Farmall Cub

Hey guys, does anyone know anything about magnetos? I have a farmall cub that is sparking on 2,3, and 4 but only part time on 1. Does that sound like the coil part of the mag or the whole mag. Does anyone know the best place to buy one..................John

Are you sure the cap is good?

yes It is brand new

Wires? Plug?

there not new but switched them around and 2,3,and 4 all spark and 1 sparks only occasionally.

You have to remember the magneto produces the same consistent spark every time. Whether it is a good spark or a weak spark, the mag has no idea where that spark is going, that is left up to the distribution system.

The coil directs the voltage to the center tower of the distributor cap. The rotor then directs the voltage to the outlying distributor towers in time with the engine.

From there the plug wire delivers the voltage to the plug which is where the spark happens.

Since you report good spark at 3 of the 4 plugs, the cap is new, we know the mag is capable of producing enough juice for a good spark. The rotor is transferring the voltage 3 of 4 times, it is being distributed to 3 of the 4 plugs so by elimination, it can only be the high tension lead or the sparkplug itself.

Double check number one high tension lead and plug.

Normally a 4 cylinder cub will start and run on one dead hole. A running cub will develop a miss if it loses one cylinder and not stop running.

Since you changed the coil, I am thinking that was the original problem that caused the tractor to stop running. In the process of changing the coil it is possible the high tension lead was damaged, not intentionally but from age.

I responded to your question after reading the other mag problem posted... Add the distributor cap to the suspect list which I believe Barnyard already mentioned.

Make sure the high tension leads are actually snapping all the way down into the cap towers, and there is no corrosion on them. Your problem is somewhere between the cap and the plug, not in the coil.

What is the history on this magneto?

Are the symptoms a new occurance or is this your first time using the magneto?

Are all spark plug wires solid (metal) core?

Remove the coil to distributor wire from the distributor - place end about 1/4" from engine block. Hand crank the engine watching the spark. This will seperate the spark distribution system from the spark generation system.

Eugene wrote: This will seperate the spark distribution system from the spark generation system.

This is very, very sound advice. You have two completely separate systems contained in one unit, this applies to both magneto and electric sparked engines.

Trouble shoot each system separately. Make sure you have a good crisp arc from the mag or coil every time it snaps. If it does, then and only then move to the distribution system checking from the rotor forward to include the cap, wires and plugs.

Pulling components from both systems at the same time leads to increased confusion when the engine won't fire when your repair is complete.

I always try to impress upon people to never work on more than one component system at a time without first test firing to make sure what you did works. The distributor or mag often fool people into thinking it is one system.