Jaguar Repair: ignition timing, crank pulley, jaguar s type

Question
Hi
I have a 67 Jaguar s type and the car has a 3.4 XK engine.  I checked ignition timing with a timing light, but I saw the mark on the crank pulley pointed 0 degree and 5 degree on the next turn.  It kept going back and forth between 0 and 5 degree.  I hooked up the clip from the timing light to NO.6 cylinder spark plug wire which is the first cylinder from the front.

Answer
Hi Yonghee,
There are several things that can make the timing change on each revolution. The distributor is driven off of a worm gear on the crankshaft so you could have free play in that or even in the distributor itself. Check for loose play in the dist. shaft and even the points plate. If the rubber in the front balancer is bad and coming apart it may change the timing marks too or even a loose front bolt in the crank as it can cause the keys to beat out. So check the front large bolt.
Howard