Topic has been discussed well, everyone should know by now that a YZ-cam has a longer decomp duration (almost no compression to work against)
However, a WR-cam needs some intake charge to be left inside the combustion chamber to actually fire up the engine.
Reason is: on good kick will have the decomp pin retract, on both bikes.
But the slow churning of the e-start does require a decomp setting,
perfectly balanced between "no-compression at all" and "way too much for the e-start to overcome"
To be able to add compression to the YZ exhaust cam we need to trim the decomp pin down so that the cam's decomp pin
will close the exhaust valve at exactly the same amout of "degrees before compression TDC" as the WR cam was doing.
That very moment when the decomp pin stops touching the exhaust valve bucket has to be found.
I was using a slab of flat aluminum, app. 20mm high, so the cam's gear is clear of the working table's surface.
I wanted to simulate the valve clearance as well. So I searched for a sturdy material that would be 0.2 mm thick.
I used a piece of hard plastic I found in my errm... garage (note: never clean up your place, last day's garbage might be todays tool )
1) lay the cam onto the flat aluminum, having the lobe close to the decomp pin (precisely the lobe's backside) touch the 0.2mm strip.
Then the aluminum would be where the valve bucket's surface normally is, one clearance away from the cam.
2) press down at the cam with your finger, behind the 0.2mm strip, so that the cam now rests on strip and far end
3) while you keep pressing down the cam, somehow note the position of the 9 'o clock punch mark
(that is the only thing telling us how the cam will be installed).
Rock the cam back and forth to find the exact decomp pin contact position.
As that position is somewhere in the air, hard to reference against, I picked the 3rd tooth left of the 9 'o clock mark instead,
because that tooth points nicely at 9 'o clock when viewed from sprocket side, and colored it black with some felt tip marker.
4) create a reference. I used the cam's packaging box and marked the exact position of the 3rd tooth.
(weight down the box or use something more sensible )
5) grab the YZ cam, find the 3rd tooth left of the 9 '0 clock punch mark, color it black.
6) now when you position the YZ cam at "decomp pin touches simulated bucket" you will see how far the 3rd tooth is OFF of the marking on the reference box.
7) grind it down, frequently rechecking, until the third tooth is exactly at the mark.
8) use a 1000 (or 800?) grade sand paper to smoothen the scuffed surface.
9) really clean the YZ cam very, very well, reassemble and enjoy the e-start working with the YZ cam
Total time, includung photo documentation: 2 hours.
Make sure you know which punch mark is the "9 'o clock" type. (see pichtures)
The 0.2mm strip could be any material as long as it won't budge when you hold down the cam.
I know that one could use another strip at the far side lob to make the cam parallel to the "bucket simulator", but I simply didn't want to be anal about it.
WARNING: altough the decomp pin looks indestructible your dremel grinder will work through it like butter! Be very gentle. Look at the scars I made and do better.
The pin cannot rotate more than 30° around its movement axis. So I only took off material at the bucket touching side.
I was worried that reducing the overall lift of the pin might make the decomp less effective. And I only wanted to fix the decomp detachment point, nothing else.
WARRRRNING: 1mm is WAY TOO MUCH! That must be the number for triming the huge WR cam's pin, I only took off 0.4mm at best!
Most important: Even when you ducktape the bearing you will contaminate it with particles, audible if you turn it by hand.
I did take the cam to the tap and rinsed it generously with hot(!) water until all oil, grease and grime were gone.
I excessively spun the bearing while holding it into the streaming water.
Don't forget to actuate the decomp pin a coupe dozen times while douching it. Dry well with a hair dryer.
Result should be that when given a good push with your thumb it should keep spinning for 10 seconds flat!
Don't get to axious because a completely dry bearing when held at an angle might develop a rattling sound comming from the fact that the balls change race.
If you hold the cam vertical either way it should be spinning w/o any other sound than "wheeeeeeeeezzzzzz". no rattling!
just took it outside and verfied the result:
started up "cold" (~28°C currently) in gear firt try.
stopped a mile later and restarted fine in gear.
deliberately stalled it with the rear brake and it still fired up in gear!
I like that. Feels like success to me.
left WR'12 right YZ'06
flat elevated surface, with valve clearance simulator
End-Of-Decompression position of the WR'12 exhaust cam
End-Of-Decompression position of the unmodified YZ'06 exhaust cam
app. 0.4(!) mm of grinding later
after polishing off the edges
Edited by WRF-Rowdy, June 12, 2015 - 06:22 AM.