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Do I need to rebuild??

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Hi I'm new here and bought a 2000 125 kx and the guy I bought it from had it at the dunes and the engine seized up. He got the engine unstuck and sold it to me and said it didn't run so I got it home and kicked it for about 2 hours. Then found the choke and it started right up.when it running its has a fast paced rattleing sound dose this mean I need the top end done???

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Well, if it even siezed at all, you need to get it apart and start inspecting. Being that old and not really knowing the history of the bike, you probably want to rebuild the entire thing. Just to be safe.

It will most likely need a replating or possibly repair, a crank kit and maybe some internal bearings. Unless you're planning on letting this thing nickel & dime you, do it right the first time.

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Well, if it even siezed at all, you need to get it apart and start inspecting. Being that old and not really knowing the history of the bike, you probably want to rebuild the entire thing. Just to be safe.

It will most likely need a replating or possibly repair, a crank kit and maybe some internal bearings. Unless you're planning on letting this thing nickel & dime you, do it right the first time.

lol ... sell it ....

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From my experienced abused bikes will kill your wallet. It starts out innocent enough. You tell yourself, "I got such a good deal on it, I don't mind putting a little money into it". By the time you're done you'll be hiding that third mortgage from your wife and trolling the forums late at night looking for that "one last add on" that will get your bike up to par, lol.

If I had it to do over again,I would just suck it up and concede to financing a new bike, since it seems to be an inherent fact of buying a used bike that either people have no idea what they're talking about or they just plain lie :-/

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Exactly!!! I am in that boat floating along right now.

We picked up an 03 kx125 for my son about 6 weeks ago. He has yet to ride it other than around the yard testing it. Granted it is still all stock and had low hours. But I'm going to start a thread on it now. So you can read about the troubles there.

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Im kicking my self in the a$$ cus I had a 2005 completly stock and not a scratch on it For the same price, and sold it cus ther was no paper work. (stupid) but oh well

and I got the 2000 from my boss! So I can't comlain lol, it was rebuilt in 2004(if it maters)

and compared to 4 strokes how hard-easy are they to do work on?

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I have a similar situation with a 2005 KX100. Got a good deal on it a few months back, and bike seems like it really had very little use - all stock, still had original tires. I serviced everything up, and it has run really well for my son. Until it locked up yesterday. Warmed up but not overheated, going down the trail and suddenly it locked up to the point that it was dragging the rear wheel. He kicked it over and it started right up, but now sounds really strange - kind of loose and rattling. He said it did the same thing just a few minutes before. So we shut it down to prevent urther damage and brought it home. The wierd part is that it seems to turn over nice and easy still, and fires right up. I tore down the top end tonight, and everything looks great - piston, rings, and cylinder all in good shape. I drained the oil, and no debris there that I could see. What could bind up bad enough to drag the tire and yet still let you kick it right over and fire up again? Thoughts? Also, where can I get a good service manual for this thing? Thanks for any help!

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I have never rebuilt a 4 stroke but i can tell you that rebuilding 2s is simple if you the right tools. If i have the parts ready i can do it about 2 days.

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From my experienced abused bikes will kill your wallet. It starts out innocent enough. You tell yourself, "I got such a good deal on it, I don't mind putting a little money into it". By the time you're done you'll be hiding that third mortgage from your wife and trolling the forums late at night looking for that "one last add on" that will get your bike up to par, lol.

If I had it to do over again,I would just suck it up and concede to financing a new bike, since it seems to be an inherent fact of buying a used bike that either people have no idea what they're talking about or they just plain lie :-/

I agree with this guy. Old secondhand bikes are a money pit at the best of times, but when your buying a non runner/poor running bike its almost a certainty. If your inclined to replace all and everything it takes to get it perfect then you'll have an empty wallet. If your not then you'll have a half empty wallet and a bike your unhappy with. Just my opinion, I've been there with a 97kx250 that 'just needed the jetting set up'!! I've had friends who wanted to follow me into this sport but have ended up buying budget bikes, against my advice. They end up spending more time repairing them than riding them, and consequently get fed up.

I buy new or nearly new, that way the bike always reaches my expectations. My 04kx250 that I bought 3 years ago was a good example. The kid who had it from new was scared to death of it really, the suspension felt box fresh, the tyres still had the mould knobbles on them, the exhaust and engine cases were unmarked, the chain, sprocket and handle bar grips were all original, but the bike was half the price of new. Thats the kind of bike that you want to find.

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I have a similar situation with a 2005 KX100. Got a good deal on it a few months back, and bike seems like it really had very little use - all stock, still had original tires. I serviced everything up, and it has run really well for my son. Until it locked up yesterday. Warmed up but not overheated, going down the trail and suddenly it locked up to the point that it was dragging the rear wheel. He kicked it over and it started right up, but now sounds really strange - kind of loose and rattling. He said it did the same thing just a few minutes before. So we shut it down to prevent urther damage and brought it home. The wierd part is that it seems to turn over nice and easy still, and fires right up. I tore down the top end tonight, and everything looks great - piston, rings, and cylinder all in good shape. I drained the oil, and no debris there that I could see. What could bind up bad enough to drag the tire and yet still let you kick it right over and fire up again? Thoughts? Also, where can I get a good service manual for this thing? Thanks for any help!
Most likely the lower rod bearing is bad. Very common on the 85/100 for riders that run lean oil ratios. That engine likes lots of oil.

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What is the typical failure in that bearing? Broken needle, flat spot? I have a hard time imagining how it could jam up tight then be all nice & loose again. FWIW, the rod does not seem discolored toward the base (ala heat), but there does seem to be a lot of play. If it is a crank (main) bearing, the bottom line would still be the same - new crank assembly and bearings? If I have to split the case anyway, is it worth replacing any other "notorious" parts while I'm there? Thanks for all the help! (and sorry for tracking off the original 125 thread, but it seemed relevant) - Jack

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What is the typical failure in that bearing? Broken needle, flat spot? I have a hard time imagining how it could jam up tight then be all nice & loose again. FWIW, the rod does not seem discolored toward the base (ala heat), but there does seem to be a lot of play. If it is a crank (main) bearing, the bottom line would still be the same - new crank assembly and bearings? If I have to split the case anyway, is it worth replacing any other "notorious" parts while I'm there? Thanks for all the help! (and sorry for tracking off the original 125 thread, but it seemed relevant) - Jack
Typically, when a lower rod bearing fails the cage breaks and the needles start to come out. It's not unusual for an engine with a failing lower rod bearing to eat a needle and trash the piston, head and cylinder.

The tight-then-free spinning means one or more of the needles is missing or damaged. When they become damaged, they will roll freely for part of their circumference, then reach the damaged spot and "skid", creating the tight feeling.

Does the rod move freely? If you stand it straight up, does it fall right over or continue to stand? If you grasp the rod and spin the crank with it, does it feel smooth or rough?

Since the cases have to be split any way, replacing the mains and crank seals is a no-brainer. I would also go ahead and replace all seals, and inspect the tranny shafts, bearings, shift forks, and gear teeth and dogs. Replace anything that looks worn, and you won't have to go back inside for a good long time.

While it's apart, be sure to disassemble and thoroughly clean the power valve assembly and valve pockets. If the power valve becomes too covered with carbon the valves will seize, causing damage to the teeth.

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Hey Chokey, Thanks much for the help. The rod feels smooth throughout the cycle, but it also seems to have a lot of play. I'll try to get some measurements tonight. Y'know, I grew up working on cars & truck and have always done my own work, including engines, trans, diffs, etc., but these bikes are new to me so I just don't have a sense yet of what is normal - everything is so small and light! So I ordered a service manual and a few needed tools today and hopefully I'll know better where I stand by this weekend. I think the general advice of doing it all while I'm in there makes a lot of sense.

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Hey Chokey, Thanks much for the help. The rod feels smooth throughout the cycle, but it also seems to have a lot of play. I'll try to get some measurements tonight. Y'know, I grew up working on cars & truck and have always done my own work, including engines, trans, diffs, etc., but these bikes are new to me so I just don't have a sense yet of what is normal - everything is so small and light! So I ordered a service manual and a few needed tools today and hopefully I'll know better where I stand by this weekend. I think the general advice of doing it all while I'm in there makes a lot of sense.
That's the best money you will ever spend on the bike. :banana:

These engines are actually very easy to work on, two-strokes are the poster child of simplicity. Read the manual, keep everything organized, and you'll find it to be a piece of cake. Just remember, it's not a car with a cast iron block. Things are light and fragile, and torque values are very low, often in inch pounds instead of foot pounds.

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Hi I'm new here and bought a 2000 125 kx and the guy I bought it from had it at the dunes and the engine seized up. He got the engine unstuck and sold it to me and said it didn't run so I got it home and kicked it for about 2 hours. Then found the choke and it started right up.when it running its has a fast paced rattleing sound dose this mean I need the top end done???

I suggest that if you want to keep this bike, you should invest in a service manual ($36 MSRP) and start learning about it. I also suggest that you take the top of the engine off and inspect everything. If you don't know what to llok for, take it apart and pay someone who knows what they are doing. If you are not comfortable working on the bike, then you have just picked up an expensive hobby.

This is just a general statement, not directed toward you. :banana: Week after week, month after month, year after year, someone buys a bike for not too much money, then after not too much time, figures out that there is not too much that is correct with the bike. In general, people don't care that they have a high-performance and high-maintenance motorcycle. They neglect doing what needs to be done to keep it in tip-top shape. Buying a newer model means buying a few years of neglect instead of a decade or two worth of neglect. Again, just a general statemnet, as this thread covers a few people's bikes and stories us "regulars" see all the time. :lol:

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