Vacuum advance

[Castanley]

Hi all, I hope everyone is well and coping ok with all matters viral!

I was hoping for a little advice regarding the vacuum advance system on my old 1952 Series 2. I've known for a while that it hasn't been working correctly, the diaphragm wasn't holding a vacuum and therefore the advance wasn't... advancing!

I'm a total novice tinkerer, so learning things as I go along. My understanding is that the carb generates the greatest vacuum under idle conditions (counter-intuitively) and that's when it should advance the ignition timing to... improve the idle? My idle is actually a little weak and a bit lumpy on pickup but other than that the car runs great.
Anyway, today I decided to remove the dizzy and disconnect the vacuum unit. But before I did that, I attached my vacuum gauge to the line that goes to the carb and fired up the engine just to check a vac is actually being produced. Oddly it seems to be the opposite of what people have been saying! No vacuum is present at idle and as I increase engine speed, the vacuum continues to climb proportionate to engine speed. I'm so confused.
Well for now I've just sent off the old vacuum unit to an ignition specialist to be refurbished and I'll tackle the installation another day.
Sorry for the essay guys, I was just confused at the vacuum level being opposite of what I expected!

Cheers,
Chris.

[liammonty]

Vacuum advance is there to provide additional advance under certain conditions, and is related to engine load, rather than speed, per se. The engine can benefit from greater advance than it would get from centrifugal advance alone at light throttle openings - too much advance at higher throttle openings would lead to pinking. The lack of vacuum at idle is intentional, and is due to the location of the vacuum take-off on the carb. I don’t remember the exact design of the carb, but I believe the take-off port is covered by the edge of the throttle butterfly at idle. As soon as the butterfly begins to open a little, it exposes the vacuum take-off, and under these conditions you will have maximum vacuum conditions - the engine is sucking in a lot of air through a very restricted throttle aperture. When you open the throttle further, there is less restriction, meaning less vacuum.

Speed also plays a role, as you’ve found, though. You’ll have increased engine revs, while barely touching the accelerator (opening the throttle) as the car is stationary. As the revs pick up, the engine pulls more air in (say twice as much at 4000 rpm as at 2000 rpm) but in the circumstances you’ve described, the throttle position and therefore restriction is the same - hence you have double the vacuum. On the road, though, you would seldom achieve high revs at a very low throttle opening.

Make sense?!?!

[Castanley]

Thanks very much for the reply, that actually does make perfect sense. I was probably looking at things too simplistically by just revving the engine with no load.
Unlike IKEA furniture, the pieces are starting to fit together!

Looking forward to getting my vacuum 'trumpet' back so I can hit the road again...

Chris.

[liammonty]

Super! It’s quite straightforward really once you’ve figured it out. And more satisfying than figuring out how to put together some IKEA furniture too