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Tune-Up Tips: The Engine
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 a very good chance that at least one tractor in your collection will have either a distributor or a magneto (its predecessor). The difference between the two is that a distributor requires the engine to have a battery and charging system, while the latter produces its own electrical energy. What both have in common is their service procedures are similar and they also have a tendency to be ignored when it comes to maintenance.
When discussing an engine’s ignition system, it must be understood that there are primary and secondary circuits.
The primary circuit is low voltage (battery voltage or less) and consists of the breaker points, condenser, ballast (if used), and the primary side of the ignition coil.
The rotor, distributor cap, spark plugs, and wires, along with the secondary side of the coil, make up the secondary circuit.
An additional task that the distributor handles working through the primary circuit is the timing of the ignition event or arcing of the spark plug. The primary ignition controls when the spark plug fires through the secondary circuit.
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 the points separate is the time when the plug fires.
When the points open, the electrical field in the coil is considered to collapse. This 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 spark plug 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 circuit so the points do not arc and continue saturating the coil. The gap of the breakers controls the saturation time of the coil and also the timing of the spark plug firing. Either can be measured with a feeler gauge or by a dwell meter.
Dwell is the length of time in distributor cam (the lobes) rotational degrees that the breakers are closed and saturating the coil.
A 30° dwell means that the breakers are closed for that amount of distributor cam rotation. As the rubbing block on the breakers wear, the dwell goes longer (higher) and the timing retards.
The cam needs to have a small amount of special lube placed on it to greatly reduce the rubbing block wear. Often this step is overlooked.
Also, the points need to be set properly and precisely for the engine to run its best.
Theory behind advancing
Every distributor has a 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° of advance means that the spark plug is arced when the piston is 20 crankshaft degrees of rotation prior to reaching TDC (BTDC).
The total timing is the cumulative result of the base ignition setting, plus the advance via the centrifugal weights. Many tractors have stuck weights, and this will greatly 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 fused for fuel economy at light load.
The key 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, the wick under the rotor with engine oil to keep the cam free to advance, and 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 may be excessive at idle.