How did my timing change ??

[Elombard]

73 911T with a 78 3.0 SSIs, Stock ignition CD box, CIS, distributor etc. Took it to a track day ended up running really bad, thought I had over reved it and bent a valve. Took it to a highly recomended shop and pulled the rocker covers, compression, and leakdown etc. Leak in the #6 cylinder and some wear on the cam shaft. Mechanic offers to buy the car for $5K or rebuild the motor for $10K. I put it on the trailer in pieces and brought it home. Reassembled it and got it running again. Car ran the same, very rough idle, back fire on decel, lots of vibration.

Today I towed it to a friend who used to be a P car mechanic at a dealer. He repeated all the same tests. Found number six to be leaking some. Still doesnt explain why it is running so bad. (Oh ya within the last 6 months new plugs, wires, distributor cap and rotor. He looks under the rotor and says "it looks like this distributor is off a tooth". Loosens the nut and adjust the timing ~2/3 of the total adjustment on the distributor. buttons it up and says go ahead and start it. I fire it up and it is perfect!!! Ran like before the ill fated track day. He sets the timing with the light and it runs better than it ever has.

How could the timing have gotten that far out? He said the distributors cant jump, I cant believe the chains jumped on the sprockets. Could the nut have loosened and retightened?? What the heck happened??

I guess I should not complain and just enjoy but I am paranoid that it will recur with worse results.

Time start saving my money for a rebuild...