How to Time a Farmall H-4 Magnetoby Don Bowman
The Farmhall H-4 is an antique tractor with a magneto in place of a standard distributor. Magnetos produce their own power to create a spark, as opposed to vehicles that use a coil to store power. Magnetos are a good choice on a tractor, since they require no power to operate; in fact, the magneto-equipped Farmhall H-4 does not need a battery to function. The engine is hand-cranked from the front.
Locate the No. 1 cylinder. This is where the spark plug closest to the radiator will be on a Farmall. Pull the spark plug wire and remove the spark plug, using the spark plug socket. Bring the No. 1 piston to top dead center on its compression stroke. Place your finger in the spark plug hole so that you can feel the pressure of the air being forced out when the piston rises on the compression stroke. This is safe, and you will only feel the air being forced out past your finger. Have a helper slowly crank the engine, by hand, until you begin to feel the pressure building. At this point, stop cranking the engine.
Insert the long, thin, screwdriver in the spark plug hole. The screwdriver is lying atop the piston at this point. Place a finger on the top of the screwdriver handle and have the helper very slowly crank the engine. At this point, the piston is still rising on the compression stroke. When the piston reaches the top of the stroke, it will begin to descend again. Your primary objective is to feel and observe the screwdriver rising with the piston and note when it stops rising and begins its descent. The piston must be at the very top of the stroke. When the screwdriver begins to move downward, stop and crank the engine in the opposite direction a small amount to bring the screwdriver back up.
Remove the screwdriver when top dead center has been ascertained. Install the spark plug and tighten it snugly with the spark plug socket. Do not over-tighten it. Remove the distributor cap on the magneto, using the screwdriver.
Insert the magneto into the engine about two inches and stop. Rotate the rotor so that it is facing the front of the engine. Install the magneto and loosely tighten the magneto hold-down clamp, using a wrench.
Install the distributor cap by hand only; do not tighten it down. Slowly lift the cap and make a note of exactly where the rotor is pointing inside the cap. Make a mark on the cap to reference this. Look at the cap and note which spark plug terminal is the closest to the mark. If the mark is to the left of a terminal, rotate the magneto to the right so that the rotor is pointing directly at this terminal.
Lift the cap slightly to confirm that it is pointing directly at the chosen terminal. Install the cap and tighten the screws. Rotate the magneto counterclockwise 15 degrees. Rotate the magneto clockwise slowly and feel and listen for it to make a “snap” noise. When it snaps, stop and tighten the magneto hold-down clamp.
Install the spark plug wires in the cap with a clockwise rotation. The magneto spins clockwise, and the firing order is 1-3-4-2. The first cylinder near the radiator is No. 1 and the cylinders are numbered in series to the back of the engine. Install the No.1 plug wire, followed by the No. 3 wire, which is the third cylinder back, then continue installing the plug wires clockwise.
Start the engine and loosen the magneto hold-down clamp slightly to allow the magneto to move. Rotate the magneto slowly, only a slight amount in either direction, until it runs smoothly, then tighten the clamp again.
Things You'll Need
- Spark plug socket
- Set of wrenches
- Long, thin, common screwdriver
Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).