The balancer is a 2-piece unit, with a layer of rubber bonded between the hub and the outer ring. The bond can fail, and allow the ring to slip, making the timing mark inaccurate.
Here's a quick check: Take an old spark plug, and knock the porcelain center section out of it. It's usually necessary to grind away the rolled edge where the shell meets the porcelain first. Thread the shell for a 3/8-16 carriage bolt, with a locknut above the plug shell. Install the modified plug into the #1 cylinder, and screw the bolt in far enough to touch the top of the piston before it reaches the top of the stroke, and lock the nut down. Carefully turn the engine by hand until the piston touches the bolt, and stops its travel. Scratch a mark on the balancer outer ring under the timing pointer. Turn the engine backwards until it stops again. Mark the balancer under the timing pointer at this position. The TDC mark should be exactly halfway between the marks, if the balancer is in its original position. If not, replace it. There's no way I know of to reposition one, and once it slips, it will do it again.
It's a little fuzzy, but here's a tool I made for checking TDC:
[img]
http://www.hunt101.com/img/192632.jpg[/img]
Jerry
