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My kx250f $ Bucket!

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My 2005 KX250f is costing me a lot of money lately! Like everyone else im having top end problems.

OK heres the deal, the motor dropped a valve during a hard session, this event proving to be catastrophic to the motor (bent crank and rod, bad piston, hole in the head and bent all the valves).... All new parts were bought except the head, the head was repaired and milled; .004 taken off **** (so I was told)... Now to the point!!! This weekend the motor went down again, after about 4 hours of ride time on it!. The motor didn't break a valve off but there was a loud tapping sound coming from it. Taking apart the top end once again I found the exhaust cam looking like it was starting to seize up and lovely intake valve imprints in the top of the piston. What the F! am I doing wrong here?

My engine guy said he adjusted the valves at:.007 and .005, My question to everyone is if there was .004 taken off the head should that be made up somewhere? Also can a head even be milled and still work correctly! This is costing me way to much time and way too much money, boohoo right? Let me know what im doing wrong guys!

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For starters I hope you have your numbers wrong. Those numbers don't fit anything for a KXF, whether mm or inches. If your numbers are correct I think we have found your problem.

Assuming you have confused the numbers and that the engine was correctly put back together you must be riding in the rev limiter one heck of a lot. Also would take a look at the amount and type of oil you are using. Valves need a lot of oil and (especially if you are going to run them hard) a very good quality oil.

Formula for longevity on a KXF is very simple. Change the oil every ride, clean the air filter every ride, change/clean the oil filter every ride. Stay out of the rev limiter. Enjoy yourself. I've got three with bookoo hours and have had NO valve problems.

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.04mm off the head? or .004 thousandths of an inch off?

Anytime you change that valve to piston relationship you MUST check the clearance. That said theres a few other things that could have happened.

Cam timing could be a tooth off. Or the cam sprocket could have spun on the cam. Ive also seen them jump time on the Kawis for no apparent reason.......be sure you have the updated guide down by the crankshaft.

If clearance is an issue, most aftermarket pistons offer deeper valve pockets than stock.

Did it make the noise when 1st put together? if not id seriously look at cam timing....also be sure valves and valve springs are NEW along with retainers and keepers.

Steve

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The motor ran great with no noises for about 3 rides. But the three rides were in trails and the motor messed up when I brought it to the track and rode it harder. I was told .004 thousands was taken off the head but can tell for sure because I don’t have a new one to compare it too. I don’t do any of the engine work but that’s not going to change b/c I’m tired of trusting people. It's weird b.c this has been a continuing problem, this has happened 2 other times since the original melt down. Thank you for the reply; if you have anything else to add please do>.

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I had this problem with once on the same bike after it blew up......It just kept jumping time over and over again....sometimes after 10 min and sometimes after several rides. We replaced the crank, cam chain and tensioner all in an effort to cure the problem, to no avail. I finally made a manual tensioner and set the tension on the cam chain as I do my drag motors.......6 months without a problem so far.

When the motor blew up last time did you replace the chain? the crank is new so you know its running true......be sure you have a new chain and tensioner....they can be damaged in a way you cant see when you drop a valve......other than that you can box it up and send it to me and Ill have a look......but after past experiences Im not 100% ill find anything wrong.

.004 off the head should not be an issue as far as clearance is concerned

Steve

Cycle Dynamics

623-581-2900

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What are the updates for the 04?

I've read here about the cam sprockets said to be sliping on the cams!

Thanks

Doug

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no updates for the 04 that I know of......06 used that crappy chain guide down at the crank. In my opinion its the tensioner thats the weak link.....ive seen bunches of the 06s grab that guide and break it off, then all hell breaks loose...the only way I can see the chain grabbing that guide is if the chain is not being controlled properly.

As far as the cam spinning in the sprocket, we weld all of them if we happen to have the cams out. That said I dont think they spin unless something happens like say valves hitting the piston...then the extra forces involved on the cam cause them to spin. Be sure to replace valve springs often, especially if you like to rev the bike to the limit

Steve

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they suck is about the best i can describe.

i am feeling the same pain as you my friend. i have a 04 kx250f and my entire top end needed to be sent to kibblewhite in california. we replaced the valves, valve springs and valve buckets. the original valve buckets cracked apart. after the first fix the bike ran great for a half hour. then after an inspection one of the buckets became dished out in the middle and destroyed the cam. after another round of new parts the engine started but sounded terrible. that is when the entire top end was sent out. i have been out all summer. i am now in the process of getting a 07 yz450f. good luck.

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suck it up four strokes suck money down like theres no tomorrow. my whole motor somehow got ruined.. $2,000 to fix.

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When you replaced you valves and springs did you go factory or aftermarket? Im looking at the kibblewhite valves and springs but I dont know if its worth the $... Titanium vs Stanless Steel, I just dont know. The valves in it are all new but the intake valves wacked the pistion, so im not sure if there any good... It doesnt look like they hit hard but you can see on the piston where they struck. As far as the springs there all new also with new cams, I have been told to go to the heavy duty springs springs but is that really something to think even about? In this case would aftermarket be the way to go for the 3rd time putting this bike back together?

Also I have just ordered a new head for the bike so it sould be in next week, I will mik it and see just how much was taken off the head I have on the bike now.... Let me know what you think

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