Magnetos are the heartbeat keeping your engine running smooth. Good magneto health is critical to making sure you get the most performance out of your engine. A good understanding of magneto theory is not important for the owner of aircraft. It is important to understand the warning signs that your magneto is telling you and that you can watch the trends of how your engine/s are running.
A fresh set of magnetos and a well-maintained ignition system normally gives an even magneto drop between left and right. There should be an expectation of a magneto drop. The combustion in the cylinders is not as efficient when only one magneto is firing. Noting the magneto drop on a healthy system should be remembered and used as a benchmark for the pilot. Differences in the magneto drop can mean different things.
When one magneto has a less than normal drop the first impression is that a particular magneto is performing better than the other and all is good. This is not the case. The magneto to engine timing is changing. Experience shows that it is likely your timing is advancing on that magneto. Advanced timing on engines is the most dangerous condition for timing. The engine is producing more power but if gone unchecked detonation can occur. Advanced magneto to engine timing normally happen when the contact points wear at a faster rate than the cam or cam follower (depending on magneto configuration).
By design, contact points and the cam or cam follower are designed to wear at the same rate keeping the magneto to engine timing constant. This is the goal of the magneto manufactures. However this does not always happen. Over the years we have seen contact points with improper contact surfaces allowing accelerated wear.
When a magneto has more than normal drop is when pilots normally get concerned because of the loss of power. The cause may not be with the magneto and we will discuss this in a moment. When a magneto is the cause of the drop the problem usually is retarding of the magneto to engine timing. This condition normally is the result of the cam or cam follower wearing at a faster rate. Both advanced and retarded timing can be adjusted on engine IF the adjustments can be made within limits. Magnetos cannot be adjusted on engine and unlimited number of times.
Some of the other contributors to a higher magneto drop or rough engine can be spark plug fouling, spark plug resistance, ignition wire failure or other internal magneto problems. Proper troubleshooting should be performed on your ignition system to determine the best way forward.