What is the point(s)? Servicing a distributor.June 3, 2020
Author’s note: This is Part 1 in a three-part series. Part 2 covers servicing a carburetor, while the last installment will bring it together to establish how to get the engine to run like new.
Growing up on a small family farm that I still operate today, it was many years before we ran our first diesel. Thus, I became very intimate with the ignition system of a gasoline engine and its needs.
There is an excellent chance that at least one tractor in your collection will have either a distributor or its predecessor a magneto. The difference being a distributor requires the engine to have a battery and charging system. At the same time, the latter produces its own electrical energy. What both have in common are their service procedures and the lack of maintenance they usually receive.
When discussing an ignition system, it must be understood that there are primary and secondary circuits. The primary circuit is low voltage (battery voltage or less). It consists of the breaker points, condenser, ballast (if used), and the ignition coil’s primary side. The rotor, distributor cap, spark plugs, and wires along with the secondary side of the coil, make up the other.
An additional task the distributor has via the primary circuit is the timing of the ignition event or arcing of the spark plug. The primary ignition controls when the sparkplug fires through the secondary.
An ignition coil can be considered an electrical bank. Just as you need to put money into an account before you can withdraw it, the same goes for a coil. The deposit is made via the breaker points into the primary windings. This is called saturating the coil. When the points are closed, the coil is being saturated. Just as they separate is when the plug fires. When the points open, the field in the coil is considered to collapse. It then steps-up the energy from either 6 or 12 volts to a figure in the thousands. It takes approximately 1,000 volts to bridge the gap of the sparkplug at idle and more under load. This step-up in voltage occurs through the theory of inductance.
The condenser is used to absorb the energy from the primary, so the points do not arc and keep saturating the coil.
The breakers’ gap controls the coil’s saturation time and the timing of the sparkplug firing. It is either measured with a feeler gauge or by a dwell meter.
Dwell is the length of time in distributor cam (the lobes) rotational degrees, the breakers are closed and saturating the coil. A 30-degree dwell means that the breakers are closed for that amount of distributor cam rotation. As the breaker’s rubbing block is worn, the dwell goes longer (higher), and the timing retards.
The cam needs to have a small amount of special lubricant placed on it to reduce the rubbing block wear significantly. Often this step is overlooked.
The points need to be appropriately set for the engine to run its best.
Every distributor has the means to advance the timing as engine speed increases. This is the centrifugal advance system.
The flame in the bore expands at a slower rate than the piston travels, so it needs to get a head start to keep up. It is always referenced from when the piston is at TDC. An engine with 20 degrees of advance means that the sparkplug is arced when the piston is 20 crankshaft degrees of rotation before reaching TDC.
The total timing is the cumulative result of the base ignition setting plus the advance via the centrifugal weights.
Many tractors have stuck weights, which will significantly limit the power and the way the engine runs at idle and under load. The rate of advance and the total timing can be checked with a timing light.
A car or truck engine supplements the centrifugal system with a vacuum advance. It is for fuel economy at light load.
Lube it up!
The critical service points in a distributor are the gap of the points along with a small amount of cam lube on one lobe when the breakers are installed. Service the wick under the rotor with engine oil to keep the cam free to advance along with the centrifugal mechanism pivots.
The condition of the return springs also needs to be confirmed. If they are weak, the advance will come in too soon. If they are stuck, the weights will not advance the timing or maybe excessive at idle.
The first step in making a tractor run as intended is the proper distributor function.