yz cam swap, hotcam update
Posted April 06, 2012 - 07:05 AM
wr cams: 168m, red-4, 45p, 1 1/8 fuelscrew, 50 leak
yz cams: 170m, red-5, 45p, 1 7/8 fuel screw, 55 leak
Posted April 06, 2012 - 07:42 AM
Awesome! Thanks for doing real measurements on this for the community. When I ground the decomp pin on our WR exhaust cam to run it 1 tooth retarded, all I had to go on was "about 1mm shorter" and trial and error with cranking compression. The HC exhaust profile isn't the same as the YZ and thus the pin lengths shouldn't be the same, but it's good to have concrete numbers.
The starting problem is definately the pin length on the decompression system. the hotcam measured 23.00mm and the stock wr cam measured 23.73mm.
Yamaha advanced the exhaust cam 22.5 degrees on the WR and YFZ to knock valve overlap down to about 20 deg. to better cope with a restrictive exhaust. The exhaust valves open around 67 BBDC for a more blowdown time into the restrictive system and close around 3 ATDC so the high pressure difference between the intake and exhaust tracts doesn't fill the cylinder with residuals. When you fix the problem that is the stock exhaust, the long blowdown time is no longer necessary and you can take advantage of exhaust scavenging with increased overlap.
Here's the difference going from the stock exhaust, cam timing, and jetting to a retimed (1t retarded) exhaust cam, good exhaust, and rejetting made on our dyno:
Posted April 06, 2012 - 12:12 PM
Posted April 07, 2012 - 10:54 AM
If you kick your bike, you should be fine following the exact steps listed in the link. If you depend on electric start, you'll need to remove some material from the decompressor pin--between .75 and 1 mm. With no material removal from the decomp pin, our motor only had 10 psi of cold cranking compression and wouldn't fire. It was up to 50 psi with 1mm removed and starts as it did stock.