my timing changed!!
 

my 76 911s, started running poorly today, so i checked the timing and it's off, the dwell is at approx 40 (38 -+ 3 is acceptable), and i can't rotate the distributor enough to get the timing correct! anyone have any suggestions, fwiw, it's incredibly hot here (NY) the last few days, thanx in advance, andrew

Re: my timing changed!!
 

Are you timing off cylinder number one? What are you trying to achieve? 5 degrees ATDC at 950 rpm? Has the distributor been out lately? Is the mechanical advance working correctly? Has the distributor ever been cleaned and relubricated? If the above have been addressed and you really can't reach the correct timing then get the engine at TDC on cylinder one (Z1 at case centerline mark and rotor pointing at notch on base of distributor) and pull the distributor out and reinstall it one tooth over to get closer to the middle of the adjustment range. Jim

as the points rubbing block wears, due to no grease on the cam, the points close up, which results in the dwell increasing, and the timing retarding. so adjust the points.

i'll regap the points, but i did that 2 weeks ago and i was able to get the correct timing for the 911, and it's changed, any reasons? thanx again, andrew

See J Walker's post above; no Bosch grease on cam. Other possibility is that the movable half of the points wasn't tight enough and gradually changed over two weeks.

Check for cracks or tracking marks inside cap or rotor.

But since the 40 deg is still within spec (38 + /- 3)... check to see position of rotor at TDC number 1. It should be at the mark on side edge of distributor. If not, reset distributor at Z1 TDC number 1 cyl and go from there... good luck

anc
It has been my experience that for some reason, the fiber block on new Bosch points wears at an incredibly fast rate. I use Bosch distributor cam grease and have found that in as little as 600 miles, a new set of points will close up due to wear on the fiber block, resulting in retarded timing and a lower idle speed. After a subsequent point adjustment at the 500-600 mile mark, the fiber block seems to wear less, and the points stay where they should for several thousand miles.

Good luck.