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Engine rebuild? What to buy? Need help

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I have a 2013 kx250f and Its right about 40 hours from last rebuild and I'm riding fast and it's tarting to lose a little bit of power and I'm sure it's about to rebuild befor any damage can come...not sure what to buy? New piston and rings? Valves? Cams? What should I buy for a good engine rebuild that will last another good 40 to 50 hours hopefully!

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Woah woah slow down pal. I'm no motor expert but the only thing I could say is that you'd want your valves checked upon rebuild. And depending what you see and the condition of the valves base your decision off that. Cams. I dont think you should have to worry about them except for the timing chain. Which is usually checked when valves are checked. Otherwise new gaskets. A piston. But bearings can slow you down too. So I'd recommend checking those out. You should be able to get 75-100 hours but you said your riding fast. Which is another term for aggressive. So 50 hours would be my max for a hard ridden motor.

Someone on this forum who knows the whole system of your motor will likely respond. Just give it a few hours .

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I have a 2013 kx250f and Its right about 40 hours from last rebuild and I'm riding fast and it's tarting to lose a little bit of power and I'm sure it's about to rebuild befor any damage can come...not sure what to buy? New piston and rings? Valves? Cams? What should I buy for a good engine rebuild that will last another good 40 to 50 hours hopefully!

I'm in the same situation except I have a 04 yz 250 2T.  don't know what to do.  My top end is toast after 8 tanks  I believe it was toast before, apparently my local shops cant measure. now I think im going to do a full rebuild but not sure I trust aftermarket cranks, so guess im going to have the crank rebuilt. IDK..................

 

 IMO the best thing you could do is buy the tools, gauges, pullers,  etc. to inspect every thing part by part... start with the top end.

valve clearance adjustment really helped my 660, it was hard to start cold and lacked some power. the intake valves closed up. but I don't know much about 250/450 race bikes. I'm sure others have better answers tho good luck.

Edited by yz250474

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Just a FYI, i bought a 2004 KX250F about 3 months ago, Now 12 years old it had over 400hrs on the piston and crank.. :jawdrop:  the valves were done once at 221 hrs.... he ended up blowing the crank bearing at about 425 hours! Those are insane numbers for a first generation 4 stroke. and it wasn't a Sunday rider either, its was rode hard and stored wet.. :lol:  for the rebuild you never really know what needs replacing until you get right into it. but for a reference, My 04 it will cost about $900 top to bottom in parts. and thats with a 269-270cc Athena big bore kit installed (cylinder, piston and rings). all new crankshaft w/rod and bearings complete. and new stainless valves for longevity. :applause:  

 

Don't think these bikes will only go 40-50 hrs on OEM parts. keep up on oil changes and remember to oil your air filter as well, if you have a 2011 or newer, two big ones people forget about... look at your inline fuel filter if its not clear white, REPLACE IMMEDIATELY! as this will restrict the fuel going into the injector causing a loss in power (Replace every 30 hours Afterwords) and clean and inspect injector/injectors. frequent valve checks every 25-30 hrs and she will be good for years to come :ride:

 

I have a 2013 kx250f and Its right about 40 hours from last rebuild and I'm riding fast and it's tarting to lose a little bit of power and I'm sure it's about to rebuild befor any damage can come...not sure what to buy? New piston and rings? Valves? Cams? What should I buy for a good engine rebuild that will last another good 40 to 50 hours hopefully!

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My 13 will get the same treatment this winter as well... how do you know if the timing chain needs replaced? Mine looks completely fine.. or should you just replace it just to prevent having to do it later? Xmas put a hurt on the wallet so I'm reconsidering how much I can put into her right now. I'm thinking new ss intake valves, (exhaust are still well within spec but 1 intake has closed up), new piston/rings... also going to rebuild the forks

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My 13 will get the same treatment this winter as well... how do you know if the timing chain needs replaced? Mine looks completely fine.. or should you just replace it just to prevent having to do it later? Xmas put a hurt on the wallet so I'm reconsidering how much I can put into her right now. I'm thinking new ss intake valves, (exhaust are still well within spec but 1 intake has closed up), new piston/rings... also going to rebuild the forks

 

Well the timing chain is fairly easy.. with the chain tension-er in you take the valve cover off, look where the cam chain goes over the cam sprockets. if you can pull the chain up off the sprocket and it has play on the teeth it needs replacing.. a good snug chain wont lift up between the teeth on the cam sprocket. :thumbsup: 

 

 How many hours on the bike? You should be able to just re-shim your valves back in spec, that intake that has closed up wont be an issue, The springs that hold the valves are really tough.. after a few hours they loosen up and relax. they should find a seating position in there quite quickly. RE-shim that intake back into spec and ride it. :ride: A good rule of thumb on valves... :prof:  if you have to re-shim that's fine, you can go all the way down to a 1.20 shim before they will need re-placing.. but shim it more than 3 times within 10-15 hours they are sinking into the head and needs re-placing. 

 

A factory piston will last somewhere around 200 hours with average use.. :shocked:  By all rights, It should be smoking like Chernobyle before a piston ever goes :lol: the rings will be shot. Or they get installed incorrectly! (Most people do NOT know how to clock the rings properly) unless you live on the rev limiter you will get much more than 50 hours on a factory piston.. Wiseco is rumoring 400+ hrs on there pistons 

 

 Forks are easy.. take them off, unscrew the caps pull the spring/springs dump the oil and replace.. There is alot on here about setting up forks just give a search..

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