i found this on another forum which suggests a shorter decomp pin:
Just got a yzf cam and did about an hour of measuring against yfz cam
Measurement across cam lobe yzf .887 yfz.887
Measurement top to bottom of lobe yzf 1.220 yfz 1.220
Put cams back to back to check profile Exactly the same again.
I checked the pin location for the decompression(this took some doing).With the yfz set one tooth retarded(cam mod) the pins are in the same spot.
So whats different? The pin for the decompression on the yzf is .022 taller than the yfz cam ,which opens the exhaust valve farther thus dumping more compression off.This could be the reason some pepole have had starting problems when using the yzf cam(dumping off too much compression on the stock compression piston).
Also different is the number of teeth between the alignment dots on the cam gear.There is 7 teeth between the dots on the yzf cam and 6 teeth between the dots on the yfz,so if you ever need to tell which cam is which now you know.
Another thing that could be different(but I have no way of precisly measureing) is the degrees of cam rotation .I measured as best as I could but it could be a degree or so either way.
i also found this from grayracer513 that suggests the pin is too long and requires grinding down:
The pin length is how the decompression timing is accomplished. On earlier YZF quads and WRs, the cam shafts themselves were the same, and the cam timing was the ony difference between those and the YZ450F. To get enough of a reduction in the compression stroke, these cams have longer pins in the AD mechanism to compensate for the advanced timing run in the YFZ/WR engines. What some quad guys will do is retard the exhaust cam to YZ specs, and then grind down the pin to match the YZ pin, but I do not recommend doing this.
my problem with this statement is that none of the quad guys grind the decomp pin when retarding the cam and it works fine.
my other theory is that if the decomp pin is shorter on the yfz cam (possibly because they have a shorter stroke) then the pin wont open the valve long enough to release enough compression making starting difficult??
im probably just going to buy a YZ cam but i have been looking for so long that i need to know so i can sleep