Who is the YZ426 expert?

7 replies to this topic
  • Jason21

Posted January 20, 2008 - 05:16 PM


I have a 00' YZ426 with an aftermarket decompression cam. The bike has always been bullet proof and kicked over with very little effort in the coldest weather. Recently, the bike has been hard kick over at which point I took it to a new mechanic. He said a preliminary check of the motor showed low compression. He said I should have approx 150lbs but was only showing 75. He said I needed a new piston/rings and would check the valves while he was in there. My question is would the aftermarket cam give him a false reading on the accurate compression. If I do need a piston and valves, would should that run me?

  • grayracer513

Posted January 20, 2008 - 08:31 PM


Conventional compression checks are useless once auto decompression is installed. Pay no attention to them.75-100 lbs. is about what you should expect from an AD equipped engine.

A leak down test is the only valid way to test for compression on such engines, and is more accurate than a conventional test even on "normal" engines.

  • mkporn

Posted January 21, 2008 - 08:31 AM


Side note to this........ I would find a new mechanic, cause the one you took it to, is not sounding very good. I find it odd that he would automatically say that it was piston and tell you he will check valves when he is in there.

Even if you didn't have a auto decom cam in there, an assumtion that the rings are bad without further testing and inspection tells me that your mechanic is a parts changer and looking to solve a problem without any real thought..

all that said, pay attention to grey, he hit the nail on the head....

  • 642MX

Posted January 21, 2008 - 09:52 AM


[quote name='Jason21']Recently, the bike has been hard kick over at which point I took it to a new mechanic.QUOTE]

Explain in a little more detail. Is it physically harder than normal to kick thru? Or its harder to get it started?

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  • Jason21

Posted January 21, 2008 - 11:40 AM


Not harder to kick through at all (it may even seem a bit easier). just don't want to start. It's always had a history of kicking right over......Jas

  • 642MX

Posted January 21, 2008 - 12:42 PM


Not harder to kick through at all (it may even seem a bit easier). just don't want to start. It's always had a history of kicking right over......Jas

Check your valves. It may have a tight valve on the intake side.

tight valve = hard to start

  • USED YZ426F

Posted January 21, 2008 - 12:47 PM


You may also want to consider cleaning the carb. You could have a gummed up jet or circuit in there that may contribute to the hard starting. Also, how old is the plug? They don't last forever.

  • Jason21

Posted January 23, 2008 - 02:40 AM


I ended up taking bike to another mechanic who did the leak down test. he confirmed there was very low compression and replaced the piston/ring. he said the carb was fine and had nothing gummed up. he also said valves looked brand new claiming yamaha makes bullet proof valves. anyway, bike kicks right over now with little effort and seems to be ok. $430 bucks later. thanks for you help and input

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