Ignition troubleshooting

by Had Robinson
updated April 27, 2020


The following information is primarily for the Top 80, Minari, Vittorazi, and the Polini Thor models.  Other motors are similar but the wire colors may be different.

Generally, the most overlooked ignition problem on all engines is the condition of the spark plug.  They are so inexpensive that replacing a spark plug should be one of the first things you do, especially if it has more than 10 or 15 hours on it.  Pilots should always carry a spare and have the necessary tools on hand to change the spark plug and accurately set the gap.  Use a torque wrench to ensure that the plug is not over or under-tightened.

If the spark plug is not torqued down enough, it will leak and engine performance will suffer.  It can also cause overheating.  You can tell if it is leaking by removing the plug and checking the aluminum spacer washer.  If there is oil on it, you have a leak.  Excessive torque can destroy the threads on the cylinder.  Pilots should invest in a torque wrench if they plan to service the spark plug.  A modestly adequate torque wrench is available from Harbor Freight for around $10 (in the U.S.).  Plugs out of the box may not have the correct gap – check them before installation.  Use a spark plug gauge to set the gap correctly (it uses a gauged wire to check the gap rather than a flat tab).

Paramotors do not have high energy ignition systems (e.g. capacitor discharge/computer controlled system) so they are very sensitive to plug, coil, or wire problems.  Here are the proper specifications for the type of spark plug, gap, torque, etc. for the Top 80.  See this site for the specifications of other paramotors.  All values are similar except the spark plug gap which can vary greatly.

For some introductory information about engine ignition systems, visit this helpful site.  Here is the specification sheet from IDM (the manufacturer) on the coil used in the Top 80 engine and some others, like the Minari, Vittorazi, Simonini, and Polini.  The typical paramotor ignition is very simple.  There are only 2 or 3 wires coming out of the coil.  The thick wire is the secondary wire which goes to spark plug.  The thin wire is the primary wire which connects to the kill switch.  The other thin wire if present (usually black) is the ground wire.  (Note: Some coils do not have this wire and are grounded internally.)  If the primary wire touches ground (the engine or frame), the coil shorts out and no spark is generated = the kill switch.

Tools needed: Digital voltmeter & some test leads.  Harbor Freight has these tools for less than $15 U.S.  Sometimes these tools are free if you show up at the right time and fog a mirror.  If you do not know how to measure resistance, Fluke has this site for the basic "how-to".

A. Primary wire test

An engine manual may needed if help is needed to identify the primary wire.  Measure the resistance of the coil primary wire to ground (e.g. an engine cooling fin).  Disconnect the primary wire from any other wires.  Some coils are polarity sensitive so you may have to reverse the leads of the test instrument in order to get a reading.  If the primary wire has infinite resistance to ground: 1.) the coil is likely bad and will have to replaced OR 2.) the primary wire could be broken before it enters the coil.  Low primary resistance is particularly hard to measure accurately but, thankfully, it is a rare problem.

Thor Magneto  The Thor 100, 130, 200, and 250 have a magneto that drives the coil.  There are (3) wires coming out of the magneto: the black wire (the exciter voltage going to the light blue wire on the coil), the blue wire (ground), and the white wire (the battery/accessory charging wire).  The resistance of the magneto exciter (the black wire) should be 290 Ohms to ground ± 10%.  The black wire must be disconnected from the coil in order to measure its resistance.  If the resistance is infinite (open) or less than specification, the ignition system will not function.  Thankfully, it is rare to have an open magneto wire circuit.  What is more common with the magneto is that the internal wiring begins to short out e.g. has a resistance of 150 Ohms.  When this happens, magneto output drops enough so that there is not enough voltage to induce a spark in the coil secondary.  The engine will not start or run in this condition.

The normal output of the magneto exciter at idle is around 90 volts AC.  To measure this voltage, disconnect the kill switch, connect the digital voltmeter to the black wire and the engine ground, and run the engine at idle.  If the engine cannot run for some reason and the magneto needs testing, disconnect the black wire from the light blue wire going to the coil and crank the engine.  The voltage may be less from the magneto may be less.

Thor Charging system  The white wire (used for powering accessories) has a voltage at idle of around 21 volts AC.  NEVER CONNECT THIS WIRE DIRECTLY TO A BATTERY!  This wire must be connected to a regulator first.  The regulator is an optional part available from Polini (or from us).

B. Secondary wire and coil test

With a digital multimeter (<$10 from Harbor Freight), measure the resistance from the metal tab inside the spark plug boot to one of the cooling fins on the engine (ground).

Other paramotors, including early Top 80 models, may have a secondary wire that has a resistance of less than 100 Ohms (a non-suppression type wire) and will have a resistance of between 7K and 10K Ohms between engine ground and the spark plug cap terminal.

If you get a low value, you should determine if the coil is OK with these steps:

Stick a sharp pin or needle into the secondary wire about 3" from the coil and measure the resistance from the pin or needle to the engine ground e.g. one of the aluminum cooling fins on the cylinder.  This will make a harmless pinhole in the wire.  It should be about 10K Ohms (the resistance of a good coil PLUS the resistance of 3" of secondary wire on a Top 80 – other engines may vary).  If you do not see any resistance at first, withdraw the needle and stick it in again closer to the coil.  If it is less than 10% of 10K Ohms (or open/infinite) in a Top 80 coil, the entire coil assembly must be replaced.  Other engines should have a coil resistance value close to this.

Remember that an engine may start and run if the coil is bad but top end performance will suffer.

If the secondary wire is open (no resistance to ground), the coil is probably OK.  If this is the case, you can replace the wire rather than go to the trouble and expense of replacing the entire coil assembly.  If the coil must be replaced, to the this page.  An internal break in the secondary wire will cause missing and a drop in engine output at high engine loading.  It becomes more acute as the engine heats up and internal pressures increase.  When the break gets sufficiently wide through burning of the ends, the spark cannot jump the gap in the wire, the engine will not start, and the coil may be damaged from internal overvoltage.

If you choose to replace the secondary wire, it is first a good idea to find out where the open or break is.  To do this, take a sharp sewing needle and gently shove it about 1/2 way through the wire and about 1/2 the way back to the oil.  With the digital ohmmeter, see if there is resistance and how much.  If there is resistance, it means that the coil is probably OK.  If there is no resistance, move the needle 1/2 way back towards the coil.  Continue moving the needle back towards the coil until you have resistance of some value.  The actual value will depend how far back you go to the coil.  The value can range from about 9K-17K Ohms.  If there is no resistance when measured 4" or less from the coil, the technique used to temporarily repair the secondary wire will not work as we need a short stub that is good coming out of the coil.  (Note: the complete replacement of the secondary wire does not require a wire stub.)

Failed Test?

Here is a photo of a bad ignition coil from a recent Top 80.  The resistance reads 7.7K Ohms from the spark plug boot to the coil ground, less than half of specifications.  The coil was new but was incorrectly installed so that the tips of the coil contacted the flywheel.  This quickly destroyed the center post and heated up the coil until the internal wire insulation failed and the coil shorted out to ground.  The resistance of the carbon-core secondary wire used by Miniplane is 8.5K Ohms ±10%.

Bad Top 80 ignition coil

Sometimes a small crack or cut in the secondary wire insulation will allow the high voltage to jump to ground rather than be carried to the spark plug.  To check for this, remove the spark plug, reconnect it, and hold the base of the plug firmly against the top of the engine.  Place the paramotor in a darkened area and pull quickly on the starter while looking at various parts of the coil and secondary wire.  There should ONLY be a bright spark across the spark plug gap and nowhere else.

If pilots want to do a quick check of the ignition coil, get a used spark plug and set the gap to +0.045" and repeat the above test.  The greater gap will insure that the coil has the ability to produce a good enough spark while under load.  If there is a good spark, the coil is OK.

As the load of the engine increases, the density of the fuel/air mix in the cylinder increases.  As the density increases, the spark gap breakdown voltage increases.  This is why the ignition system must be in top condition if full power is to be realized.  Those who are curious about the principles of how the spark plug works in an engine can read this technical document from the Society of Automotive Engineers.

Note: Miniplane recommends that pilots never remove the spark plug from the secondary wire and then crank the engine because very high voltages can be created which can destroy the coil if these voltages are not shunted to ground directly through a spark plug or a test lead connected from the metal clip inside the spark plug cap.  Always have a spark plug connected and touching ground before cranking the engine.

The use of a carbon-core instead of copper increases the resistance of the secondary circuit and helps suppress ignition noise in nearby radios caused by the spark.  This special kind of high voltage wiring has a resistance of about 3,000 Ohms/ft.  The downside is that the core is fragile, will break, and then eventually burn out.  The engine will run with a broken core but high end performance deteriorates eventually as the spark weakens.  The ignition will eventually fail with time because, as the engine runs, the gap between the broken core ends will widen as the ends burn.  That is, the high voltage arc will gradually burn the core ends away from each other increasing the gap until the spark voltage is insufficient to jump across the gap in the core.  When this happens a new secondary wire can be installed.  Replacing the entire coil is much more difficult than replacing/fixing the secondary wire.

Pilots often notice some soot under the spark plug cap.  It is actually pulverized rubber that is created by the severe vibration of the spark plug chewing away at the inside of the spark plug cap.  You can replace the secondary wire with an automotive type heavy duty wire and cap and this problem will be much less.

C. Secondary wire permanent replacement

Please go this page for a permanent replacement of the flimsy, lawnmower grade secondary wire made by the Italian company IDM.  This is what all pilots should do if their secondary wire has gone bad, rather than replace a perfectly good coil.

D. Secondary wire temporary field repair

A field repair of a bad secondary wire can easily be made if the coil is good.  Most of the secondary wire is located far from any metal parts so using high tension replacement wire is unnecessary.

Use the following procedure to find the break is in the secondary wire (where to repair it) and, at the same time, to check whether the coil is good.  It is expensive and difficult to replace the coil and a waste if just the secondary wire is bad.

Connect a fine needle to one of the probes of the ohmmeter and, starting at the spark plug  connection, stick the needle into the wire every 1/2" or so, moving back towards the coil.  When some resistance is indicated (see specifications), that will be the approximate location of the break in the carbon-core.  If zero resistance or less than specifications is found going all of the way back to the high tension post in the coil, the coil is defective and this repair is useless.  A new coil must be installed.

The side towards the coil from the last pin location will be useable for the repair.  Carefully split open about an 1.5" of the wire at this point with a razor blade to expose about an inch of the end of the broken core that goes to the coil.  Pull up about 3/4" of the intact carbon-core.  The good part of the core must be lifted out of the insulation.  Do NOT pull on the core or it can break.

Cut a piece of ordinary #18 stranded automotive wire long enough to go from the break to the spark plug cap.  Add about an inch or two.  Strip the ends about 3/4".  Place one stripped end of the new wire in the bottom of the split in the secondary wire.  Now, firmly press the carbon-core down on top of it.  Carefully hold the split closed so that the new and old wires are firmly touching each other.  The core and the wire should overlap each other about 3/4".  DO NOT USE ANY SEALANT OR GLUE IN OR ON THE SPLIT!  However, for the obsessive types, go to the hardware store and buy a tube of ALNOX brand conductive grease (used by electricians) and coat both wires with it.  It will guarantee a good connection between them.  Now, close up the split in the secondary wire with electrical tape.  Wrap it very tightly.

Drill a hole the same diameter as the new wire in the top of the spark plug cap all the way through.  Stick the other end of the new wire in the cap until it sticks out about 1/2" inside.  Firmly wrap the new wire to the non-working secondary wire with electrical tape.  Reconnect everything.  Be sure to firmly push the cap onto the spark plug using a slight twisting motion.  This will crush the stranded wire out so that it will firmly touch the top of the spark plug.  Now you can seal the hole in the cap with silicone so that the wire does not shake out.  DO NOT SEAL ANYTHING ON THE INSIDE OF THE CAP!

This repair will hold up for about 50 hours of engine time especially if the new wire is firmly wrapped to the old and the spark plug cap is not worn out.  The reason the field repair fails is that the new wire going into the top of the spark plug boot gets chewed away by engine vibration.  However, a new wire could be spliced to the field repair wire and re-inserted into the cap.

Obsessive-compulsive types could do away with the spark plug boot entirely, solder the new wire to a ring-terminal, and put it under that screw-on terminal.  This repair would probably last 100's of hours.  Anyway, insulated spark plug boots in ultralight aviation serve no purpose except to keep noodle-heads from shocking themselves when the engine is idling and on the ground.  Protection from shorting out in the rain?  If you fly in the rain – good luck anyway.  The reason for any of this is due to Miniplanes use of lawnmower-quality ignition parts.

The break in this wire occurred right next to the wire anchor on the spark plug side.

Secondary wire failure in the Top80 engine

Note:  The Top 80 has severe vibration and you must be careful to anchor the secondary wire at its original locations.  Failure to do so will cause premature failure of the secondary wire.

E. Testing the Kill Switch

When the kill switch is pushed, the primary wire of the coil is grounded.  Disconnect the wire going to the kill switch and measure its resistance to ground when it is pushed.  It should be 0-1 Ohms.  If it is always grounded or always open, the kill switch wire/button will need to be serviced.  Rarely, the primary wire or the connection to the kill switch wiring can be defective.  Just because a connector feels "tight", the wiring inside may be severed or pulled away from the conductive parts of the connector.

An alternative to test the kill circuit is to set things up per the above (remove the spark plug, reconnect the spark plug, etc.).  Push and hold the kill switch while pulling the starter.  There should be no spark at the spark plug.  Release the switch and pull the starter.  A spark at the plug should be seen.

F. Spark plug Problems

This is another major source of problems for pilots. Time after time I see spark plugs incorrectly gapped. The ignition systems on paramotors are NOT high energy systems like in a car. Therefore, pilots must be certain that the plug gap is correct and that the plug is functioning properly.  Failure to replace/gap spark plugs is the most common mistake made by pilots.

Just recently, a experienced pilot was complaining about high end performance on his Top 80.  We took the plug out and measured the gap.  It was over .030” – way too much.  The spark was being quenched under high load (high cylinder pressures) and the engine would stutter badly. The proper gap is .020” - .024” (0.5mm – 0.6mm). The pilot properly gapped his spark plug and the engine ran perfectly – with more power than he had hitherto experienced.  Plugs should be replaced after 25 hours.  Of course, how will you know how many hours the plug has unless you keep a log and have an hour meter/tachometer?

The spark plug below (out of a Top 80) had more than 60 hours on it and is worn out but the engine still ran fairly well, even at high loads.  However, the engine experienced sudden failure due to a piece of lead phosphate (the byproduct of the action of TCP additive on the tetraethyl lead in AVGAS).  The flake of lead phosphate dislodged from the spark plug (most likely) and got caught between the electrodes of the spark plug, shorting it out.  The pilot had to do an emergency landing (no damage or injury).  But all ultralight engines can fail at any time, right?  We should always be prepared to land.  Never assume that your engine will give a warning when it is about to fail.  For more photos of spark plugs and the different engine running conditions they show, go to our fuel/oil specification page.

spark plug shorted from lead phosphate deposits

The severe vibration of a running paramotor can not only destroy the secondary wire after many hours but can also damage the plug itself by breaking internal connections inside.  When troubleshooting ignition problems, you should also be sure that the spark plug internal resistance is not more than about 30 Ohms.  If it is open (no resistance) change the plug even though the engine may still start.  High load performance will also suffer.  Always use the spark plug specified by the engine manufacturer.  They engineered the engine and know which spark plug will give you the best performance.

G. Grounding Problems

Some paramotors (not the Top 80) may fail to start/run because of ground loops or open grounds. Generally, you want to be certain that the motor and frame are bonded together electrically, e.g., a #14 wire with ring or spade connectors attached at each end which are firmly attached to screws/bolts on the frame and the motor (remove all paint, if necessary, beneath the screw/bolt).  If this is not done, connections for grounding the coil and other electrical devices may not work reliably.  How the ground is done varies from paramotor to paramotor.  The rubber mounts between the motor and frame effectively isolate the two which is why they need to be bonded together with a bonding wire.  The resistance from the ignition coil to the aluminum frame of the engine must be less than 1 ohm.  When you disconnect the kill switch from the coil, the kill switch circuit must be “open” = no resistance and 1-3 Ohms when the kill button is pushed.

Turkey Vulture