Few problems with my 2002 YZ426F

3 replies to this topic
  • ben-yates

Posted April 28, 2014 - 11:43 AM


Hi all,


i bought a cheap YZ426F to get me back in to my bikes, i bought it having some problems but seen as im quite handy with a spanner thought id take the risk, anyways its throwing quite a lot of bluey smoke out the back, is the best thing to strip it down and inspect or will a compression test show the problem, i think its going to be either piston rings or valve stem seals, now i have more of a car back ground so if it could be anything else please tell me??


if the smoke was to be caused by the piston/ piston rings can the rings just be replaced or is it common practice to replace both at the same time??



next s it boggs down mid range, im thinking it may just be some shit in the carb but again if there is anything else just let me know??


cheers in advance

  • grayracer513

Posted April 28, 2014 - 12:38 PM


Your description of the bogging is insufficient to offer any suggestions.


As to the smoke, it's almost certainly rings.  Here's the thing:  YZF cylinders are Nikasil plated aluminum bores.  If worn out or damaged, they have to either be replaced, or bored out slightly, then replated, then precision honed back to their correct size.  You can also use the damaged or worn cylinder as a core exchange for a big bore kit if you roll that way. 


At minimum, you need rings.  It's likely that you also need a piston.  You will need to carefully evaluate the condition of the cylinder, and that I suggest you have done by a shop or a machinist familiar with such engines.  The wear limit is .002" (.05mm).  While you are in the engine, replace the valve guide seals as well.


Cranking compression tests will tell you relatively little beyond whether compression exists or not.  A far better test is the "leak-down" test, which most shops can perform for you.

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  • ben-yates

Posted April 28, 2014 - 10:33 PM


It boggs down when accelerating about half way through the rev range??


Are the chances of bore damage quite high? as its not worth doing, i may aswell just run it till it stops as dont want to put alot of money into the bike


Cheers for any help

  • grayracer513

Posted April 29, 2014 - 06:39 AM


The possibility of bore damage exists, but you should have a look into it anyway.  Running it until it stops may well take it from a $250-600 repair to more like $2500

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