Low compression '98 400f

22 replies to this topic
  • 1998YZ400F

Posted April 22, 2008 - 05:45 PM


my 1998 yz400f has fairly low compression. the valve clearance is good and the timing is right. the rings are fairly new too. the compression release isnt stuck open either. its still good enough to ride but i can kick it over as fast as a two stroke. what could it be?

  • Qwik Gixxer

Posted April 22, 2008 - 06:13 PM


May have a scar on the cylinder wall. so it only gets compression above that point.

  • grayracer513

Posted April 22, 2008 - 07:12 PM


Have a shop do a leak down test on it and find out for sure.

  • 1998YZ400F

Posted April 23, 2008 - 12:37 PM


can i just do a simple compression test with a generic tester? the kind that screws into the spark plug hole with the gauge on it?

  • grayracer513

Posted April 23, 2008 - 01:40 PM


Yes, if all you want to know is whether it's high or low, but you'll notice there is no specification given for that.

A leak down test shows precisely how much is leaking, and lets you find out precisely what is leaking as well.

  • 1998YZ400F

Posted April 23, 2008 - 01:57 PM


ok thanks. i know its low, im just curious how low. so there is no given psi level it should be at?

  • grayracer513

Posted April 23, 2008 - 02:24 PM


It isn't specified because when the bike is healthy, you usually can't kick it through the compression stroke anyway.

  • 1998YZ400F

Posted April 23, 2008 - 02:52 PM


yeah, thats the only way i figured out something was wrong.

  • 1998YZ400F

Posted May 07, 2008 - 10:21 AM


i found this home made leakdown tester on the web. seems like it would work to me.

Posted Image

could i make it into a dual gauge by placing a second gauge between the pressure regulator and the damper valve?

  • grayracer513

Posted May 07, 2008 - 12:31 PM


Yes, depending on the damper valve, but by the time you get done with it, it won't have been that much less than buying one ready made.

Also, applying air pressure to the cylinder will blow the piston down to BDC, and no matter which BDC it goes to, valves will be open. Because of that, the engine must be locked at or near TDC on the compression/power stroke to do this test. DO NOT attempt to hold it by hand or with a wrench by hand.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • 1998YZ400F

Posted May 07, 2008 - 11:54 PM


so how do i "lock" it at TDC?

  • grayracer513

Posted May 08, 2008 - 05:59 AM


The simplest way, if it works, is to use a breaker bar and a socket on the flywheel nut. Lash the handle of the bar to the foot peg with a hose clamp or 2-3 zip-ties.

  • 1998YZ400F

Posted May 08, 2008 - 08:19 AM


could i just put it in gear up against something so it doesnt roll? maybe push the rear brake too?

  • grayracer513

Posted May 08, 2008 - 09:23 AM


That likely won't work because the engine will probably be able to move more than the 120 degrees or so required to open one of the valves.

  • 1998YZ400F

Posted May 08, 2008 - 09:25 AM


ok so i will try the way you suggested. but where can i get a spark plug fitting for the gauge that will fit my bike?

  • grayracer513

Posted May 08, 2008 - 09:49 AM


From a compression tester, or by making on from an old spark plug.

It would really be a lot simpler just to go and have the test done, or at least just buy the tester.

  • 1998YZ400F

Posted May 08, 2008 - 04:51 PM


well since i have the gauge i might as well try my best to use it. i found adapters but what size would i need? 10, 12, or 14mm?


  • grayracer513

Posted May 08, 2008 - 05:22 PM


10mm. I should mention that the tester will not be accurate unless the orifice in the buffer valve between the gauges is of the right size.

  • 1998YZ400F

Posted May 14, 2008 - 10:52 AM


so i found out i have leaky intake valves which doesnt surprise me as they have never had anything but adjustment and the exhaust valves have only a season or two on them. is there any way to tell if rings are bad? when its hooked up to the leakdown tester, where will bad rings leak? i know a bad head gasket bubbles the coolant.

  • grayracer513

Posted May 14, 2008 - 11:19 AM


when its hooked up to the leakdown tester, where will bad rings leak?

Into the crankcase. Air would escape through the breather, and through the timing plugs, if they are out.

Related Content


2016 YZ450 by CaptainKnobby

Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  59 replies

Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

Snake pit oct 30th by The Anvil

Dirt Bike   Dirt Bike Regional Discussion   California
  • Hot  293 replies

James Stewart back on a YZ450F by YamaLink

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies

BIGBORE FOR YZ450F 2006? by Taattie94

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

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.