical so now im wondering if there is a common problem that caused both seizures. Flooding the engine before the 2nd seizure definitely adds some confusion because that could have also caused the 2nd seizure.

I know the obvious answer is to take the bike to a professional, and I might still do that. But first I want to see if there's anything obvious that I could identify and fix, I?m already knee deep in it and I like working on the bike as a hobby.

If anyone has any advice or can help me out with the following questions I'd be very grateful:

1) The pics of both pistons are abelow (1st seize on the left of the images), they've both broken by the exhaust port and in the identical place. Any ideas on what could have caused this type of damage? From my research online it looks like a possible coolant leak, but I haven?t lost any noticeable coolant and did use a new gasket & o-rings on the rebuild. I guess the 2nd seizure could be from water left in the fuel anyway.
2) The conrod has a lot of play left to right (but not up and down) on the main bearing. I?m not sure what the usual tolerances are here, could this be a problem?
3) If that "thump" while the revs drop is worrying is there anything I can test without opening the bottom end, if it comes down to that I might just send in the bottom to a professional.
4) Any idea other theories or things I can check will help point me in the right direction?







Thanks for taking the time to read through the post and provide any help.

[cnrcpla]

The first seizure looks to have been the exhaust valve opening to far into the cylinder and coming into contact with the piston (based on the large scrape down what I'm assuming is the exhaust side of the piston). That would have probably caused the first seizure because the shards of metal from the chipping piston would have found their way into the rest of the engine. The second seizure looks to be more catastrophic. Between rebuilds, you mentioned a reed cage screw had made its way into the engine? Was it ever located? It could have been sitting in the bottom end out of sight. Assuming you haven't already, take a flash light and look into the bottom end to see if there is any metal debris from the failure (there probably is, look at that chunk missing  ) If you see any which you probably will, it would be a good idea to split the cases. I know that's not what you want to hear, but if that metal finds its way into the main bearings like it may have already for the 2nd seizure, than that can cause another seizure, and another, ect... Welcome to the forum  If I didn't cover something up there than I'm sure another member will nail it. Good luck man, seizures suck.

[dale.tho]

Thanks for the reply cnrcpla, thats correct its the exhaust side, I did check the power valves when they were still lose but they dont seem to be able to reach into the barrel, they just stop against the outside of the barrel.

The screw had come lose before the 1st seize but it never made its way into the engine, it just allowed the petals to move out of place and break the tips off. I cant see any debris looking into the bottom end, but your right there will definetly be something there so I'll make sure its opened and checked, dont want to risk another seize

[cnrcpla]

Yeah that's the only way to make sure and be certain. And while its open check the main bearings and crank and all that, if it needs to be rebuilt than you may as well do it while its open.

And if it wasn't the power valve that caught the piston, it was the port itself. Next rebuild for the top make double sure the piston is the correct size for the bore by measuring it yourself with a pair of calipers. Hopefully the bill isn't to expensive for you, when I replated my engine for complete top end failure it costed about $300, when a $30 water pump gear crapped out  But I would rather spend $300 for a two stroke than about 1k on a four stroke 

[arnego2]

In the whole thread I could not find a premix ratio number. IMO after the first seizure I'd have done bottom end regardless of how good or new the bearings and seals were. With a lost screw floating around there I would not take the risk. A 125 ridden normally should get a new piston at intervals closer to 15 hours than to 30. A good premix ratio is 30:1 a better one is 25:1 specially when running in sand, dessert or open roads. 

A rebuild has a lot of specific specs you have to take into consideration. Piston clearance and the clearance of the valves in regards how deep they wander into the cylinder. Those 125 are race engine and one should treat them as such

[2STROKEREVOLUTION]

It is certainly worth getting the bottom end checked and cleaned now. That looks worse than just running low on oil or running too hard on a cold engine.

What does the exhaust port look like on the inside?

When you get re-plated the shop should supply the piston and they will match the clearances.

Was the piston installed the right direction with the ring in the right spot? That can cause the ring to catch the ports. Just another idea of the cause, throwing out possibilities.

How do you ride? Trails, mx? Do you abuse the bike? You shouldn't notice much loss at 33hrs and if not abused you can easily get over 100hrs without a rebuild. If anything, just new rings to keep power from dropping off a little.

[arnego2]

over 100 hours on a 125 2 smoker piston? I find that hard to believe.

[dale.tho]

@cnrcpla, sucks about that water pump! Agreed though 2 stroke is the way to go, simpler engine and loads of fun to ride

@arnego2, im using a premix of 40:1 with a fully synthetic oil, the manual suggests 32:1 but I ended up increasing the ratio because I was fouling plugs. A few experienced riders told me they run the same so I didnt think it would be a problem, maybe its time to get back to 32:1 and change the jetting.

@2strokerrevolution, the exhaust port looks good on the inside, it just has 2 indentations where the valve stops against the barrel, I have fiddled with the valves and there is no way they are able to reach into the barrel.
Unfortunately we only have 1 engineering shop here that can do the nikasil plating and they dont provide any parts, but I did measure the diameter which was right for the stock size.
Good question on the piston direction there might be something to that, on both seizures the piston is in the correct direction but for some reason on the original build the ring gap is facing the exhaust port. I know from the OEM piston I put in that the gap should be facing the intake because it has a ridge so you cant get it wrong. So this could be a reason for the 1st seizure, as the ring wore down it got caught in the exhaust port.

I ride trails and dont give the bike much abuse, I've heard of people getting 100hrs out of their engine but im not that brave lol, seizures are an expensive risk

Just to clear the screw thing up, it is not missing in the engine, it came lose from the reed valve assembly but was not able to move enough to escape into the engine.

[2T Institute]

That piston has got mighty hot and I think cnrcpla is correct about the PV hitting the piston. Has been installed correctly. First thing to check would be the piston to bore clearence. should be 0.05mm

[gpnewhouse7]

over 100 hours on a 125 2 smoker piston? I find that hard to believe.

My first 125 did a whole year on a single topend without any faults at all and that was while averaging atleast an hour a day and had water coming over the front mudgaurd on more than one occasion. Modern bikes can take some serious abuse before they wear out.

I wouldn't try doing that again though.

[arnego2]

im using a premix of 40:1 with a fully synthetic oil, the manual suggests 32:1 but I ended up increasing the ratio because I was fouling plugs. A few experienced riders told me they run the same so I didnt think it would be a problem, maybe its time to get back to 32:1 and change the jetting.

So far I know the premix has not that much to do with fouling plugs but with the an engine not hot enough when ridden.  Oil premix ratio has a lot to do with the rider style of each of us. The harder you ride the more premix you should use. Side effect would be a better ring seal with a higher premix ratio. When I started on my 2 smokers I ran 50:1 and had some problems with lean seizure along the way I went to 40:1 then 30:1 and now I'm at 22 to 25:1 no spooge no fooling plugs but you need to ride the bike hard to get that.

I sold my trusted RM125 to a kid, jetted for me, premix oil ratio at 40:1. Long story short got the bike back a week later spooge ouzing out of almost every possible opening. Went from needle in 4rth to Needle in 1rst and lowered the main from 180 to 165. That suited his riding style way better. 

You will hardly find anyone riding the way you are except you are in the pro class level So each rider takes care of his/her bike in a different way. Some people change pistons at 50 others @ 100 and I changed the piston on my RM 125 @ 10 to 15 hours. Now I re-plate @ 150 hours per cylinder, do a bottom end @ 150 to 180 hours and won't re-plate a cylinder more than 3 times.

[dale.tho]

Hi Guys, had a better look at the power valves last night and your right, they are just managing to reach into the bore because the tips have worn down. So thats the mystery solved, feel a lot more confident now to rebuild the top end. Thanks for all the help.

@arnego2, interesting post on the remix ratios, I'll stick to 40:1 for a bit but will decrease the ratio as I start getting more out of the bike.