Jon, Just to double check, True or false?
- TDC (white) should be showing up with timing light at idle at 750rpm and thats 0 degrees.
- Static timing mark should be 10 degrees (at what RPM)?
- Mechanical advance at 3000rpm should therefore add 25 degrees = around 35 degrees
- Mechanical advance does not rely on vacuum but is some kind of additional amount added by the distributor itself not the vacuum unit.
Hi Nick -
Not quite - at idle, the distributor will just have static advance - so 15 degrees or whatever (15 degrees before TDC).
Then just as you come off idle, vacuum advance kicks in and adds something like 10 degrees more advance.
Then as then engine continues to accelerate up towards 2500 RPM mechanical advance progressively replaces vacuum advance as the vacuum in the manifold falls away, so by the time you've got full mechanical advance, total advance is static + mechanical (and vacuum is zero).
Mechanical advance works by some weights in the body of the distributor moving out under centrifugal force causing the shaft to twist (advance again).
In terms of the marks, I would guess that each serrated point represents 5 (since they do on mine) degrees, and the furthest right serrated mark (to right of red mark) will be TDC.
So if that's the case, then the red mark is only at 5 degrees, so don't know why that would be picked out in red. It could be that the engine builder actually marked that as the true TDC and so the white mark would be the static timing mark at around 12 degrees or something. In which case your 'all in' 35 degree mark would be two imaginary serrated teeth beyond the end...
If the timing light shows it on white mark at idle, my theory seems plausible.
This engine is weird, it runs badly at low revs but goes completely berserk at high revs, that and the compression (Mike I checked with camera from work, and yes, it's got flat top pistons, good news, I think). Idle is erratic, but when foot down positively demonic, can overtake easily, accompanied by the smell of gas and rubber...
I think if you've got no vacuum advance, and only 10-15 of static advance, then it's not going to pick up well at low revs. You need more advance. you could test it by just adjusting the idle advance to the furthest left mark - it should pick up better, although don't run the engine fast because you'll probably have too much total advance giving you preignition etc.
the erratic idle is probably down to a small air leak or imbalance between the carbs.
Mike, I guess the additional timing indicator allows 1) markings to be accurately applied to large surface while engine running for tweaking 2) allows for the fact that there are no dots on the belt pulley thingy, only one mark visible (will clean it up) 3) all can be done at the business end of the engine easily visible while fiddling with carbs at least that's what I'd do if I built it.
And then, bear with me while I learn (limited understanding I only manufacture telescopes for a living), but are we saying that:
a) there is no need for vacuum advance with these Jag carbs, and there never will be because this engine is designed to run full-tilt most of the time?
B) Or is it because there isn't enough vacuum with a 2" bore designed for 4.2 litre jag (Venturi rules, aperture too small) and therefore, no point in even using the vacuum advance?
C) Totally on wrong track post is utter rubbish
I'm all ears! Nick
I'm inclined to go B or C
it could be something else, like it once had triple carbs (where the manifold runners for each cylinder aren't connected, so unless you get very complicated plumbing, the vacuum advance line only gets to see vacuum of one cylinder).
I'd suggest next steps to confirm total advance is somewhere near 2 imaginary teeth beyond the end, then get the carb balance/mixture/air leak all sorted out then come back to optimising the ignition timing. (which would involve either getting a replacement vacuum advance unit, or setting higher static advance and modifying the mechanical advance to add less to keep the total at 34/35.
