Was searching for vacuum leaks the other day and noticed the dizy cap has a pipe on it, closer inspection reveals two and the one on the top is broken. Fine, glue and prayers seem to have worked for the moment though I will get a new one asap, just don't like the look of changing the caps!

But, why does the cap have vacuum pipes on it? How does it all work, anyone got a picture of the inside of the cap? Seems very strange and without taking it apart I can't work out why and I do think I have a leak.

 

The two pipe spigots are for ventilating the inside of the dizzy to ensure no build up of inflammable fumes. The bottom one should be connected to a flexible tube and it should have a small filter in the end which runs to the LHS wing, along the bracer strut. The top one should be connected to a tube that runs to a rubber inlet moulded into the AAV air inlet rubber.
https://www.jaguarclassicparts.com/u...up-to-v-157115

https://www.jaguarclassicparts.com/u...alve-5-3-litre

If your AAV pipe (4 in diagram) is in poor shape, it is NLA; but you can rig something up in a suitable 90 degree pipe using a special spigot that can be inserted into a rubber pipe. The engine then sucks air through the dizzy when all this is rigged up.

 

It’s for ventilation.
One line goes to LH air filter box, the other to a simple inline filter
i believe it should be to prevent explosion of fumes inside the distributor...

 

Thank you so much, I hate mysteries! Makes total sense once you see it and I am pleased that all is coorect now I have repaired the top exit pipe.