The length of the pin, all by itself, is not the issue until it gets too short to even raise the lifter at all. What it really comes down to is a matter of timing. I had occasion to degree the cam on a YZ450 recently, and after determining that it was where it was supposed to be, I checked the point at which the decomp pin re-seats the valve, for my own curiosity. The valve was reseated at 27º BTDC.
The way the decompression system works is simple enough; a flyweight rotates shaft that extends a pin that raises the left exhaust
valve off the seat before the start of the compression stroke and seats it before the end of the stroke, which shortens the compression event and lowers the cranking compression to a reasonable level. The pin extends through a bore in the camshaft perpendicular to its axis. The '03-'05 YZ450 exhaust cam shaft
is identical to that of the '03-'05 WR450. The only difference is the position of the sprocket, which changes the timing of the cam, with the WR being more advanced by about 19-21º. If the same decomp pin were used in the WR, the exhaust valve would then be reseated at around 47º BTDC instead of 27 or so, and you'd have too much compression to crank.
There are two ways in which this could have been fixed: Yamaha
could either have made a different camshaft for the WR with the hole for the decomp pin drilled 20º farther retarded, or they could have done what they did; use a longer pin in the WR, which would cause the valve to reseat later in the cam's rotation, and correct for the change in timing.
What does that have to do with the Hot Cams
or any other-than-stock cam? Just this: pin length may not be the best way to compare these when addressing starting problems. Checking the reseat point, both OEM
and aftermarket, with a degree wheel
will give you a much better idea of whether and how much modification the pin would need to match up. If the cam manufacturer indexed the decomp pin bore in such a way as to reseat the exhaust valve at 25-30º (or whatever the figure is for an OEM
WR cam) when using a short pin, then it has no need of one longer or shorter than that.
Hopefully, that understanding may help eliminate some trial and error.