yz426 valve adjusment?

this is my first post after reading countless pages about this cam mod. i hate to have to resort to asking but..... i did the mod but my shim numbers were 182 and 184!!! those aren't even referenced in the manual i have. so i said to H@#$ with it and didn't replace the valve shims. i only had a standard feeler gauge and the clearance was .007 for both valves. now when i go to start it cold in 85 degree weather it wont start. before the mod it started 2nd kick. now even with choke on, off, a couple of throttle twists nothing i have to bump start it to get it started. it bump starts easily BTW. and will hot start no problem. any ideas or suggestions, are my valves to tight/loose

First, your shims. These are original factory build shims. Service shims are only available in increments of .05mm, but the factory has access to shims in steps of .01mm. Sometimes, these will continue to turn up in engines that have had a valve adjustment because the technician who took them out of one engine saved them and used them on another one. You calculate the clearance the same with these as with any of the service shims, but you have to do the math instead of using the chart.

Your exhaust valve specs are .20mm - .25mm, which is roughly .008"-.010", so you exhaust valves are a little tight. That won't cause your cold start problem.

Go back over your cam timing, carefully. Review this thread:


If all that checks out, you may need to richen theidle mix a bit, and a larger pilot jet could be in order. It sounds a little lean.


Doesnt Motomans valve adj. guide (the timing exactly) only refer to a stock cam 426?

i thought the cam lobes, after doing the 450 cam mod, would be in a slightly different position than stock...something like a 10 and 2 o'clock position??

Use the 450 Cam Mod PDF to "time" the cams....but checking the clearances are the same once they are timed. Time the cams, then check the valves using Motoman's guide.

450 cam mod pg 1

450 cam mod pg 2

Thanks for the input. i broke down and bought a set of MM feelers guages re-checked clearances and they were .40 off so i ordered a 195 and 200 shim which sounds like what most people had to switch to. ill update when i get them installed. Thanks again everyone

Hey Satch, got my new cam in the mail now and about to do the mod...Does the same clearance chart work with the new (450) cam? Just double checking so I can have all parts ready when I breakdown the bike, and rebuild.

Hows Dallas these days????

the same chart works .....

Dallas is good....riding NoconaMX Saturday and Badlands Sunday......we miss Mosier Valley though!

You can get the Hotcam for the yz 426 already with the aut decomp.. for like $165

Sad to se the old place go...My family goes back to its inception, nad the history and stories are awesome...

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