Starting Woes, Part 2

By Vic Syracuse, EAA Lifetime 180848

This piece originally ran in Vic’s Checkpoints column in the September 2021 issue of EAA Sport Aviation magazine.

As luck would have it, just as I sat down to write Part 2, I received a text from a friend in another state. Sure enough, it’s Saturday morning and his airplane won’t start after being shut down. I have him describe the situation and determine it is flooded. He tells me he tried a flooded start already with no luck. I ask him to try it my way. Voilà, it starts! As promised, this month we are going to cover the scenarios like this where the engine is cranking properly but won’t start. I am going to break it down by carbureted and injected engines, as the techniques will be different.

Let’s discuss cold starts first. Most engine manufacturers recommend the engine be preheated below a certain temperature. Lycoming uses 20 degrees Fahrenheit as an absolute must. Personally, I preheat mine whenever the outside air temp is going to be below 40 degrees Fahrenheit. It just makes things a lot easier, both in starting and in warmup time. Lycoming engines don’t like to sit and idle for long periods. The lead in the fuel can play havoc with the valve stems and cause sticking, as well as spark plug fouling. Most engines have a recommended oil temperature prior to runup, so preheating can reduce the time spent idling waiting for the oil to warm up.

Preheating helps in a number of ways. First, the engine will turn over faster, especially if you are using the right viscosity oil. I run 20W50 weight oils in all of my aircraft all year long, so I don’t have to comply with seasonal oil changes. Spinning the engine faster will help generate a better spark if using magnetos. A warmer engine environment will also help in the vaporization of the fuel. The fuel really needs to be vaporized in order to burn. That’s right; liquid fuel at the right temperature will not burn. You can actually put a lighted match into it, and it will snuff out the match. I have seen it demonstrated, and it is true. But it has to be at a temperature such that there is no vapor above the liquid fuel. (Please don’t try this yourself.)

Primer lines need to be checked. Copper can become work-hardened and brittle. A crack, such as the one seen here, will cause raw fuel to be dumped into the engine compartment. This sets the potential for a fire.

Carbureted engines are especially grateful for the preheat, as the carburetor usually sits on the bottom of the engine (called an updraft carb). To get things going with the engine, we have to suck the vapor up into the intake. On a really cold day, this can be almost impossible. Many inexperienced pilots will start pumping the throttle while cranking, just like in the old days with carbs on car engines. Pumping worked because the carb had an accelerator pump in it that squirted raw fuel into the intake. On a car, the intake is below the carb, so the fuel ended up in the intake system, where it was sucked into the cylinders and could ignite.

Guess what happens in an airplane when the throttle is pumped? The raw fuel is squirted up, and then it tends to fall right back down into the airbox, thanks to gravity. Couple this with a slow-cranking engine and a backfire, and the stage is set for an intake fire. Most of the time, you won’t notice that this has occurred unless someone from outside the airplane sees it and warns you. If it does, the best thing to do is to keep cranking with the fuel off in the hopes that it will draw the fire into the engine before any major damage is done. However, you will want to check the intake system prior to flight if you suspect an intake fire has occurred.

So, for carbureted engines, a priming system is really necessary for those cold, winter starts. This will put a shot of fuel directly into the cylinders. Of course, you want to make certain that you use the proper primer fittings. They help to vaporize the fuel as it is squirted into the cylinders. It is important that you are ready to start when you prime the engine, so don’t prime it and then continue down some checklist. You should prime and immediately be ready to engage the starter. Otherwise, the fuel will just puddle in the cylinders and not do any good, except to wash the oil off the cylinder walls, which is not any good for your engine.

If you find yourself making three to four start attempts with no noise at all coming from the engine, it might be time to look at the spark plugs. Oily, contaminated, and fouled plugs such as these will make ignition nigh near impossible.

Injected engines use the fuel pump and the mixture control as a priming system. Normal procedure is to turn on the fuel pump and then push the mixture control full forward with the throttle open about one-fourth of the way. Do this for about 3-5 seconds, or look at your fuel flow meter as an alternative. Once fuel flow rises, you should be good to go. Turn the pump off, bring the mixture back to idle cutoff, open the throttle about one-eighth of the way, and then hit the starter. As it starts, move the mixture to full rich, and then back to a leaner position once it is running steadily.

Unless it’s really cold outside, cold starts are not usually a problem. However, if you have done everything right and it still doesn’t start, then it’s time to check the ignition system. Spark plug gaps with magnetos are very critical, and the spark plugs should be checked for proper gaps at least every 50 hours. Remember, the engine is turning over quite slowly during the starting process, so magnetos are not really at their peak power yet. Sure, impulse mags can help, but the plugs need to be gapped properly.

Most aircraft with magnetos will have one mag called an impulse mag. It has a mechanism that “accelerates” the mag whenever rpm is less than 500 in order to help generate a more powerful spark. You can hear the snapping sound made by an impulse mag whenever you pull the propeller through by hand, but make sure the mags are off first. If you know you have an impulse mag and you don’t hear the snapping noise at the top of every compression stroke, then there’s a chance it has failed and you will want to get it looked at prior to further flight.

This next check is only for new installations, or perhaps if you’ve changed out one magneto for an electronic ignition. Most aircraft engines with magnetos are timed at a fixed setting, which is usually around 20-25 degrees before top dead center. If we let the mag fire at this setting during the starting sequence, the engine will kick back and never start. Things can break as well. To eliminate this, the mag with the fixed timing is grounded during the starting sequence and only the “retarded” or impulse mag is allowed to fire. It will fire at some delayed timing, usually top dead center or later. This is accomplished through the key switch, which has provisions on it to ground out the normal mag when the key is turned to “START.” I have seen this happen more than once. Someone replaces a mag with an electronic ignition source, forgets about the START grounding function, and inadvertently grounds out the new ignition source.

Let’s move on to hot starts. Usually, carburetors are the easiest to start when the engine is hot. It is just a matter of opening the throttle a bit and engaging the starter. Sometimes, one pump of the throttle while you are cranking will help as well. The warm temps help vaporize the fuel, and it is going up, right where needs to be. The most common problems I have seen with hot starts on carbureted engines are usually caused by improper spark plug gaps or weak magnetos, most notably a failed coil inside the mag.

Hot starts with injected engines are the bane of all pilots. I’m willing to bet more curse words have been invented during this scenario than any other. This is also the main source of weekend calls to me. The primary reason is a flooded engine by the pilot upon starting, so let’s understand the cause first.

Here you can see the data plate on an impulse-coupled magneto. It shows a lag of 20 degrees, which means the engine timing is set at 25 degrees before top dead center. It will fire at 5 degrees before top dead center during the starting sequence, eliminating the chance for kickbacks and making starting easier.

Injected engines usually have the fuel distributor and injector lines right on top of the engine, where it is the hottest after shutdown. If you look at your CHTs prior to shutting down, they are usually around 250 degrees. You can bet that soon the top of the engine will be close to that temperature, which is clearly above the boiling point of the fuel. After shutdown, there is still some fuel remaining in the injector lines — some of it is due to normal leakage from the fuel distributor, and some is just remaining there because the engine shut down. That remaining fuel now begins to boil in those lines. You’ve probably heard the sound yourself after disembarking if you get near the engine compartment.

Boiled, vaporized fuel bubbles in the injector lines are larger than the diameter of the injector lines, so now the lines are effectively blocked from allowing fuel to pass. You do not want to turn the fuel pump on and prime a hot, injected engine. Instead, just start cranking with the throttle opened up one-eighth to one-fourth of the way, and wait for the engine to start firing as it sucks the fuel out of the injector lines. Then, slowly move the mixture forward and the engine should catch. This procedure will work 95 percent of the time.

If you add fuel too soon, you are going to flood the engine. The procedure now is to fully open the throttle with the mixture completely leaned and start cranking. The engine should catch within 10-15 seconds. You have to resist the desire to enrich the mixture. Let the engine slowly catch. It will be rough at first, but as it begins to smooth out, move the mixture forward.

Airflow Performance makes a purge valve for its injection system, which I really like. It makes every engine start cold by effectively removing the hot fuel from the fuel lines in the engine compartment prior to engine start.

One of the most effective systems to deal with hot starts is to add some kind of electronic ignition system. The larger, hotter sparks usually power through a slower-cranking engine and can make things much easier. However, as I mentioned in the last column, it all depends on having a good battery that will move the engine at the proper speed.

Hopefully, by now you have some understanding of the mechanical/electrical side and pilot technique as it pertains to starting your aircraft. It takes both of them to keep the fun factor alive!

Vic Syracuse, EAA Lifetime 180848 and chair of EAA’s Homebuilt Aircraft Council, is a commercial pilot, A&P/IA, DAR, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more 9,500 hours in 72 different types. Vic also founded Base Leg Aviation and volunteers as a Young Eagles pilot and an Angel Flight pilot.

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