JDOROUR is quite correct.
Also, do these, listed from easiest to hardest. Do only ONE at a time...
Check the operation of the vacuum advance. First, temporarily disable it by disconnecting it from the distributor and plugging the hose. See if the timing steadies-up. Find a piece of hose, connect it to the vacuum chamber on the distributor and put the other end in your mouth. Start the engine and have a friend watch the timimg mark with the timing light. Create vacuum by sucking in on the hose. The timing should advance as you change the 'amount of suck'. With the engine still running, reconnect the hose to the engine and move the accelerator on the carb. If the timing doesn't change exactly the same way it did with you sucking, you've probably got a vacuum leak.
Next test, remove the cap and rotor and lubricate the centrifugal weights. Occasionally they will bind-up and if you time the engine with them in the advanced position, you'll get 'bad' numbers when they return to a non-advanced position. This actually happens a lot since folks rarely lube these. With the cap off, have someone crank the engine and check if the shaft rotates without wobble. A bent dist shaft (rare) would cause your problem.
With the engine cooled off, pull the coil-to-distributor lead. Position yourself over the radiator and place each of your hands on opposite sides of the crank pulley. Pull up and down alternating on the left and right sides by grasping the belt that goes around the crank. There should be no movement; if there is, then the keyway or key that mates the pulley to the crankshaft is bad. In this case, you really don't have a timing problem, it just looks that way because the timing mark is moving. This rarely happens but I HAVE seen it.
Hope this helps.
[ July 25, 2004: Message edited by: stuart ]</p>
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