Another YZ400 Noob Needing Some Assistance...

24 replies to this topic
  • grayracer513

Posted August 15, 2013 - 12:46 PM


As the head heats, valve clearance increases.  This is because the head, being aluminum, expands more than the valve does, moving the cam up away from the valve seat.  The valves heat more quickly, and run hotter, but at full operating temps, the clearance is slightly greater than when cold


Your intake valve clearance is .10-.15mm, not .15mm.


Trimming shims is not a good approach.  There is no possibility that you will get both surfaces parallel to each other.  Just buy one. 


What size shim do you currently have?

  • Fastest1

Posted August 15, 2013 - 01:40 PM


Without a surface grinder and the know how to set it up. Trimming as you call it would be a joke at best. This is a hardened piece of steel. Some shops will swap shims, if not they are very inexpensive. If he can't afford a shim, probably can't afford gas or a helmet.

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  • WrightStuff

Posted August 16, 2013 - 10:39 AM


The shims are currently 1.80 and based on my measurements I need 1.05 for proper clearance, however, per my understanding the smallest available is 1.20 correct? If I put in 1.20 shims I would still be off proper clearance but it still may run is that what you are suggesting?


Otherwise I am looking at replacing the intake valves which is still pretty dang cheap just time and effort is multiplied. If I did replace the intake valves would I need to have them properly fitted to the head at a metal shop or could I simply install and go?

  • grayracer513

Posted August 16, 2013 - 12:17 PM


Your math is seriously honked. 


Your clearance is .075mm when you want between .10 and .15.


.10 - .075 = .025

.15 - .075 = .075


So you need a shim that is between .025 and .075 thinner than a 180


A 180 is 1.80mm thick, so...


1.8 - .025 = 1.775

1.8 - .075 = 1.725


That means that a 175 shim (1.75mm thick) will put you at .125mm clearance (.075 + .05), exactly perfect.


Looks like a major case of decimal slip, among other things.

  • WrightStuff

Posted August 17, 2013 - 01:27 PM


My math isn't the only thing seriously Honked about me! ;) Thank you GR for taking the time to figure this out for me.I had the time to go to Yamaha today and get the shims, put them in and bike fired up and stayed running. I adjusted the idle and fuel/air to reduce some back fire and so it would idle. It ran for a good 4 mins and I shut it down. Started it two more times on the first or second kick. Only issue I have now is some smoke from the header pipe - I think I read some threads on this and oil may be coming from where the decompression used to be. It also boiled over a little but I'm guessing that's because it was idling and I was revving it up pretty good and no air was getting through the radiators. I'm sure i'll have some other items pop up but yeah got it running! Thanks again!

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