By Vic Syracuse, EAA Lifetime 180848
This piece originally ran in Vic’s Checkpoints column in the February 2023 issue of EAA Sport Aviation magazine.
I recently had the opportunity to work with some customers who were having engine problems on new installations. While working with them to help diagnose the problems, it became clear that not everyone understands the nuances between magnetos and electronic ignition systems (EIS). Hopefully, this column will help with some simple explanations for you.
In one case, a whole lot of money was spent, and a lot of time was lost getting a newly completed helicopter airborne. During the initial hovering test, exhaust gas temperatures (EGT) were extremely high at more than 1,600 degrees. My experience has been that it is hard to achieve 1,600 degrees EGT on a normally aspirated piston aircraft engine. Cylinder head temperatures (CHT) were also high, forcing short test periods. Another data point was that indicated manifold pressure during the hover was close to 28 inches rather than the normal 19-21 inches that is usually needed for this model.
The helicopter had two magnetos, and I queried the owner about the ignition drop when they were independently selected during the run-up. The answer was that he wasn’t seeing much of a drop. In the meantime, the fuel system was completely cleaned and the fuel servo and distribution unit were sent back for testing and calibration. Next, the engine was suspected, even though it was a brand-new Lycoming IO-540. It was difficult to get a smooth idle, which is not totally abnormal for a new engine before breaking it in. I finally asked the owner to disconnect the P-leads at the magnetos, start it, and go hover. Guess what? It worked. All engine temps and power settings were normal.
Now it was time to figure out why. When I was in the United States Air Force tech school at Chanute Air Force Base in 1975, one of the things they told us was that there would never be two causes for the same problem. My experience has been that not all theorems hold up in real-world situations, as we would find out in this case. Now that we knew it worked properly with the P-leads disconnected, it became a wiring problem. Why? Well, magnetos are a simple, self-contained ignition source that require no power. The only reason that we have a wire connected to them is so we can ground them to stop them from firing.
A quick check with an ohmmeter showed that one of the P-leads was shorted to ground. In other words, this magneto was prevented from ever firing, regardless of the key position. That would explain the high EGTs and lack of power, as the engine was only running on one magneto. For those of you who are curious, pay attention to your EGTs the next time you do an ignition check. You will see that all the EGTs rise. If any don’t, then most likely you have a fouled plug.
The cause of the shorted P-lead is a story in itself. This was a new builder, and the whole avionics and engine harness was provided by one of the well-known shops. As with most new builders, and A&P mechanics as well, electrical wiring is not really a strength. P-leads to magnetos are made of shielded wire. The shield is connected at the magneto case with the center lead attaching to the magneto post. Not knowing it was shielded wire, the builder crimped a ring terminal to the shielded wire without separating the shield from the center connector. Now there was a short between the center connector and the shield.
Some of you are now probably wondering why it didn’t show up in the ignition check. For those who don’t know, if this was the only problem, the engine should have quit running during the ignition check when the key was selected to this magneto. Why? Because selecting LEFT or RIGHT with a key switch grounds out the other ignition. With both magnetos grounded, there should have been no source of ignition. However, this engine kept running.
This was the second cause of the problem. There was a clue given earlier in the discussion to which we should have paid more attention. Remember he stated there wasn’t much of a drop when the ignitions were checked? Hmm. Discounting that clue due to it being a new engine was a mistake. It turns out the ground wire had not been connected to the key switch. So, neither magneto was being grounded during the ignition check. It kept running on the single, properly wired magneto. Fixing the short and connecting a ground lead at the ignition switch has everything working properly now.
The most common ignition setup we see on RV aircraft is one magneto and one EIS. It’s also my favorite setup. The initial installation of these two systems also causes some problems for first-time builders. Remember the magneto needs a ground to STOP it from firing, but most EISs need power to START them firing. Some have a key switch option so you can still use a key switch. For those using toggle switches, you need to remember that the magneto switch needs to be OPEN for it to work, while the EIS needs to be CLOSED for it to work. We’ve seen some builders forget this, and install both toggle switches the same, then wonder why the magneto won’t work.
There’s another nuance between magnetos and EISs that can be confusing to new builders, and even some A&Ps. When timing a magneto to an engine, two timing operations need to be completed — an internal magneto timing and a magneto-to-engine timing. The internal magneto timing usually only needs to be done at installation or removal and replacement, such as for a leaking magneto gasket. The internal timing should be done by a knowledgeable person. Of course, there are plenty of YouTube videos out there on how to do it. You will need a magneto timing pin, such as T-118 for Slick mags or a Mag Lock tool for Bendix mags. It’s a simple matter with Slick mags to look at the magneto data plate for left or right rotation and then insert the timing pin into the proper hole on top of the mag until it seats. Then make sure you keep that pin in place until the magneto is mounted to the aircraft.
Be careful not to move the propeller until you have removed the pin from the magneto or you will damage the magneto.
The next step is timing the magneto to the engine. For this you need to look at the data plate on the engine. It should specify the angle before top dead center. For most Lycoming engines, this will be either 20 degrees or 25 degrees. It is important to place the No. 1 cylinder at the same setting as specified on the data plate. You can do this by lining up the hole in the starter case to the timing marks on the flywheel, or by using one of the various timing indexing tools available. Again, please be sure you are on No. 1 cylinder and at 20 or 25 degrees before top dead center. Verify this by placing your finger on the spark plug hole and feeling the compression as it comes up to top dead center, then back it off to the proper setting.
I have seen one person just move the flywheel to the timing marks without checking if the No. 1 cylinder was on the compression stroke. The engine did not run, as the magneto was not firing on the cylinders at the correct time.
Warning: Be sure the ignition switches are off or at least one spark plug from each cylinder is removed so the engine won’t fire and potentially kill someone with a prop strike.
Once you are in the correct position, the magneto can be secured to the engine, but not tightened down completely. The next step is to use a “buzz” box connected to the magneto, with the ignition switch in the ON position (or the P-lead disconnected), to fine-tune the magneto timing to the specified engine timing. Then torque the two mounting nuts to the proper value, and you are done. Be sure to reconnect the P-lead if it was disconnected, and to turn OFF the ignition switch.
With EISs, the timing is easier, but done differently. Most of them are timed at exactly top dead center, and you need to be sure to do just that. If you time them at the 20 or 25 degrees before top center like a magneto, you could damage the engine. I test flew an RV-10 once in which the builder had mistakenly done just that. The cylinder head temps were almost redlined by the time I was 100 feet above the runway. Reducing the power to a minimum that would get me back around for a landing worked. On the next flight I quickly shut off the electronic ignition, and everything stabilized. That’s how we discovered the problem.
The main difference is that magnetos have “fixed” timing, meaning the timing does not change, regardless of power settings. It always fires at the same 20 or 25 degrees before top dead center. The real advantage of an EIS is that the timing will change according to power settings. At high power settings (such as for takeoff) at sea level, the timing will probably match the magneto. But as you climb to cruise altitudes, the timing will advance, sometimes to as much as 35 degrees. It makes for a smoother, more efficient engine. Starting is also much easier, with less chance for kickbacks, and the idle can usually be set lower.
Hopefully, this short column has helped some of you understand the operation of magnetos a little better. They are really simple devices, quite old technology-wise, but dependable if set up properly. Without some basic understanding, simple mistakes when installing them can really put a damper on the fun factor.
One piece of advice I would give for new builders with initial engine problems is to take a step back and get someone who understands your setup. Start with basics before spending a lot of money trying to fix things. For 95 percent of the amateur-built aircraft being completed today, the engines have most likely already been run on a test stand. Your aircraft is what has changed. So, have someone thoroughly look over your installation of fuel, electrical, and oil systems prior to that first engine start. It may help keep the fun factor alive.
Vic Syracuse, EAA Lifetime 180848, is a commercial pilot, A&P/IA, designated airworthiness representative, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more than 10,000 hours in 74 different types. Vic founded Base Leg Aviation, authored books on maintenance and prebuy inspections, and posts videos weekly on his YouTube channel. He also volunteers as a Young Eagles pilot.