Short circuit across points

Short circuit across points

Postby Steve6 » Tue Apr 23, 2019 10:02 am

Hello all, my 1948 P3 is misfiring and I have been looking at confirming the timing is correct. I was puzzled to discover that although the engine does run, I still seem to have continuity across the contact breaker points when holding them apart with the probes of my multimeter. Removing a wire or two seems to suggest the continuity seems to travel back through the coil to the SU fuel pump ( which runs off the switched side of the coil and is only 3 ohms resistance to earth) This doesn't seem right to me, am I doing something wrong? I can't figure how the engine runs, albeit badly, when the contact breakers don't even...er...break!
Any suggestions gladly received, thank you.
Steve
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Re: Short circuit across points

Postby TonyG » Wed Apr 24, 2019 8:19 am

Steve,

Electrical issues are almost impossible to determine remotely. However, while I’m not saying that what you are seeing is correct, it sounds likely that you will have continuity testing the way you are. With the 12volts removed the components that it feeds will have continuity to earth or they won’t work when the power is connected. As such, I suspect you are misleading yourself a bit here.

Time to go back to the problem which, I assume, is poor running? I’m also assuming that nothing new has been fitted and that all wiring is as it was when the car was running ok. Also that you have new petrol in tank since unleaded degrades over time. What is the spark at the plug like when you put a spare in one of the leads and earth to the cylinder head and turn over on starter? If looks ok, next step is to check the points gap and that the contacts are clean and not pitted. Once done, check and set the timing and you will need to take the CB or +ve (assuming pos earth) wire off coil and connect your meter so you can see earth through points and adjust to just opening when on correct timing mark.

If car fails to run correctly, new plugs may help. I’ve found that NGK work ok but seem to fail more often than the old Champion ones so worth changing. Pulling off each lead in turn while engine running will help identify problem. If points were burnt and pitted a bit it might be worth changing the condenser in the distributor.

At this point you will have correct timing, clean points, plugs and, obviously, all HT and LT connections sound plus a good spark at all plugs and new clean fuel. If it still runs badly, is it one cylinder? If so, a compression test to rule out head gasket or sticking valve. Then onto the carburettor, which should only be looked at once you know everything else is ok, so hopefully won’t be necessary.

Hope that helps a bit.

Tony.
Tony Gilbert

P1 12 Tourer
P2 12 6 Light Saloon
Discovery 3
Discovery Sport
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Re: Short circuit across points

Postby Steve6 » Thu Apr 25, 2019 12:14 pm

Tony,
Thank you so much for taking the time to post such a comprehensive reply. I think you may be correct, I'm fooling myself and not thinking things through correctly. Since I posted, I decided to have Martin Jay at the Distributor Doctor rebuild my distributor. It may not be the cause of the poor running, and I might be wasting my money but at least I will have total confidence in the distributor. Once it's back and refitted, I will follow your suggestions.
One thing I noticed as I removed the distributor, is that there are 2 possible location holes for the contact breaker plate to engage with the pin on the advance retard mechanism. They look about 10 degs apart so obviously a significant timing difference. One is marked something like 6A so is probably the one for my 75.
Thank you again, I will report back what I find.
Regards
Steve
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