The bob weights were partially seized.
I removed the gubbins and the top plates and just doused it in wd40 and worked it back and forth. It moves nice and smoothly now.
I have removed that damper / check valve / non return valve unit. All I can see it doing is inhibiting the vac advance unit. It is a non return in the wrong direction?? No other vac advance car I have owned has had a similar valve.
I timed the dizzy to 10 degrees btdc and it seems a lot happier, but seems to have a slight flat spot at 3000 rpm. Would different advance springs cure this??
Changing the springs is dodgy.
There will probably be two springs, one thinner than the other, and the thin one will be tight right from the start whereas the fatter one will have one end (or both) with some longways free-play on the end pins it sits on.
On tickover or starting up the little spring stops centrifugal advance from working at all, and the engine can fire up with the ignition as far RETARDED as the static setting allows. This makes for good starting but lousy running.
As soon as the engine starts, the bobweights try to fly outwards, but to do so they must stretch the first spring. Up to perhaps 2,000+ rpm the thin spring will control the advance, and it will allow a rapid build-up of more advance as soon as you drive away and get the revs up, BUT while you are accelerating the vacuum device will hold the advance BACK a bit.
Max advance on light throttle is good for economy and cruising, but max advance on light throttle at any speed will be WAY too much advance for sudden full throttle. The VACUUM device RETARDS the ignition in those circumstances, which stops you blasting holes in pistons from over-advance at low revs with full throttle.
After that modest first rev-limit on the light spring, the stronger spring takes over, and by about 3,500 rpm it will be at full stretch and no more advance can happen. The revs limits vary between engine makes and types, so be VERY careful about changing things. It's dead easy to blast a hole in a piston if you get this wrong, especially if you enable MORE advance at lower revs.
By far the best way to time an engine is to first make sure all the bits are working (you did that) and then time the engine at a good speed, say 3,500 rpm or so, when you know that MAXIMUM advance should be in play.
Below that speed it will be less critical of small variations.
I suggest you buy a new one-way capsule if you have any doubt about it. It is there for a GOOD REASON, and you may do serious harm in the long term if you just dump it.
CharlesY
