Weak Ignitions
 

Hey Gang

There have been several posts here where people are having a hard time with some of their engines performing correctly, as a rule I think most are carburetor problems but sometimes it is in the fire. I think for the most part it will be in the batteries but even CDI ignitions and coils go bad. But sometimes there are other problems that cause weak or no fire for the engines. Usually weak fire is the most difficult to locate. I wanted to let you know how I check for some problems. I am not saying my way is the only way, but this information may help someone.

I don't think breaker type ignitions are used anymore so there's no need to mention points.

When you do have the older style engines that use a magnet in the flywheel with a coil like the earlier Zenoahs. One thing that usually is overlooked by most people is sometimes the area where the coil is mounted to the engine will corrode or oxidize and the coil will loose or get a weak ground. Another possibility for weak fire on it is if the contacts that pass by the magnet rusts that will also cause weak fire. In either case remove the coil and lightly sand the corrosion off both the coil and the mounting posts, and a good ground will be reestablished.

If the coil goes bad, after running the engine a short time, the engine starts running crappy and gets worse as it runs or dies, as a rule the coil will be extremely hot to the touch, If that's what happening the coil is shorting inside itself and breaking down as the engine runs, it will be so hot, if you touch it, it will sear your skin, it will be brutally hot.. A good way to test that is put a drop of water on it, the water will tell you quickly if that's what happening. Then sometimes a coil just goes bad and wont get any fire at all.

The newer electronic ignitions do solve a lot of problems but they have their own failures, some are just not good quality out of the box as I understand. But any mechanical thing wears out or fails. There are a lot of people here who do know more about these than I ever will but if I have an engine that starts running erratically in the air after I fly it a short time. I first test the battery, if that doesn't do the trick, check the spark plug for it's fire. I'd have to assume the ignition module on these can corrode like the coil type on the engine so you may need to clean it also.

The way I check for weak fire is, Have a pair of insulated pliers, pull the spark plug out of the engine, then reattach the plug into the plug wire, now ground the spark plug against the engine block holding it with the insulated pliers. and spin the engine over. If you have weak fire no matter what ignition system you use. You can usually tell by the color of the fire the spark plug is putting out just by looking at it.


if the spark plug is getting good fire in any ignition, the spark will be a pretty blue in color and you will be able to hear it spark strongly as the engine turns. If it is weak then the spark will be yellow in color or maybe a little blue with yellow in it. If the coil is breaking down in flight again check the temperature of the coil, If a CDI is breaking down in flight, the only way I could think of to test that is to have an extra plug with you and unplug the one in the engine as soon as you land and do the spark test while the ignition is still hot.

As I stated earlier, I really don't know that much about the newer styles of ignition, and maybe some of the other members here will have other suggestion on how to find problems in them that I'd be interested in knowing, so if you have any tricks up your sleeves about them I'd appreciate your sharing your knowledge.


Sometimes when you take off with your plane and it won't run it's full rpms or looses power, it's because the ignition isn't providing enough fire to completely burn all the gasoline going into the cylinder

Bassard

Good post

W8YE Thanks for the compliment,

Quick check on a battery CDI. Take an old spark and cut the side electrode off. Using one of the adjustable ignition tester set the RPM for 10,000. If the ignition will fire the 3/16 gap at 10K rpm then it will run an engine. I keep both in my flight box.
It seems wrong, but it takes a lot more spark energy to start an engine than it does to keep it running.

I will agree it is a nice post. It used to be that the ignition coil was separate, so one could measure the coil resistances. But nowadays the CDI units have everything integrated into one module and it may even be potted (filled up with non-conducting epoxy resin) too. So it may be impossible to check the coil on the inside.

One thing to watch out for though, is that the coils are made more small as they increase the voltage driving them. That makes the coil really susceptible to damage from high voltages beyond what it was designed for. You never want try to see how big or how long of a spark you can get off the ignition unit. You want the check the spark at the gap the spark plug is supposed to be set for. Sometimes it happens when the spark plug cap comes loose or has a poor ground return path, but the spark may have to jump a larger combined gap, and that causes the high voltage to peak out at really high voltage levels and that causes the coil wires inside to start to short out. I have seen some spark plugs with the cap etc, sparking across the plug tip OK but there were also sparks across the sparkm plug body to the cap too, which is not good. Some car companies have firm rules setup that one does not unplug a spark plug cap or COP unit while the engine is running as it will immeditately cause its failure.

Bill from CH Ignitions shared this one with me. I had an engine that was extremely difficult to start when cold. (No, it wasn't a lean problem.) Once it got running, it would then re-start fine, until the next dead cold start. Bill suggested trying a 6 volt battery in place of my 4.8 volt. That immediately solved the problem. He said sometimes a weak ignition would slip through the network, and using a higher voltage would point it out. (The unit in question was an RCEXL)

I also like your idea for testing them Terry. That probably would have trouble shot the problem I was having as well.

AV8TOR

I also had a engine where it would cough, sputter, pop and tried to start and run, but never quite got there. Eventually it turned out that the two LiFe battery packs I had were not capable of providing the meager amount of current needed to run the CDI unit. When I tried a different NMH pack the engine fired right up and ran like a champ. The two liFe packs charged up Ok and showed they had a nice voltage to them, but they had no current carrying capability. I since then used a better brand LiFe pack and the engine ran great with it.

I also ran into the situation where a NMH pack lost some capacity and had gotten weak at the flying field. In this case the engine would start right up and run OK, but as I went to full throttle the engine would suddenly quit. That was another one of those gotcha things. But fortunately I had it happen on the ground and not up in the air with the plane in a bad position for as dead stick landing.

Another one I ran into was an intermittent sensor. Sometimes it would run great; sometimes it would run terrible. After lots of trouble shooting, I finally changed the sensor and that fixed it. Bill from CH said they usually either work or don't work, but about 5% of the time they will just act "screwy" when they go bad.

AV8TOR

A note,no need to ground the spark plug while testing on our electronic ign. units while plug is in cap.Ground is the cap body itself.

That's true for the RCEXL and some others with the braid/shielding wire connected to a metal spark plug cap. The CH Ignitions units with the rubber spark plug cap/connectors need to be grounded. In fact, repeatedly turning the engine over with the ignition on but without the plug being grounded can ruin the sensor.

AV8TOR

Good post. ALSO the spark plug wire can be bad. On a G38 you can unscrew the hi tension wire out of the coil & replace it with a new wire. It makes for real good starting & running. Give it a go.