compression question

8 replies to this topic
  • rmxtgh

Posted August 22, 2006 - 02:40 PM


i had a leakdown test and it showed 98%.....about 6 hours of riding later some dufus crashed into me.......the impact bent my bars and knocked the spark plug cap off my when i go to start it....i dont need to use the decompression release cuz the kickstarter kicks almost as easily as a two stroke...i know if you dont clean the air filter etc you'll lose compression but seems very strange to me to lose it pretty much instantly due to one crash.........any ideas???

  • BlackSheep

Posted August 22, 2006 - 03:02 PM


I assume its a 400 or 426 because it still has a manual decompressor, is the lever bent or cable hung up, is the decompressor arm bent holding the exhaust valve open?

  • rmxtgh

Posted August 22, 2006 - 03:13 PM


my bad...its an '01 426 with very low hours on it.

  • rmxtgh

Posted August 22, 2006 - 03:16 PM


hte lever/cable seems to work fine....doesn't look like its bent but.....i dont have another yz to compare it to in order to see if its returning all the when i release the seems to be....with my finger i can't push down the arm down any furthur on the top of the cylinder.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • rmxtgh

Posted August 23, 2006 - 02:49 PM


any body????

  • YZ426F Rider

Posted August 23, 2006 - 03:36 PM


Damn...I can barely pull the spark plug cap off my bike by hand!

Does the bike run? You could have a kink or bend in your compression release where it enters the head...but if you were losing compression (sounds like you are for sure) I'd think the bike would not be running very well. How is your right radiator? Is it bent back into the compression release lever?

Probably should pull the valve cover and take a look to see if the compression release is sitting flat on the lifter. Check your valve clearances also...if the release is engaged at all your right exhaust valve clearance will be WAY off.

  • David37

Posted August 23, 2006 - 07:52 PM


I would try disconnecting your compression release cable from the compression release arm on your engine. If your compression comes back you have a problem with your compression release cable. If you still have no compression, check your compression release arm on your engine and see if you can manually disengage the compression release. If the compression release arm does not move you found your problem. IMO from your description sounds like your compression release mechanism on your engine may have been damaged/bent during your accident.

  • rmxtgh

Posted August 24, 2006 - 03:15 PM


it does run.....took me about 20-25 minutes of kicking to get her to fire....thats not a surprise though since this happened way back in april and i just parked her and rode my trail bike in the woods since then.....she seems to run decently....had a little popping on startup but again with the time down and without stabilizer its not surpriseing....i just went up and down the street once so not like i tested her out real well but seemed to run just fine .....yes hte right radiatior is bashed in a bit too..

  • rmxtgh

Posted August 24, 2006 - 03:43 PM


how much is a tool to test the compression and what is the right psi?

Related Content



Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  30 replies

BIGBORE FOR YZ450F 2006? by Taattie94

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies

The "going back to the other darkside" adventure... by Monk

Dirt Bike   Make / Model Specific   KTM   250/350/450/505 SX-F & XC-F (4-Strokes)
  • Hot  113 replies

James Stewart back on a YZ450F by YamaLink

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies

Yamaha YZ450F 2017 by Chris.GVS

Yamaha YZ450F 2017
  • * * * * *
  • 1 review
  • Last review by Tommyk_55
  • On January 15, 2017

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.