WR Timing & YZ Timing Question


7 replies to this topic
  • SCRC John

Posted August 28, 2009 - 11:41 AM

#1

What makes the difference? The way the cam gear is indexed on the cam? I have a hotcams cam set for my WR, and am curious as to how the gears are indexed. I've also read that Hot Cams uses their own indexing (i.e. neither WR or YZ indexes). Am I on the right track here? I can't find a clear answer in my searches (geez there are TONS of threads on this, but no clear description)

  • ww44wrf450f

Posted August 28, 2009 - 11:53 AM

#2

yes on how it is indexed and also the grind of the cam.

  • grayracer513

Posted August 28, 2009 - 04:07 PM

#3

You didn't specify the year of your WR. In models '06 and earlier, the camshafts themselves were the same as the same model year YZ450 (or 400/426) cams, but the sprockets were pressed onto the cams in a different position so that the exhaust cam on a WR is more advanced.

In 400 and 426 models, the stock WR cam could simply be retarded one tooth and the timing effectively changed to be close to YZ specs. In the 450 models, however, this results in a loss of cranking compression that keeps the bike from starting, since re-timing the cam also re-times the auto decompression unit.

  • SCRC John

Posted August 28, 2009 - 08:26 PM

#4

Mine is a 2004. I had the hotcams installed and the Auto Decomp was not functioning. You can noticeably tell the decomp pin is shorter than the stock unit. They have agreed to install a longer decomp pin. My question was more focused towards the timing, as in, if I timed the cam differently would the decomp work better. All of this is thrown out the window since hotcams uses neither the WR or YZ cam index, but their own index.

Either way I appreciate the feedback.

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  • CanadianWR450

Posted August 29, 2009 - 03:42 AM

#5

Strange, I put in Hotcams on my 04, and yes, the pin IS much shorter, but as you said, it is because the new cam's dump way more compression and are indexed more.....
I have no e-start issues at all..

  • grayracer513

Posted August 29, 2009 - 08:12 AM

#6

The Auto Decomp system works by means of a pin which is extended by spring force on an eccentric inside the camshaft at speeds below 700 RPM, and retracted by a flyweight at speeds above that.

The pin extends from the camshaft near the heel of the left exhaust cam lobe, and lifts the left exhaust valve off its seat as it rolls over the lifter. The intent is to have this occur as the piston is moving upward from BDC at the start of the compression stroke, releasing compression as a result. The pin is timed by both its position in the camshaft and its length so that it will return the valve to its seat at somewhere around 60-80 degrees BTDC, which cuts the length of the compression stroke roughly by 40 percent, leaving enough compression for the engine to start, but reducing it enough so it can be cranked.

With that in mind, you can see that if the pin is too short, too little compression will be released. Too long, and the valve will close too late, and too little compression will remain to start the engine with. Likewise, retarding the cam without altering the pin will delay the valve seating and reduce cranking compression. Advancing the cam will seat the valve sooner, and have the opposite effect.

  • geegee

Posted September 01, 2009 - 02:02 AM

#7

The Auto Decomp system works by means of a pin which is extended by spring force on an eccentric inside the camshaft at speeds below 700 RPM, and retracted by a flyweight at speeds above that.

The pin extends from the camshaft near the heel of the left exhaust cam lobe, and lifts the left exhaust valve off its seat as it rolls over the lifter. The intent is to have this occur as the piston is moving upward from BDC at the start of the compression stroke, releasing compression as a result. The pin is timed by both its position in the camshaft and its length so that it will return the valve to its seat at somewhere around 60-80 degrees BTDC, which cuts the length of the compression stroke roughly by 40 percent, leaving enough compression for the engine to start, but reducing it enough so it can be cranked.

With that in mind, you can see that if the pin is too short, too little compression will be released. Too long, and the valve will close too late, and too little compression will remain to start the engine with. Likewise, retarding the cam without altering the pin will delay the valve seating and reduce cranking compression. Advancing the cam will seat the valve sooner, and have the opposite effect.


i have just re- shimmed my wr 250 - 05 , this was my first attempt at working on my bike !! - i put everything back in the way it was meant to go in , TDC etc, i tried to start my bike with the e - start - no go !! but i could start the thing by kicking it over - WHAT HAVE I DONE WRONG !!!!
i have a powercore 4 pipe on if this helps any

  • grayracer513

Posted September 01, 2009 - 11:36 AM

#8

WHAT HAVE I DONE WRONG !!!!

Your cam timing, or you have a tilted shim.




 
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