balancer shaft weight

just wondering what effect changing the weight of a balancer shaft would have ie fitting a ligher one? does it just not cancel out vibration as efficiently or throw everything out completly.

Assuming you are speaking of a single cylinder engine, the results would be hard to predict. Singles present a major challenge in balancing, anyway. You have essentially two groups of components in such an engine; the rotating, and the reciprocating. The rotating group is the crank and everything attached to the ends of it, the rod bearing, and a percentage of the bottom of the rod. The rotating mass can be pretty much perfectly balanced with relative ease.

The reciprocating mass is another matter. This is the piston assembly and the top of the rod, and it produces a vertical vibration with the change in momentum at the top and bottom of the stroke. This is further complicated by the fact that the piston travels faster in the top half of its stroke than the bottom due to rod angularity, so the same counter balance that would resolve one vibration won't resolve the other.

As a result, before counter balancers, we used to have to deliberately imbalance the rotating assembly to counteract the vertical reciprocating imbalance. The trouble is that even if you canceled the vertical imbalance this way, you created a horizontal imbalance in the process, so the game became one of compromise in which you had to decide at what rpm you wanted to tolerate some fairly heavy vibration.

Counter balancers, then, can be used in a couple of different ways, but they are intended to produce an imbalance, and thus a vibration, in a direction opposing the vibration of the crank and/or piston, depending on the approach taken. If you give the engineers credit for getting it right the first time, any change you make will make it worse, but it might also improve it.

Here's a good place to start:

http://en.wikipedia.org/wiki/Engine_balance

All based on a constant crank speed:

if the centerline of the cylinder is on the same plane as the crank, the maximum velocity of the piston would be reached at exactly mid stroke. The deceleration to zero velocity would be equal from there to either end of the stroke.

A velocity difference would only exist if the cylinder plane is offset from the crank center. and the net result would be a slower velocity in one of the upper half directions, and a faster velocity in the opposte half. mirrored in the bottom half

All based on a constant crank speed:

if the centerline of the cylinder is on the same plane as the crank, the maximum velocity of the piston would be reached at exactly mid stroke. The deceleration to zero velocity would be equal from there to either end of the stroke.

A velocity difference would only exist if the cylinder plane is offset from the crank center. and the net result would be a slower velocity in one of the upper half directions, and a faster velocity in the opposte half. mirrored in the bottom half

Not true. Maximum piston speed is reached at the point it the rotation of the crank where the rod (center to center) is at 90 degrees to a line drawn through the centers of the crank pin and crankshaft. Because the rod must angle back toward the cylinder center from the crank pin, this always occurs before 90 degrees of crank revolution, and therefore, the piston travels farther and faster during the first 90 degrees from top than it does in the remaining 90 degrees to the bottom.

In a YZ450, for example, the piston is halfway down the bore at 82 degrees ATDC, and peak piston speed is reached at around 79 degrees ATDC. The YZ450 has a 2.5" stroke, and at 90 degrees, the piston sits at 1.431" down from TDC, leaving only 1.069" to be traveled in the second 90 degrees. With crank speed relatively constant, the piston must travel the upper 180 degrees in the same amount of time as the lower 180, so it obviously goes faster over the top.

This non-sinusoidal speed curve, and the accompanying change of momentum that naturally goes with it is why the same counteracting force won't cancel the vertical vibration in both directions; they're different.