426 Compression?

12 replies to this topic
  • slacksaw

Posted April 09, 2007 - 03:01 PM


I want to check the compression in my 2001 426 what should the psi read on the gage I know the ratio is 12.5:1. but the snap on compression kit is in psi any help would be great

  • slacksaw

Posted April 10, 2007 - 01:31 PM


thanks for all the info appreciate it.

  • grayracer513

Posted April 10, 2007 - 01:44 PM


Thanks for your patience. Shall l tell you what I know about it?

  • King_Air

Posted April 10, 2007 - 02:34 PM


Thanks for your patience. Shall l tell you what I know about it?

Maybe you should direct him to the search feature so he doesn't have to wait on us old slow guys gray.:thumbsup: :worthy:

  • slacksaw

Posted April 10, 2007 - 04:09 PM


sorry not trying to be a jerk. and I have tried to search this forum for It and didnt find it. just seemed like no one wanted to respond on this topic but everyone will reply on what tire to run or what oil to use. just frustrated that local dealer wont help so I tried here yesterday and got no response. I apologize

  • grayracer513

Posted April 10, 2007 - 06:08 PM


One reason for that is that compression tests are rarely done anymore. The test, or the test compression are neither one listed in the manual, and no one really knows what it ought to be.

In order to do a compression test, you'll have to kick it through the full compression stroke 4-5 times. The YZ426 in good condition should produce, I would think, something right at 200 pounds of compression, but it could vary from that considerably. The real test is not in how high the numbers are, but in how many strokes it takes to reach its max. A healthy engine will run up to 95% or more of full pressure in 3-5 strokes (hold the throttle open for the test).

Cylinder leak down tests are a more accurate measure of combustion chamber sealing, and make diagnosis of the origin of any excess leakage easier.

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  • slacksaw

Posted April 11, 2007 - 05:56 PM


thanks for the help I really do appreciate it. :thumbsup:

  • mxgene

Posted April 12, 2007 - 09:20 AM


Cylinder leak down tests are a more accurate measure of combustion chamber sealing, and make diagnosis of the origin of any excess leakage easier.

Dumb question here, I bought a Snap On leak down tester, and need to know what psi I should put into the cylinder? I know that more than 10% loss indicates a problem and to check for leaks at the exhaust, intake, and breather. I don't want to put too much pressure not knowing if that will give me false reults.

BTW- the Clymer manual doesn't indicate what the pressure should be set to.

  • grayracer513

Posted April 12, 2007 - 01:11 PM


Since you're asking what to set the input pressure to, I'll guess it's a two gauge unit with an adjustable regulator on the input side. Most of the time, 100 PSI is used, simply because it makes the percentage loss calculation so easy. See if the instructions say anything about it.

Bear in mind that the engine has to be locked at TDC; it will not stay there by itself, and don't get in the way if the air starts to turn it. The simplest way to accomplish this is to put the bike in a higher gear, then use a wrench to turn the engine forward to just past (like 1 degree) TDC with a load on the drive train (the engine has started to move the bike). Then, have someone firmly hold the rear brake and apply pressure.

Also not that you WILL hear air passing the rings. It's normal because the rings have an open gap at the ends, and cannot seal perfectly. Your concern is with the percentage loss, and with any leakage you can hear in the intake or exhaust.

  • mxgene

Posted April 12, 2007 - 02:14 PM


Thanks Gray...<<<<:thumbsup: :worthy: :ride:

  • 02yzrider

Posted April 12, 2007 - 03:16 PM


?????? How do you build up compression on a yz426f??? Everytime before you kick the bike over, you must use the decompression lever, which releases the pressure... I have never tried a compression test on a 426 because I thought it could not be done without the autodecompression cam... Please let me know

  • grayracer513

Posted April 12, 2007 - 04:08 PM


You're serious!?

You know how if you don't do it right, the kick starter comes to a complete halt? That's the compression stroke that you would be testing. As far as releasing the pressure between strokes, the valves do that anyway. The pressure that is shown on the gauge is stored by the gauge until you release it.

But that is one of the problems with trying to do a conventional compression test on something like a YZ426. How exactly do you propose to kick it through the unreleased full compression stroke, anyway?

  • slacksaw

Posted April 12, 2007 - 05:15 PM


Dont know if this will help or not but I called yamaha and they said if you have more than a 5% loss than there is a problem.

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