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MG MG Y Type - YA will not start

YA will not start.
Car has been running 4 weeks ago, but with backspitting of petrol clouds through both the twin SU carbs and out through the air filters.. Ignition problem diagnosed and entire 50+ year old ignition system replaced: coil, coil HT lead, distributor cap, condenser, points 12/1000", rotor arm. Carbs balanced with 1000rpm idle.
Choke sticky, but made to open, and good.
Battery good. Starter motor good.
12.5V at coil switch. 0.5V at CB switch with contacts closed. Closed contact flick test on points generates no spark to good earth.
I've already sent one dud coil back. I can't have another dud, can I?
A J R Felthouse

Check the rotor arm and replace it. Very often they fail these days. Had that happen to me one time. Spark was going straight down into the sump rather than thee plug leads. Hair-line crack in the top that you could hardly see!

Paul
Paul Barrow

Hi When you do the flick test on points you are opening the circuit,is there a insulate on points in the wrong place? You can test at the coil by removing the CB wire and flashing it there.Some new contacts need the faces cleaning with a rag to make them work from new.
Bob Wood

Sorry,removing the CB wire will no work as you will have taken the condenser out of the circuit.
Bob Wood

Can I make a few comments which may help. The first thing that struck me was that you say there is 0.5 volts on the points when they are closed. If we can assume that the meter is accurate and you have a good connection on both leads there should be zero volts here at this time as the points short down to earth when closed. The half volt must therefore be being developed across some resistance between the points and the earthy side of the battery. this could be due to dirty points (hold a screwdriver across them) or due to them being incorrectly assembled, the fibre washer under them is often missed. If this is OK look at the circuit back to the battery earth side. As this circuit is through the metalwork of the engine and chassis I would double check the engine earth strap and the battery earth connection. A quick check would be to run a wire from the battery earth connection to the body (metalwork) of the distributor as see if the voltage disappears. Otherwise don't assume that just because something is new that it is serviceable. Many modern reproductions are not a patch on the quality of originals, especially the capacitors/condensors. It is good practice to swop things one at a time and put the original back on the grounds that the chances of two faults occurring at once are smaller than fitting a faulty part, leaving it in and thus now having two faults. Once you have found the fault the new parts can be fitted and checked one at a time or carried as spares. As I reread your symptoms it strikes we that the spitting back through the carbs could easily be down to incorrect timing or mechanical problems. loosen the distributor and turn it both ways and see if that improves things. The cam drive one tooth out or sticky valves are also a possibility. Good luck in you quest for reliability.
Ian Thomson
Notts. UK.
ian thomson

Update.
Coil now registers battery voltage at CB terminal with LT wire removed and points open. Zero with LT wire attached.
Earth inside distributor.
I'm not sure that I have the LT terminal isolated correctly, but I can't find a picture of whether there should something missing e.g. insulating washer, somewhere? It seems odd to me that the only insulation at the LT terminal is the small, shouldered plastic cylinder.
Am I missing something obvious here?
Wouldn't be the first time by a country mile.
A J R Felthouse

Hi If you click on technical centre on this website there is a video of a set of points being fitted.There is a insulator on both top and bottom of the wire post. Some brands of points had a insulator under the pivot post.A little dab of grease on the cam plus a drip of oil under the rotor helps keep the advance weights in good working order.. BOB THE POM
Bob Wood

A salutary tale:
When overhauling the distributor contact points, don't forget to make sure that the condenser and points contacts with the LT terminal are on the inside of the insulating space, or the CB current will earth out to the distributor body, reading zero at the coil CB terminal with the LT lead attached, and battery voltage with it off.
Many thanks for your help, gents. I can heartily recommend the Lucas Overseas Technical Correspondence Courses, Part 3 covers Coil Ignition, to help one to understand the theory behind these fascinating machines.
Mind you, what a palaver; if it hadn't have started on Sunday night, I was up for buying an electronic ignition conversion kit on Monday.
Anyway, the engine runs as sweet as a nut, the back spitting problem has stopped, and now it wants leaving alone for a bit, while I watch the dynamo dying.
A J R Felthouse

This thread was discussed between 05/06/2020 and 10/06/2020

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