But now I put my engineer's hat on, and get all serious. So come to order, class. :-) There will now be a little dissertation on cams as used in the infernal combustion engine for the opening and closing of poppet valves.

The camshaft is an eccentric lobe rotating about a concentric centre. It has a number of salient features, and several distinct parts to the lobe. These are, in order of rotation, the base circle, the opening ramp, the flank, the nose, flank, the closing ramp and then back to the base circle.

The lobe height is a measurement of how far it is from the base circle to the tip of the nose. This measurement will tell you if the cam is worn, or not. It does not tell you anything about lift, or opening and closing timings.

Cam lift is the distance the tip of the nose is from the rotational centre of the cam. A cam with a smaller lobe height may in fact have a higher lift because the base circle is closer to the centre of the cam. The cheap way to increase the lift of your cam is to grind the base circle. You put a thicker shim in the bucket to compensate for the material you have take off, and so the valve will lift further as the ratio between the distance from the rotational centre of the cam to the tip of the nose and the base circle has been altered.

Valve timing is determined by the shape of the cam. When the valve opens and closes is a function of the opening and closing ramp. Picture the cam lobe side on. It should be roughly egg shaped. the base circle is the round bottom. The rotational centre is about a third of the way up. The ramps can start above or below the rotational centre. A cam with early opening and late closing will have ramps that start below the centre of the cam.

The characteristics of the cam are determined here. The rate at which the valve is lifted off its seat is done on the ramp and the flank. A cam that is gentle on the valves has nice easy ramps and doesn't accelerate the valves off their seats quickly, and deposits them back down gently.

The shape of the nose determines how long the valve stays at full lift. If it is wide and flat, the valve will be open a long time. This usually goes with quick opening and closing. It is good for lots of high rev power.

Anyway, the explanation above is aimed at me saying that the lobe heights given in the Clymer manual for the 750 and 850 cams may be the same, but the characteristics may be completely different.

I have two cams for my XT500. Actually, I have three different grinds, but two of them have the same lobe height. One has lift of 0.502", and the other is 0.470". The first cam has a duration of about 310 degrees, the other is 282 degrees. But they have the same lobe heights.

End of lecture. I recommend a very good textbook called "Performance Tuning in Theory and Practice" by A. Graham Bell. Published here by the Haynes Publishing Group. This is a very good book and covers all parts of motor development theory and practical examples. Recommended reading for anyone building a hotrod. Has a large section on Mikuni carbs.