My mechanical advance setup was initial timing approx 17°, total timing 34° at just under 3000rpm which is fairly conservative on the rate but is nice and safe. With a car as light as this I have found that increasing the rate and getting the timing in by 2500rpm, as many recommend, makes little difference to the "seat of the pants" dyno.
I know that my idle vacuum on purely mechanical advance is somewhere around 8-9" mercury so if I add vacuum advance from the manifold source, in order to avoid hunting at idle it needs to be fully deployed at 8". If the vacuum cannister is in the middle of it's range at idle it will wander around and give an unsteady idle as the timing is always changing.
I set the vac unit to 0 turns (fully clockwise) on the adjuster. This was loose enough for the vac advance to start coming in at 5.5 and be fully in by 7.5.
I expect that the extra advance at idle will make it run better, with a higher vacuum which means that I can probably bump up the vac setting to come in higher. I want to avoid any crossover at light acceleration when the vacuum advance is still slightly activated but the mech advance is coming in with rising revs.
I started the car and immediately the idle vacuum jumped up to 11". This was great news. I gave the vac cannister 3 turns on the adjuster. This now means that the vacuum advance comes in at 7" and is fully deployed by around 9 to 9.5". My idle timing with vacuum connected was 26°. This means I'm getting roughly 9° vacuum advance which is fine for now. It takes a bit of fiddling to get the limit adjuster in just the right place and is largely trial and error. Having the vacuum "start" at higher vac levels means that the vacuum advance will definitely have completely dropped out when I accelerate lightly so there is no risk of pinging due to too much timing when the engine is under load.
I was able to lean off the idle mixture screws which gained another 1" of idle vac to give a total of 12" but due to the fact the engine was now running much better, the idle speed had come up to 1050rpm. One quick adjustment on the idle screw and it was back down to about 850/900rpm with 11.5" vacuum. This is the best its ever been.
Out of interest, to make sure I fully understood everything, I lowered the idle speed and as soon as the idle vacuum dropped below about 10" the idle speed started to vary and wobble around a bit. I got a timing light on the damper and could see the timing changing, due to the problem described above - the vac can isn't fully deployed and the timing varies, giving an unsteady idle. As soon as I bumped the idle speed back up again, the manifold vacuum returned to above 11" and the timing was then steady and all was well - the vac can was now fully deployed and the timing wasn't changing. The engine will idle steadily with the vacuum advance fully deployed at 800rpm for 10.5" idle vac but when the fan comes on the extra load drops the idle just a little and then the vacuum advance starts to drop off and leads to an unsteady idle. That's why the final idle speed setting took this into account and was set a little higher to give some margin. The final value that I settled on was as follows:
Idle speed: 900rpm
Idle vac 11.5 in.hg
Base timing: 17°
Vac timing: 9° fully deployed at 9.5-10 in.hg. Starts at 7.5 in.hg
As an extra bonus job, I removed the steering column bracket and welded some triangulation onto it to make it stiffer as the steering wheel had been vibrating a bit at certain revs (about 1600rpm). Only took a few mins, didn't bother painting it as I wanted to drive it first to see if it needed any more welding first.
Went for a drive and it was really very nice. Smoother down low although it doesn't like accelerating hard from below 1500rpm as its out of the operating window of the cam. The vac at cruise has raised from 13" to 15" which would indicate that the increased cruise advance is well received and is allowing the mixture to burn more completely. The steering wheel also wobbled a lot less so I will remove the new bracket, paint it and refit. The car has always been rich on cruise to prevent surging, as suspected I think I can go another stage leaner (4%) on the mixture rods for cruise because the vac advance allows the leaner mixture to burn properly and save maybe 1mpg which is nice. No surging except at extreme driving conditions e.g below 1200rpm in 3rd and it bucks slightly but good heavens if I think back to how it used to be its like a different car. All these small continuous improvements have added up to make a massive difference.
Next problem to solve - rich bog during hard cornering and braking.
