Tech Topics

The New England M.G. ‘T’ Register has a long history of providing its members with advice and technical expertise on the restoration and maintenance of our special cars. We present some of our favorite technical articles and tips here for members and visitors alike.

Peter Roberts question:

Q: Why did the coil pack in on a perforat­ed carb float? Or, more to the point, why did running the engine three miles on two cylinders cause the coil to die?
A: A coil is a transformer and any abnor­malities in the secondary will be reflect­ed as abnormalities back into the pri­mary. With a flooding carb the plug may wet so that it looks like an electrical short. Maybe Peter's coil was about to fail and the abnormal operating condi‑
tions caused the primary to fail. The condenser across the primary will nor­mally provide some degree of protec­tion if the coil is sound.
Just a thought,
Peter Ross, TC8892
Reminds me: When driving across Wisconsin almost 20 years ago, laziness casused a late downshift for a hill and the engine lugged in top for a moment. Pop! down to three cylinders. A few minutes later, the thumb on plughole test indicated loss of compression on number 1 cylinder and it seemed to run a bit better without the plug installed. Best guess was a burned valve. Off we went. About 5 miles later it quit. Distributor rotor failed. Installed a new one which lasted a further 5 miles. Fortunately we had another, but what was the problem?
When the distributor contacts open, the magnetic energy stored in the coil is transformed into a high voltage in the secondary windings. It has to go some­where. Like lightening, it tries to find the shortest or easiest path to ground and this is designed to be the plug gap. We had a dangling plug wire so I fig­ured the closest path to ground was through the rotor to the center spindle of the distrib­utor. With each discharge, a spark through the rotor would deposit some carbon to the point where a per­manent conductive path was formed and no sparks could get to the remain­ing three plugs.
Our solution was to tape the plug to the generator, grounding it, and to reconnect the plug wire so the spark had somewhere to go rather than through the rotor. Did we have an "external combustion engine"?
The solution worked and we made it the 200 miles to Madison where we pulled the head and replaced all four-exhaust valves before continuing on schedule the next morning. We are ever grateful for the help the local MG enthusiasts provided.



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Disclaimer: All Tech Tips presented here are copyright of The New England M.G. ‘T’ Register. Be advised that every effort was made to verify the validity of each tip. But as in any free advice, it’s up to the user to judge the usefulness of each tip presented here.


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