V12 Distributor Cap

Irma Jean Morgan wrote:

My '73 XJ12's distributor cap has two vent nipples; one on the side and one on the top. When I purchased the car I was aware it was running a little rough. Compression was within limits so I figured carbs, spark, or emissions which I thought I could dope out. When I got to the distributor the vent nipple on the side was open. When I took the cap off I was suprised to find that one of the screws stayed in the mount along with a 2"X1/2 inch slice of the cap base.

I went to the Haynes manual and some of my parts catalogs to check prices and now I'm not sure a vented cap is supposed to be fitted to this model year.

1. Was the wrong cap fitted?
2. Should the side vent hole be open?
3. Can you glue a cap together and save the $50.?
4. Is a cracked cap bound to cause misfires...I can't believe the PO missed it!
5. I'm in the dark here(token Lucas shot)and would appreciate any help you can offer. Thanks in advance for your time.

The distributor cap fitted to your car is what was fitted to the later HE V12 engines. The idea was to vent the distributor with fresh air to prevent fuel fumes building up inside which can happen under certain circumstances. The result being that when the engine is started the distributor cap literally blows up and usually starts an under-bonnet fire which is very undesirable, dangerous and expensive. The 2 vents on your cap on the HE engines one vent goes to the rh inlet manifold and the other to a filter. Yes you will have to replace the cap you cannot glue them back together as the spark will track to earth along the crack, in fact I am surprised the engine ran with sudh a large broken piece. You should be able to buy an earlier cap without the vents, if you can only buy a vented cap you can either block the vents or try to copy the HE set up.