From Moss Motoring 1984
Function: The function of the points is to make and break the electrical circuit to the coil. Each time the points open, the circuit is broken, causing the magnetic field around the ignition coil to collapse. When this field collapses, a high voltage spark is created that fires the spark plug. The points must open sufficiently to break the circuit and minimize arcing. The points must remain closed long enough for the magnetic field to regenerate before the next spark plug is fired. The distance the points open is called “gap”, and the amount of time they remain closed is called “dwell”. On a four-cylinder engine operating at 3,500 RPM, the points open and close 7,000 times per minute.
Problem Areas: Each time the points open, a very small amount of metal is transferred from one side of the points to the other. This transfer is uneven, and in effect closes the point gap. The second problem area is the fiber block that rides on the distributor cam and opens the points. This fiber block not only seats itself on newly installed points, but wears on points that have been in use for long periods of time. The combination of wear and metal transfer will eventually close the points completely, and the engine will no longer run.
Symptoms: As the gap begins to close beyond some rather broad limits, the engine will start to misfire under hard acceleration. As the gap closes even further, the engine will start to miss at normal road speeds. Further closing of the gap will result in an engine that is hard to start and impossible to make idle properly. These symptoms are very similar to those of fuel starvation caused by faulty fuel pump, plugged fuel line or filter, or dirty carburettor.
Conclusion: Before taking the fuel pump apart or tearing into the carburetors—both messy jobs—check the point gap.
Additional Information: During the past summer, I assisted at least nine different owners that thought they had fuel starvation problems, when in reality they had points that had closed beyond their limits. It is a good idea to carry a spare set of points in the car, as a badly burned set of points are nearly impossible to set.
Submitted by Paul W. Johnson Moss Motors Club Representative Lakewood, Ohio
'Tech Tip – Distributor Breaker Points' have 3 comments
July 26, 2013 @ 7:33 am Mark Kurtz
I had a similar problem last summer with my 1967 MGB – thought I had fuel starvation issues (pulled the SU’s and cleaned and rebuilt; swapped out the fuel pump; put in an in-line fuel filter…all to no avail) It turned out to be a $3.99 condenser.
July 27, 2013 @ 5:58 am Anthony Rhodes
Back in the old frugal days you didn’t replace the points, you used a points file to flatten out the mountain of the transferred metal. I have a few of those files around. Right now with a Pertronix, I don’t need them, but I plan on going back to points, but I probably will also add an electronic switching unit, so the points do not conduct much current, and there is no charged condenser adding to the arcing.
-Tony
July 30, 2013 @ 2:15 am Rob medynski
You may want to note that the fiber block is plastic on many of the new points. If the points seem to close prematurely say within 20 or 30 miles, the plastic block may be melting usually it is over heating of the points. This is usually caused by the wrong coil. Points operate cooler with a 3 ohm coil. You will usually find when the newer type plastic block point are closing with signs of the melting plastic block, the coil is a 1.5 ohm. Installing a early pure 12 volt 3 ohm coil Moss part number 143-220 will usually fix the problem.
Hope this helps
Rob
British Vacuum Unit