(In the Jensen's case, the factory probably put the adjustable gears in because they: couldn't make accurate parts?..or they hadn't figured out what the specs should be?...or they figured it was the easy way to have the car meet the emission requirements in different markets. I doubt that they included it with a note in the owners manual or the FSM..."Have fun guys, the engine really rips at 10 degrees after"!)
Interestingly, in the manual there are six specifications given. The gears have 3 marks apiece and they listing timing data and ingition timing data for six combintions, although there are more combinations than this. Now, not all Jensens had this adjustability, late 73s and 74s seem to, 75s seem not too.
Still, I like the other question better - what happens if I need new cams? JH no longer exists. Lotus does. Lotus built the motor and if I call Lotus and order cams for my JH I will not get the same cams that I got back in 1974 because the manufacturer has superceeded these cams with a modern design. Is this legal? I understand in VW land Rabbits do this with some sort of G-grind cam in a similar situation. I don't need cams right now, but I will for my spare motor. My first motor is getting run as is, I'll be the limiting factor anyhow, now motor power production!!! ;-)
As for timing, well, I'm going to run the car and run the timing IN THE MANUAL that feels best.
And, from doing a lot of work on the JH I feel JH put them there to meet emission requirements. This was in the dark ages of emissions and I think the reason they specify different timings is so the things would pass emission but allow the owner to revert them to actually make a little power. For example here are three from the manual:
36 BTDC
56 ABDC
66 BBDC
21 ATDC
26 BTDC
66 ABDC
66 BBDC
26 ATDC
21 BTDC
71 ABDC
71 BBDC
21 ATDC
You'll notice this is all with the same two cams of course, just different factory spec timing. And, there are different cams depending on engine serial number, and different gears too, to make matters more complicated.
It is commonly accepted the cars never made the 140hp they were supposed to, but only around 125hp or so. Therefore, I don't think I'll be passing any of you fellows so don't worry.
Ron