Hi Andy
I was going to send you a reply so you avoid the pitfalls sometimes associated with building a nice engine. I am going to write a long winded cam choice thread, but in the meantime let me explain ( its a wet Saturday afternoon, and I have nothing else to do until the Sooty show starts later)
ALL the cams available from any source for the XS650 are really only intended for 750cc application, the sole reason being that when the cam forms were initially developed by Yamaha and others in the 70s all the engines built for performance were 750cc. The cams mostly used were cams for flat track racing, in the USA this is a long cinder/dirt track, around a 1/2 or 1 mile long. I watched a load of HD XR750s racing at San Jose over 30 years ago, it was stupendous, the noise, the dust !! . However all they are after is maximum power in a relatively narrow power band, they rev their engines between 4000 and 8000 rpm , this is the nature of flat tracking, and XS650/750 type flat tackers will be the same. The archetypal flat track cam the Shell#1 is a 750 cam, Shell (Sheldon Thuett) built race engines, all his bikes were 750. This is actually a very good street cam, the #1 cam is for 1/2 miles where you need more tractability, whereas the #2 ( rarely seen) is a 1 mile cam. Carburation and exhaust pipe/silencer choice can calm any cam down if it is a bit wild, as can advancing the inlet timing a degree or two.
When you build an XS650 and leave it 650cc, if you install any cam other than the stock cam form, the bottom end power will suffer. It is wholly possible to recoup some lost bottom end by increasing the compression, but on a 650 this is limited to skimming the head/cylinder and fitting thinner head gaskets. The only exception is to use the earlier XS1/XS2 cam form, on a stock phased engine this requires changing either the crank sprocket or cam sprocket to allow the cam to fit ( the early engines have a different cam chain pitch, and the sprockets are 17/34 not the later 18/36 from 1974 onwards) if you are building a 277 degree re-phased engine, Smedspeed in the UK keep billet cams with this XS1/XS2 cam form.
If you build a 750 cc engine ( which was always my preferred choice) the compression is always higher as you have more swept volume with the same/slightly less combustion chamber volume. The piston choice for 750s is much better, real quality pistons from Wiseco, Wossner and JE. A 750 engine will put up with much more cam than a 650, the bigger engine will "want" more duration and lift, not just put up with it.
Simply stated the more cc and compression you have the more cam you can run. The dynamic compression of an engine is all important, and the earlier the inlet cam closes the lower down the rev band the engine will make good torque. On the race bikes we juggled cam choice and inlet closing figures for five years to get the effects we wanted.
The 880cc race bike we run has a very high lift and long duration cam, in a 650 that cam would be just awful, but in the big cc race bike it makes it a torque monster.
IF I doubt about which cam to use, always look at the inlet closing figure. This coupled with the dynamic compression figure will decide how well you engine runs out of its torque band.
I do not generally speak about power when tuning engines. Power is torque x rpm. So if you have good torque you will end up with good power anyway, torque is produced by filling the cylinders full of air/fuel mix and anything that reduces that at any rpm reduces torque and thus the power.
if you email what you want from an engine, I will suggest the ways to do it. Rick will sell you all the parts of course, I am his technical resource for when he needs it ( not often)
Howard
