Harmonic Balancer Problem

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Help! I've got a '93 ES-300 with about 216,000 miles. The engine was totally rebuilt about 65,000 miles ago, and I've had no problems with it. However, I just tried to pass my smog test, and the mechanic pointed out a few things: The timing was set to 25 degrees TDC, and the car had no low end power when it was reset to the required 10 to 12 degrees. Second, you can easily see that the harmonic balancer has a serious "wobble", almost like a flat tire. My mechanic is now concerned that the timing belt may have slipped a notch or two, resulting in the inability to properly set the timing.

Just how big a job is replacement of the harmonic balancer? Is it an "urgent, don't drive it until it is fixed" type of repair, of can the repair wait a while? Also, does anyone know if the engine has to come out, or can they get to the repair via just via the top end and timing belt?

Thanks!

Rich