Vacuum advance mystery

[madformotors]

I strobe-checked my 2.8i ignition timing, and found that it made no difference to the advance reading if the vacuum pipe was disconnected or connected. It also made no difference to engine speed if the pipe was plugged or not.
The actual vacuum unit on the dizzy is working, using the 'suck' test.
I poked a needle down the tube at the engine end, and found that it hit a dead end. Looking down the opening leading to the (open) throttle plate I've found that there doesn't appear to be a drilling for the vacuum take-off, or at least none that I can see.

Is this a manufacturing fault?
Is there a deep meaning to this phenomenon?
Should I go to Specsavers?
Am I going crackers?

Ian

Edited by madformotors on Friday 24th August 18:14

[RCK974X]

Well Official Ford advice for unleaded gas is to disconnect the vac advance and retard timing a little anyway.....

OE - There should be a small hole which comes out just after the throttle plate when the plate is almost closed - that way the vac advance only operates for small throttle openings.
With a good lamp, you should be able to see it looking into inlet when throttle is wide open.....

Vac advance on 2.8 does NOT operate when idling, as far as I know.

Edited by RCK974X on Friday 24th August 20:38

[madformotors]

Hi, thanks for that,at least it means that the non-functioning of the vacuum doesn't mean a problem. And I did think that the vacuum didn't have much of an effect at idling despite the instructions to disable this function when adjusting the timing. But... there is no drilling by the throttle plate, even checking with a bright light!
Btw I've been running the advance at 9deg bdtc to take account of unleaded.
Ian

[RCK974X]

On mine (1982 DHC) running 95 unleaded, I managed to get to 10.5 BTDC, and it ran better than at 9.

Assuming the gas is pretty much the same as NZ stuff (it probably is...) ....

The vac advance on mine made the engine misfire and buck at low throttle when connected, but engine felt a bit 'flat' on acceleration at 9 btdc.

At the time I moved to NZ (20 odd years ago) there was still 4 star leaded around, so I had a good memory of how it ran.
I also changed to N7Y instead of N9Y plugs - technically too cold, but never had an issue with fouling (and it supposedly helps reduce pinking)

Of course true timing numbers depend on how good your crank marks are, but you might get away with a little more advance....

Even with the 'Premium Unleaded' here which was around for a while and supposed to be 98+ Octane (same as 4 star), engine still didn't run right with vac adv connected.
I understand that unleaded actually burns faster (and hotter) than leaded gas, even for same octane, which makes sense with retarding timing..

I simply tried mine with a little more advance step by step to the point where it pinked/detonated on full throttle, and backed it off slightly, and 10.5 is my best guess - your results may vary, but best timing is advanced as possible where it doesn't detonate (when hot). Mine still feels slightly flat on first response to throttle, but not as bad as at 9.

Sounds like a manufacturing fault on your throttle body....

Hope that helps with a bit of guidance on best timing etc....

[madformotors]

Hi, Many thanks for the detailed information. Since my vacuum advance is effectively disabled, as in 'never worked anyway', I won't fret about that. I may try experimenting with slightly advancing the timing (currently 9 deg BDTC) to the point that I get the best performance without pinking.
Ian

[madformotors]

Embarrassing update! Careful further checking shows that there is a miniscule hole by the throttle plate. I was looking for something about 1-2mm in diameter, in fact shining a bright light down the outer vacuum metal tube shows up as a pinprick dot inside.
Apologies re. talking total bks.
So, the answer to my original question is option 3. Should have gone to Specsavers.