www.jec.org.uk

I have a 1973 V12 E-Type that has been converted to Optronic ignition by a previous owner. Now that I've put a few miles on the car, I notice that she's reluctant to rev over 3000RPM. I suspected the centrifugal advance but this seems to be ok. The vacuum unit has no effect however and I can hear leaking when I test it. So a new one of those is on the way.

My question is this. What is the function of the vacuum unit? is it advance or retard? The only movement available on the base plate suggests it's the lattter. If so , why have the centrif advancing the timing while the vacuum retards it?

Also there seems to be a lot of lateral as well as rotational movement of the base plate. I'll investigate this further when the new vac unit arrives. But for now I can't find any articles that give a definitive explanation of how the set up works.

Any guidance would be appreciated.

Pat

This link explains what should happen and why reasonably well Pat.

The vac unit and bob weights work in conjunction with each other.

Your leaky vac unit won`t help the situation. Air drawn into the inlet manifold would cause lean running as a starter. Lean mixtures cause an engine to run hotter and damage to pistons and valves can occur if not sorted out.

Leaks will also cause the vac unit to work out of sync with the advance weight unit. A worn distributor bearing set (pedistal wobble) will cause a degree of ignition timing float which would be marked if the car was using points to make and break ignition current. It isn`t usually so critical with breakerless ignition but such wear would still have an effect on the accuracy of overall ignition accuracy.

http://www.hotrodders.com/forum/distrib ... ry-part-1-
a-59033.html

I found this link by typing Ignition Timing Theory into Google. There are several other sites to look at there. One of the best ones was for the 2cv. The article is well written by a man who knows his stuff.

All this gen was bashed into me when I did my licensed aircraft engineers course some years ago. I`d have attempted to explain the situation myself but others have already beaten me to it. Thank goodness for that or I`d be up all night writing myself!

Hope this helps.

Mike

Hello Pat,

I don't know what the link said as I couldn't be bothered to scroll through to find it.

On U.K. spec cars the vacuum module is an advance function.
The mechanical advance advances relative to engine rpm and each distributor is matched to a particular series of engine. (I see many posts on many forums where the distributor is identified by it's type, e.g. 25D6, but the important identifier is a 5 digit number on the side of the distributor (this is Lucas units by the way).)
The vacuum unit advances the ignition depending on inlet manifold vacuum.
It is basically an economy device as it allows the engine manufacturer to set the mixture leaner than ideal for maximum power, but this leaner mixture requires more advance of the ignition timing. Again these units are matched to the engine, and have three sets of numbers, I think, vacuum when it starts to advance, total advance in degrees and maximum vacuum for full advance.

So, essentially it is an economy device, so your new one should help the car's running. (Better if it is the correct module?)

Alec