Guys, I’m sure this has been discussed many times before. Therefore I apologize for asking yet again. My excuse it that maybe time and experience has changed some opinions.
I am looking at installing a 123 distributor, but the one that allowed fully custom advance curves only works with negative ground.
Any good reasons not to make the change to negative ground?
Prior to this I have found no advantage to change to negative ground and other than the battery my car is electrically stock. I never ‘show’ my car, as its sole function in life is to be driven around at speeds suggesting poor judgement.
You would have to either convert, or isolate the casing of your clock. Nothing much else, really… original radio? Reverse the connections to the ammeter? I think that’s it.
Generally the reason for converting our positive earth/ground cars has been to use an alternator or a modern radio or other music or phone or computer system.
Well, here’s the real reason. I noticed a lot of dwell jitter so I think my dist is pretty worn. Plus, my plugs foul a bit in stop and go and very slow driving. I think a modern ignition would help this considerably.
And. Yes I know I coul just get my original dist rebuilt and install a pertronix module.
The original clock just has a balance wheel with a wiping contact that briefly powers an electro-magnet each time it oscillates. This contact is prone to oxidation which is why the clocks become unreliable and many people modify them. Mine was done by Mike Eck on this list because I still wanted to hear it ticking. I don’t think the original clock will be polarity conscious be unless it has been fitted with a spark suppression diode. If it has, this will need to be reversed during the polarity change.
I changed my XK140MC to neg ground to install a 123 Tune dissy. The Tune allowed me to “fingerprint” my cam and valves and work out an advance curve to match the engine, which was restored before I bought the car. It made a huge difference on power and heat generation. Highly recommended.
My early 120 FHC instrument panel happen to be out now, so I took out the clock to see what’s inside. There is a number 49038 hand written on it in black ink. There is no diode inside. It works with either positive or negative ground.
I can’t get the videos to work, but in these pictures the little coin sized flywheel in the middle is moving and there is a faint ticking.
Looking in the parts books, I see there was no separate entry for the clock. It came as part of the tach (rev counter), which is part 49035/5 for 120 or 49035/6 for early 140, then 51-117-102-04 for later 140, then RN.7460/00 for early 150, then RN.7461/00 for later 150.
So it would appear that the diode that prevents ground switching came along with the later tachs, and once again we find that a piece of advice applies to some but not all XKs.
Would a stand alone battery power a polarity sensitive clock ? Something like a 9 volt PP3.
I have fitted a battery cutout switch and it would be a pain to keep setting the clock .
In my shop, I use a small 12v converter that came with some electronic gadget.
So a battery should be fine.
Either that or a hot wire that bypasses the cutout.
