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Discussion Starter #1
2004 KTM 125 EXC import?

Ok so I am looking at buying a bike and its the 125 exc model. I guess it was imported into the US because I can't find much info on it. When I run the VIN I get some sites telling me its a 2004 KTM 200EXC and some say its the 125. Really weird. Plus when I got on Rockymountainatvmc.com, which is where I get most of my parts from, they don't have the bike as an option, only the 200exc. Does anyone have some info on this bike? I cannot find as much as a wiki page on it. Curious what countries it was sold in.
 

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Dirt Wizard
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Can you give me the frame number or at least the first 10 digits?
 

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Discussion Starter #4
Went ahead and bought the bike. Looks like it had been sitting for a month but still rode well. Power valve needs adjusting and a carb clean couldn’t hurt.

Major reason I got this bike is because it’s street legal. Supermoto wheels would be sick but they so damn expensive
 

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2004 KTM 125 sx
I just bought this bike yesterday and the first time I run it under my ownership it dies. I went to get gas (93 from racetrak) 1 gallon of gas and 3 oz Lucas oil. That should be around a 45:1 mixture (within the manuals 40:1 - 60:1 recommendation). I rode to the pump so I put in the gas first then the oil then sloshed the bike around for a minute, which a couple different people I watch on YouTube do it and don’t have issue. Literally 2 minutes later riding down the road and the engine makes this awful sound then dies. I pulled over on the side of the road, where I sit now, and the kicker won’t turn at all.

Not sure where to go from here. Just spent all my money on this bike and now I’ve gotta year it appart and spend more money on it. If anyone knows of a good write up to help out with this process please let me know. My day just went from really good to really bad
 

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Hard luck or no luck?

Sorry for your troubles @Freggles that’s a tough break. I have seen this exact problem so many times and have had it happen to me as well. A lot of old school 2 stroke guys would scold you for the hillbilly method of fuel mixing but I would not be quick to blame your mixing method. I have seen that method used without any troubles you have here. I sounds like your motor had a hard failure and once you’re done being pissed off there’s nothing left but to see what broke in your motor and fix it. Once you have it repaired be sure to drain fuel tank just in case your previous fuel was suspect or possibly reacted(gelled) with new fuel. Some of the old school castor oils will gell when mixed with new tech oils. Better to drain fuel system and use drained fuel as weed killer.
 

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Discussion Starter #8
Thanks for the quick reply. I don’t think it was the method of fueling I used either but your gelling theory sounds like it could be right. Bike was running like sh!t right before so I think something clogged one or more of the jets.

I got the cylinder head off, which is a b!tch and a half to remove because of the weird sized bolts. Oddly enough they weren’t metric even though it’s was designed or assembled in the USA, but they were just barely too big for the 3/8 wrench. Still used it though, just needed a bit of persuasion. Now I can’t figure out the size of the cylinder bolts. If anyone knows that would be awesome.

Found out what needs to be fixed though. Looks like my piston melted because it’s got some edge bits missing. Found those stuck to the bottom of my head. So that’s just a new piston and rings right? How do I find out if it needs anything else? And how do I get out the little crumbly bits of piston that I’m sure fell down into my engine?

I really hope this isn’t too much of a headache to fix. Any advice on good budget rebuild parts (if they exist) would be golden as well as any other tips for a first rebuild. I’m very mechanically inclined (right word?) so it shouldn’t be too difficult if I can get the right size wrenches.
 

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off to a good start

well good for you for getting right after the problem. you may find your KTM has a few bastard size fasteners. those may use some oddball 7mm fasteners, but im not certain. just make sure your tools fit the fastener. it sounds like the top ring may have broken on piston as those pieces sticking into cylinder head are usually shards of hardened rings. i thought the cylinder nuts were 8mm stud nuts but anything is possible by now with a machine that age. 8mm should require a 13 or 12mm wrench for removal, but find something to fit fastener as you would not be the first to find non standard hardware on an older machine. take your time with disassembly and take lots of pictures for your reassembly reference. be sure to drain coolant first:surprise:nothing worse than working in a puddle of radiator piss:frown2: you may find some xc200 stuff may interchange on to your motor, but lets take one problem at a time. lets have a look at the damage and form a plan to fix er up. be sure to post up detailed pictures for forum study from some of our other forum mates here. best of luck my friend.
 

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Discussion Starter #11
Almost took the head off without draining the coolant lol that would’ve been fun. Meant to post pictures with the last post so here they are. Lmk what you think.


Also the cylinder only has 15 hours on it since replaced according to last owner. Does this limit my options for the new piston? I heard that makes Wiseco not an option. I don’t have one of those expensive bore measuring tools so best I can do is whatever result a tape measure can give me. Hope that’s enough.
 

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Identification marks?

Almost took the head off without draining the coolant lol that would’ve been fun. Meant to post pictures with the last post so here they are. Lmk what you think.


Also the cylinder only has 15 hours on it since replaced according to last owner. Does this limit my options for the new piston? I heard that makes Wiseco not an option. I don’t have one of those expensive bore measuring tools so best I can do is whatever result a tape measure can give me. Hope that’s enough.
Are there any letters or numbers stamped on the top of the piston?
The reason that I ask is that is how I was able to identify the piston in my '93 KDX and with that positively identify the size.


Rangerman:ricky:
 

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Discussion Starter #13
I don’t see any on the top of the piston but once I get the cylinder off maybe there will be some on the skirt
 

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seize the day....not your motor

looks like she cooked the exhaust side. your cylinder stud nuts look like 14 mm wrench should work. yeah. i warned you about baptism in coolant because i could imagine that in your haste you could easily forget. we all do it:surprise:
not to worry. hopefully it tied up before it scored the cylinder. i would expect the exhaust (front) part of the cylinder would be where you can expect to see the worst of it. be sure to clean it up and take detailed pictures of inside of cylinder. keep at it:grin2:
 

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Discussion Starter #15
*clever title*

The exhaust side definitely took the brunt of the damage, as expected, but it looks like the cylinder is ok. I've gotta split the cases to get all those crumbly bits out anyway so I'll post pics of everything when I get to it. Luckily I've got 2 exams on Friday and work all weekend so that'll give me time to wait on parts. Could also probably use a carb cleaning.

In the mean time I'm searching for a top end kit with everything I need. There are plenty of them out there but I think most of them are for cylinders that have 100+ hours on them. Mine only has 15. does anyone know if this will affect which piston I need? I guess ill call a couple manufacturers tomorrow in my free time and get their $0.02 as well.
 

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Dirt Wizard
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Judging by your vin, my info tells me its a 2004, 125sx.
Are you sure it's compliant? Does it have a plate or compliance sticker?
 

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Discussion Starter #17
yeah the bike is now plated in my name but in actuality its a 125sx, not exc.

I figure i should probably split the cases and clean out the crankcase because bits of melted piston/rings could've fallen down in there. while I'm in I may as well change crank bearings and big end conrod bearing too yeah? what tools do i need? will the old blow torch and hammer technique work to get the crank bearings out/in?
 

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Discussion Starter #18
Still can’t get the cylinder nuts off. For imperial a 1/2 inch is too big but a 7/16 is too small. Metric - 11 is too small and a 12 is just large enough to strip the nuts. Does an 11.5 exist? Or a 15/32?
 

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Freaky nut size

Sounds like an oddball part. Some of the industrial hardware manufacturers use what’s called a heavy nut which uses an oversize head for more surface area. They always require 2?size larger wrench. Good mechanics hate them. You may have a metric equivalent of the heavy nut on your motor. 13/32 is the only fractional size between 10 and 11 mm. 11mm is only slightly smaller than 7/16 and usually substitutes easily with 7/16. I never heard of a 13/32 wrench. But I think you may be able to bang a tight fitting 7/16 into it if done carefully. You want to try to avoid destroying the nut if possible
 

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The only way to know what bits (and how old) there are inside a motor is to fit them your self.

The cylinder has a Nikasil coating. Once you have damage to the bore you end up either fitting a new cylinder or re-coating. The 125 bikes use three size pistons: 53.94mm, 53.95mm and 53.96mm. Your old piston will have that on the top (carefully clean off the top of the piston and you will find the info there). A rule of thumb is that if motor has done less than 100hours use the smallest, 100 - 200h use middle size and 200 up use biggest one. BUT we did just that and as motor had more than 100 hours we went with mddle size... only to find out that when all got nice and warm the ring (single ring piston for less resistance) got too big and the ends touched making the ring push hard against the bore. The small area between two ports made an indent in the ring and compression went. You say the cylinder is 15 hours old, if that is correct you should use the .94 piston.

In cases when I put together a motor I don't know I use the smallest size piston as that guarantees that the piston and ring will fit even when hot. The loss of compression as result is very minimal and an average rider will never notice. There is also a little more noise from a small piston in a worn bore... but try to hear that when revving these bikes...

The cylinder, cylinder head and piston might seem simple and so a piston is put in after some damage, bike starts and off you go... only to find that same thing happens again. There are a bunch of things that can affect the life of your piston, and they all need to be considered!

- The squish (this is adjusted by fitting right thickness of cylinder base gaskets). As the KTM's have evolved the squish has changed a fraction, so I recommend you use what is recommended for your motor. (This is obviously based on that the rest of the bits are KTM bits in the motor...). If the head has had work the squish will be changed.
- Spark plug. Recommend you use what the manual stipulates!
- CDI. Make sure the CDI is the correct one for your bike! If you use the wrong one the spark will not come at right time and in worst scenarion this will melt the top of the piston (we found that out early...). Sadly the label KTM fits on the CDI's loose it's print and you have no idea what model it is!
- On your model you can adjust the timing manually by moving the timing sensor by the rotor. If you are not sure what you are doing please set it to recommendation!
- The reed valve needs to work well as these engines rev to more than 10,000 rpm. (That is 170 times the piston will go up and down in a second...) The fuel actually has more than one job to do. It also cools down the internals in the cylinder!
- Check what jets are fitted and where the needle is positioned (the clip on the needle). Again, the fuel does more than creates an explosion! Set carbi to factory recommendations. Then if you want to try some changes you know where you are!
- Crank seals. Non-sealing crank seals will give you all sorts of issues. New ones is a cheap insurance policy!
- Also always fit new head gaskets (these are two O-rings) and torque down the head to right setting!!
- If you ride in hot conditions make sure you have good quality anti freeze/coolant in your radiator! Also make sure your radiator cap is the right one that releases at the right pressure!

And this is all before you even know what modifications the past owners have done to the bike to "make it faster"... When I pick up an old/used bike and there is a big ? re what has been done to the bike I revert everything to factory setting. If not, and there is an issue, you end up chasing your tail!

But on the good side, you have come to the right place for advice as there is a good bunch of guys here who has worked with these rocket ships! Once you have the 125sx running well make sure you keep on top on things:
- Replace rings at 20-30h (serious racers replace piston at 15h... but for a "sunday" rider that is not needed.)
- Replace piston at 40-60h.
- Bottom end rebuild at 120h (or earlier if you ride it like you stole it!) Do it before you have issues!
- Every time you take cylinder off ro replace piston/rings clean the power valve!! So many issues with "runs like crap" is from dirty power valve!
- We run aviaton fuel. But 98 octane is OK. Just make sure that you do not use fuel that has ethanol mixed in!!
- We use fully synthetic oil at 40:1. Leaves the plug a fraction black, but it is a good insurance policy...

Good Luck! Come back and tell us how all went!
 
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