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kshields2

Question on yz450f

13 posts in this topic

I have a few questions. This may sound sad but I have had my bike since 2010 it's an 09 and ride it in the summer in the trails in northern Michigan and sometimes on the track. I have probably 300 hours on it and have never changed the piston. I know it's way over due but other than that what else should replace I know you really can't say without looking at the motor but in general what do you generally change at that many hours. My buddy and I looked at the valves last year and only one was out of spec but it was such a small number that probably didn't even need to be touched. What else should I do. It still runs perfect but towards the end of last season i would be just cruising down a road and when I'd come to a stop its die but then I'd start it back up first kick and itd run perfect. I'm gonna start with pulling the carb off I have a feeling that could be gummed up and my packing was burnt up. Any ideas why my bike would be doing that? And Input would be greatly appreciated.ImageUploadedByThumper Talk1424442610.519868.jpg

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I have a few questions. This may sound sad but I have had my bike since 2010 it's an 09 and ride it in the summer in the trails in northern Michigan and sometimes on the track. I have probably 300 hours on it and have never changed the piston. I know it's way over due but other than that what else should replace I know you really can't say without looking at the motor but in general what do you generally change at that many hours. My buddy and I looked at the valves last year and only one was out of spec but it was such a small number that probably didn't even need to be touched. What else should I do. It still runs perfect but towards the end of last season i would be just cruising down a road and when I'd come to a stop its die but then I'd start it back up first kick and itd run perfect. I'm gonna start with pulling the carb off I have a feeling that could be gummed up and my packing was burnt up. Any ideas why my bike would be doing that? And Input would be greatly appreciated. ImageUploadedByThumper Talk1424442610.519868.jpg

replace the valves. Even tho they seemed good they are a time bomb with that many hours. Also check the play at the big end of the connecting rod and check the crank for play in the main bearings. Also check the timing chain. If you find excessive play or any rough spots in the crank rotation, I would split the case and replace the crank and all bearings. Edited by Goforaride

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I have an '06 that I recently did the piston on at roughly the same number of hours.  What you need to replace depends on the condition of things, and to some extent, on your ability to correctly evaluate that condition.  In the case of my own, I replaced the piston, rings, wrist pin, and clips, while reusing the cylinder (honed with a brush type hone).  Piston pin retainer clips should ALWAYS be replaced. 

 

I disassembled the head, tested the valve springs (passed), and inspected the valves for wear.  I replaced only the center intake, and the remaining 4 have yet to need an adjustment from the original shims.  Bearing clearance was within the manufacturer's tolerances, so I left all of that, as well.  Connecting rod big end bearing clearance was well within tolerance, and did not vary regardless of crank position, so I left it as well.

 

Having said all of that, if it were someone else's engine, I would have pointed out that the valve springs are relatively cheap, and recommended he replace them.  Also, when considering things like the various bearings, what you cannot tell by any kind of inspection is when things like surface fatigue failures such as spalling ("flaking") will start to occur.  You simply can not see it coming before it starts, so you're taking an unquantifiable risk by not replacing them.  Then again, they may last another 1-200 hours, or even longer. 

 

Same thing with valves.  The fact that they haven't worn beyond their hard coating now gives you no clue at all to when they eventually will, so if you put them back in, you're setting your self up to possibly have to pull the head again at some unknown future time to replace one or more that start sinking.  Then too, there is a risk of one of them becoming fatigued and dropping the valve head into the works, which is shall we say, highly undesirable.  That sort of failure is normally the result of reshimming too many times once the valve begins to sink, but one can't be certain, precisely.

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replace the valves. Even tho they seemed good they are a time bomb with that many hours. Also check the play at the big end of the connecting rod and check the crank for play in the main bearings. Also check the timing chain. If you find excessive play or any rough spots in the crank rotation, I would split the case and replace the crank and all bearings.

I just checked the valves and every single one was in spec

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I just checked the valves and every single one was in spec

I understand that. My concern would be that 300 hours is a LOT of hours. Your gonna do the top end and then drop a valve and have to do it all over again

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replace the valves. Even tho they seemed good they are a time bomb with that many hours. Also check the play at the big end of the connecting rod and check the crank for play in the main bearings. Also check the timing chain. If you find excessive play or any rough spots in the crank rotation, I would split the case and replace the crank and all bearings.

up and down no play side to side slight to none

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replace the valves. Even tho they seemed good they are a time bomb with that many hours. Also check the play at the big end of the connecting rod and check the crank for play in the main bearings. Also check the timing chain. If you find excessive play or any rough spots in the crank rotation, I would split the case and replace the crank and all bearings.

timing chain is tight

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You sure about 300 hours.  I ride almost every weekend and usually fall short of putting 100 hours a year on a bike.  I would think with Michigan winters it may be even tougher to reach that number,

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You sure about 300 hours.  I ride almost every weekend and usually fall short of putting 100 hours a year on a bike.  I would think with Michigan winters it may be even tougher to reach that number,

He has had it over 5 years.
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He has had it over 5 years.

Yep I can do the math. Just asking the question since he indicated he rides it summers. I have seen plenty of 5 year old bikes where the owners claimed high hours but only rode then a handfull of times a year and way over estimate hours.

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Yep I can do the math. Just asking the question since he indicated he rides it summers. I have seen plenty of 5 year old bikes where the owners claimed high hours but only rode then a handfull of times a year and way over estimate hours.

yah we all do that if we don't have an accurate way of tracking actual run time.

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