If the shaft was broken I would bet it would be a whole lot worse,under acceleration or high rpm the timing would probably slip beyond runable.

Does your timing chain have all steel gears ? or the plastic coated ones ?

I've seen this happen once on a car where the intermediate shaft gear and the gear teeth on the cam were almost totally worn away.

Pull the distributor and look at the tang. Also pull the intermediate shaft and check for wear.

I stand corrected.
I should have stated distributor drive gear.
"it appears to be retarded. Because I have to advance it to put it back to spec.
This is with a timing light?How does it run when you start it with corrected timing and how long?
Distributor bolt is tight but is the distributor itself tight?

Once completing a short drive, maybe fifteen or twenty minutes, I stop the car and it will not restart. This is when I discover that the timing is again off. It does not do this every time, however.

My thoughts too. The only place where something like this would be likely to happen is in the mechanical advance mechanism.

R.

take off the distributor cap and grab the rotor and turn it , it should move fairly easy a small amount and then spring back when you let go.

Agreed, twist it CW/let loose/see if it retracts all the way. Then as needed open it up & good visual inside

Have you determined if the distributor housing is itself moving under the clamp after your drive?

with it being hit or miss it'll take some time to see if the new dist takes care of it. If desired send the old one to me & I'll figure out what was wrong with it & if you want it back just pay the shipping. On the new one you might note: what springs are in it, the # on the plate and the # on the can arm (& what the orig one had). EDIT when the timing does shift, how much and which way ???

Also, every time I set the timing, it had to be set ahead, because once you tightened the clamp on the dist down, the timing mark would move. Actually watched it happen while friend tightened bolt.

Did you guys miss this one?

Did you guys miss this one?

Sounds to me like either the dist has surface depressions where the hold down sits, or something isn't as flat as its suppose to be. As he tightens the hold down, its pulling the dist back to where someone really cranked the hold down to.

I had a 360 one time where the block at the distributor was not cut flat. Every time you set the timing, and tightened the clamp, it would return to the original setting. I ended up replacing the dist, the hold down bracket, and making a soft shim for under the dist!

Even after that, the idea the timing is retarding itself enough for the motor not to start is something I'm having a hard time dealing with. I suspect there has to be a timing chain issue. I've dealt with that before as well, but that is a story for another time. Gene

I saw that but it was somewhat confusing to me.

Why would someone set timing, lock down the distributor, see that it moved and leave it like that ... assuming he even checked it afterwards?

He did say he changed the chain a couple years ago in the original post ... or did you miss that ??? ... so unless he put a crappy chinese piece of junk in there, or has driven 100k miles in that last 2 years, then the chain may not be the issue ... but stranger things have happened.