Shems 426


12 replies to this topic
  • kirk_12

Posted April 07, 2008 - 07:48 PM

#1

If the shems are loose in an 01 yz426 will it lose power???

  • Qwik Gixxer

Posted April 07, 2008 - 08:58 PM

#2

Do You mean the valve shims? If so then yes because the valves wont open very far.

  • grayracer513

Posted April 08, 2008 - 08:51 AM

#3

Do You mean the valve shims? If so then yes because the valves wont open very far.

If the valve clearance was .20mm too loose, the engine would be ridiculously noisy, but there would not be enough change in lift to notice even a small drop in power. The answer to the question is no.

Kirk, why do you ask?

  • kirk_12

Posted April 08, 2008 - 07:22 PM

#4

i went ridin sunday an my bike bogged down a couple of times which has never happend, then on my way home i got out ran by a crf 250 that i have beat like 10 times!!! jus felt like i was losin power

  • Fabwiz

Posted April 08, 2008 - 07:29 PM

#5

When is the last time you changed the plug or cleaned the air filter??

  • kirk_12

Posted April 08, 2008 - 07:37 PM

#6

last monday ... but i didnt ride any that week cuz i was in louisville the whole week ...

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  • Fabwiz

Posted April 08, 2008 - 07:44 PM

#7

Do you have any ticking in the engine, more then normal i should say. Pull your plug out and check the color, black and wet is a sign of flooding could be carb troubles, grey or nothing on the plug is ok, white running lean possible air leak in the intake or other carb issues.

  • Fabwiz

Posted April 08, 2008 - 07:45 PM

#8

also check the gap

  • kirk_12

Posted April 08, 2008 - 08:20 PM

#9

actually it did start ticking like the shems are loose, i know on 426s that when the valves wear they loosen as to hondas tighten ... but i will check the plug an for air leaks ... then if all is good i will get my uncle to shems it!
thanks guys.

  • spc mike

Posted April 08, 2008 - 10:52 PM

#10

its SHIM(s)!

  • OUCR250R

Posted April 09, 2008 - 05:13 AM

#11

actually it did start ticking like the shems are loose, i know on 426s that when the valves wear they loosen as to hondas tighten ... but i will check the plug an for air leaks ... then if all is good i will get my uncle to shems it!
thanks guys.


The yamahas do just like the hondas do and tighten as they wear out. The seat surface gets worn down and causes the valve to drop further into the seat which causes the SHIMS to tighten. How many hours would you say are on the engine? Because you should have absolutely no trouble beating a crf250r in a straight up drag race.

  • grayracer513

Posted April 09, 2008 - 05:57 AM

#12

actually it did start ticking like the shems are loose, i know on 426s that when the valves wear they loosen

Wrong. Normally, the wear occurs only at the valve face and seat, which causes the valve to move up into the head, reducing clearance.

  • kirk_12

Posted April 10, 2008 - 07:43 PM

#13

well i guess the local yamaha shop is wrong if they tighten wen wore cuz they told me they loosend wen wore .. but anyway i checkd the clearence jus now and the intake was very loose an the exhaust wasnt that bad but still loose





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