Kings of timing – what do you think of this plan?

[Excaliber]

Generally speaking there is nothing special about a ported vacuum signal, OTHER THAN, vacuum is not present until the throttle plates are opened. No vacuum at idle rpm, full intake manifold vacuum above that. If your using a port that does not give you full manifold vacuum once the throttle plates are open it is somehow 'restricted' and certainly NOT a port I would choose to use. You could check this with TWO vacuum gauges but the readings may 'bounce' and the variance of the two gauges may make it difficult to reach a solid conclusion. It's an 'art' reading a vacuum gauge.

Some feel, I tend to agree, that the vacuum signal reacts faster with a direct intake source as there is inherently at least some initial restriction with a ported signal. There is nothing in the carb to ENHANCE the ported signal, to 'increase' it's strength. It will never be greater than the direct intake manifold vacuum signal.

Depending on what you want to do I can think of a couple of reasons why I would prefer direct intake vacuum over ported.
1. Faster response time (but unless your racing for that 100th of a second, a moot point).
2. To prevent 'run on' when you turn the key off.

With intake vacuum the timing is advanced immediatly upon startup. The idle rpm will go higher with it than without it. To compensate for this higher idle rpm you would have to CLOSE the throttle plate idle adjustment screw for a lower (correct speed) idle rpm. Because the throttle plates are 'more closed' than would normally be required when the engine is shut off it is more difficult for the engine to draw in fresh air\fuel through the 'more closed' throttle plates. This causes the engine to 'starve' and prevents 'engine run on' or 'dieseling' when the key is turned off.

As far as 'crisper throttle response' intake vs ported signal, that leaves me scratching my head. I think it would be extremely difficult to 'feel' the difference UNLESS there is something wrong with the ported signal, such as being 'restricted' TO much.

I to run a vacuum advance MSD dist. I've played around with both ported and intake vacuum and intake works better for my application. HOWEVER, I was disappointed to see in MY case it makes virtually no difference in gas mileage using it or not. Certainly vacuum advance CAN help many motors to get better mpg, but it just depends.

Currently I'm not using my vac advance at all. I DO have an engine 'run on' condition, which USED TO be handled nicely by the vac advance allowing for more closure of the throttle plates on engine shut down. So why did I disconnect it? My feeling is it's to much of a variable in the overall timing control. I like the idea of knowing exactly what my base timing is and my mechanical advance timing is. To compensate for 'engine run on' I turn off the key and let the clutch out at the same time with the car in gear to stop the motor dead instantly.

Why does my engine 'run on'? I have twin carbs on high riser heads with a fairly wild cam. This setup means I need an idle speed of a 1000-1100 rpm, just enough to cause run on sometimes. 800-900 rpm, no run on at all, but the engine will all but die at the low rpm anyway.