Welcome to Myrons Mopeds New Tomos Service Department. Here are some articles to help you repair or lovingly maintain your 1994-later Tomos. You can also purchase hard copies of owners manuals and service manuals, listed in the Accessories Department.

Tomos Basics is a 2 page mini owners manual, that supplements the regular owners manual.

Tomos Electrical is a 12 page crash course in understanding and troubleshooting electrical.

Tomos Trans Oil Leak is a 2 page info sheet about 2008-later oil leaks at the chain sprocket.

future articles

               TOMOS BASICS for USA Models 2007-on (A55 engine)

1. TOMOS means TOvarna MOtornih koles Sezana (motorbike factory in Sezana, Slovenia).
2. Warranty is 6 month/4000 mile limited. Owner must send in the warranty registration card located in the owner’s manual to activate the warranty on a brand new Tomos.
3. Engine is the A55 two-cycle 49cc  single, EPA and CARB compliant with catalytic exhaust. Speed is 30mph. Horsepower is 2hp. Transmission is two-speed fully automatic.
4. Pedals go forward for hybrid human/motor propulsion. Pedaling speed is 5 to 10 mph. Pedals kick backward for starting engine. Pedal forward to put pedals in kick position.
5. Gasoline is 91 octane premium unleaded. Only Sprint model is pre-mixed with oil 50:1 or 2.5 ounces per gallon. All other models have oil injection and use straight gasoline.
6. Gas tank size: LX 1.5gal   Sprint, ST 1gal.  Arrow, Revival, Streetmate .75gal. On the LX and ST locking gas lid, only the key turns, counterclockwise to open.  On ’05-08 Arrow, Revival, Streetmate the gas lid is push in. It pulls straight out. Sometimes it’s tight. After 2008 all gas lids are quarter-turn type.
7. Gas valve is a manual fuel tap or shut off. It is located above the engine on the left side of the bike. It has three positions, off (horizontal), on (down), up (reserve). Use on during normal operation. Use off when not operating. Use reserve if you “run out of gas”. Reserve lasts 4 to 10 miles. Don’t forget to put it back to “on” after you gas up.
8. Gas level is checked by opening the gas lid, looking, shaking, and listening. There is no gas gauge. On the LX you can see the whole tank. On the ST and Sprint you can see half way. On the others you can only see the first fourth. Many people choose to install a clear hose branching up from the fuel line. It serves as a gas gauge. The liquid level in the tube is the level in the tank.
   Gas tank vent: On Arrow/Revival/Streetmate models the gas tank is inside the frame, which is U-shaped. On one end of the “U” is the fill hole, where you add gas. On the other end of the “U” is a vent to let air in and out. It normally lets a tiny amount of air pass but not enough during refueling. When adding gas, the gas tank vent, a black spring-loaded push button, needs to be pushed each time the tank seems full to let any air escape allowing more room for gasoline. It makes a hiss sound. When the ¾ gallon tank seems full and the vent no longer hisses then it’s truly full.
9. Gas mileage (mpg) is 100 miles per gallon, going 37mph on city streets. If you’re larger, have hills, rough roads, stop and go, your mpg (and range) will be less. If you’re smaller, more aerodynamic, and go slow, your mpg (and range) will be more.
10. Gas range:  LX 125mi  Sprint, ST 85mi  Arrow, Revival, Streetmate 60mi. Means topped off to hitting reserve, 160lb rider on smooth flat city streets.
11. Oil is two-cycle oil. It’s slowly consumed by burning along with the gasoline. Use a good brand, preferably synthetic, like Champion weed wacker oil. Without the oil the two-cycle engine will get hot, melt the sides of the piston, and “seize”, with a scary skid.
12.  Oil tank is under the seat, except Sprint, which is premixed in the gas. It doesn’t matter how much oil is in it, only that there always is some. Check the oil level at each gas fill-up. After several gas fill-ups it will need an oil fill-up. 13 ounces lasts about 5 gallons.
13.  Steering lock is on the left front of the frame. To lock, put the steering almost all the way to the right, push the key in while turning it to the right. Move the steering slightly until it finds the hole and goes down 3/8 inch. To unlock, turn the key to the left, pull up, and wiggle the steering if necessary.
14.  Keys: Sprint = steering lock only (2), no ignition key. No battery to turn off.
15. Keys: ST/LX = steering lock (2) and gas lid (2), no ignition key. No battery to turn off.
16.  Keys: Arrow = steering lock and ignition (2). Turn off ignition and battery after use.
17.  Keys: Revival = steering lock, seat, and ignition (2). Turn off ign & battery after use.
18.  Keys: Streetmate = steering lock, seat, trunk, ignition (2). Turn off ign & bat after use.  Main key switch, also known as “the ignition”, on Revival is on left top of the fake gas tank. On Arrow and Streetmate it’s under the left side of the seat.
19.  Choke is manual. It’s needs to be on (flipped up) for starting when the engine is cold. After a few seconds, when the engine is warmed up, it needs to be off (flipped down). In winter, temp 40 F, the choke is left on for the first two blocks. For normal temp 70 F, the choke is left on for 10 seconds. In hot summer, temp 100 F, it’s not needed at all. Never leave it idling for long periods with the choke on or it will become “flooded”. Never use the choke when the engine is warm or it will become “flooded”. It’s better to under use the choke, than to over use it. If the engine becomes flooded, then it will need no choke and full throttle (maximum air) to compensate for the excess fuel that has accumulated. Running fast cleans it out, and is better for a two-stroke than idling.
20.  Starting:  Turn gas valve to “on”, arrow down. (or “res” if it’s very low on gas)
21.  Starting:  Turn engine stop switch to “run” symbol. It’s on the right handlebar.
22.  Starting:  Revival/Arrow/Streetmate only. Put the key in and turn it on.
23.  Starting:  Cold starting only. Put the choke on by flipping the black lever up.
24.  Starting:  Stand on the ground with the bike on its center stand. Pedal forward until the pedal on your side is in the 2 o’clock position. Have your left hand fingers on the left (rear) hand brake, ready to stop in case it comes off the stand and takes off. Have your right hand on the throttle, with the twist grip turned only just a little, or not at all. Now kick backwards. A broke-in new Tomos starts in one kick.
25.  Starting:  After it starts, rev it up by twisting the throttle. The engine needs to be revved up either stationary on the center stand or moving down the street. Starting it up and then letting it idle to warm up is not necessary or recommended. Its two-stroke crankcase already has the oil film, so it’s ready to ride immediately after starting.
26.  Starting can also be done with the tires on the ground, off the center stand. Pedal the bike forward, maybe 10 or 20 inches, to put one of the pedals in kick position.
27.  Starting can be done while moving. Just stop pedaling and kick backwards.
28.  Electric starting is on Arrow/Revival/Streetmate only. Use the above starting steps, but replace “kick backwards” with “hold the left brake and push the start button”. The start button is where your right thumb is. Then ride for 10 minutes to recharge the battery.
29.  Storage: Ride it with the gas valve turned off, to evacuate the gas from the carburetor. After about a block when it runs out, put the choke on and it will go another block. Doing this will prevent the carburetor from getting coated on the inside with tar from dried up gasoline during storage.
30. Flooded Starting: Occasionally the moped can become “flooded”, a condition where too much gasoline and not enough air is causing the engine to not start. When this occurs, a different starting procedure must be used. First the gas valve should be turned off. The carburetor holds enough gas to run for at least a minute. Instead of minimum throttle with choke on (up), it needs maximum throttle with choke off (down), to get the most air. Once it fires, it needs the maximum throttle for awhile, maybe 5 or 20 seconds, until the rough running goes away. Then it needs 15-20 minutes of fast running to heat up the exhaust fully, to boil off any unburned gas and oil. During this period it will smoke a lot. If it does not fire within a few full kicks, then the spark plug will need to be removed and the gas or oil dried off it. If the white or brown porcelain insulator is shiny or black, then it will need a new spark plug, NGK BR7ES. Here are the causes of “flooding”. Knowing these before it happens will help prevent the problem.
31. When a moped is leaned way over, especially with the gas left on, such as when it is being transported, gasoline can spill into the air filter or into the intake port of the engine, and result in not starting. This also can happen on Revival and Streetmate when gasoline spills over during filling. When the engine is run with the choke on too long, the spark plug can get wet with too much gasoline, and result in the engine not starting. Idling for too long, or going slow all the time can also cause this. When the engine is kicked over many times without having a spark, such as when the kill switch is left in the off position, the spark plug can get wet with gas and result in not starting.
32. If the float valve inside the carburetor malfunctions, gasoline can spill into the air filter. A tiny fiber can cause that, or tar from a long time sitting, or a worn needle valve and/or seat. If the oil injector leaks oil into the engine during storage, then the spark plug can get wet with oil. Remove the spark  lug, kick over the engine to push out any excess oil, and clean the oil off the plug.

Basics of Tomos Moped Electrical Systems

by Shaun Strahm, Myrons Mopeds, December 2010

The following is a “crash course” on how to find and fix electrical problems on late model TOMOS mopeds.

            Table of Contents                             page

 Everything is circuits……………………………….…   2

Shorts and opens…………………………………….     3

Volts and amps………………………………………..    3

Batteries and generators…………………………     3

Magnetos………………………………………………..    4

Magneto versus battery……………………..…..    4

Testing without a tester…………………………..    4

Example 1: Prince of Darkness…………………     5

Example 2: The Flasher……………………………     6

Make predictions…………………………………….    6

AC lights and DC lights……………………………..    6

Example 3: The Double Crosser……..……….     7

Ignition ……………………………………………………     8

How to test a coil…………………………………….    8

How to check for spark……………………………    8

Ground the coil wire to stop the spark…..    9

Tomos A35 and A55 CDI ignitions…….……..    9

Ignition timing…………………………………………   10

Ignition symptoms………………………………….   11

Everything is circuits

     Every electrical system is composed of current loops or circuits. The electricity flows out, goes through the device being powered, and then flows back. That is why all electrical cords have at least two wires. One wire is out and one is back. Coaxial cables look like one wire, but they’re actually one wire surrounding another. If you follow that pair of wires in an appliance cord all the way to the transformer on the power pole, you’ll see they’re joined in the windings of the transformer, forming a closed loop.  In some electrical systems such as automobiles and mopeds, the metal frame is a “ground”. In this context a “ground” is a path for the electricity to flow through that is shared by many loops. If the cars in a big city were electricity, then each current loop would be one car, going to work in the morning and then coming back in the evening, making a loop. Then the freeway would be the ground, where the separate current loops all share the same roadway.  When several devices are connected with each having its own current loop, they are said to be connected in “parallel”. Unplug one device and the others are unaffected. Everything in your house is wired in parallel, and has its own current loop. Similarly, on a moped, the head light, tail light, brake light, and horn are wired in parallel, and have their own separate loops or circuits. The tangled nest of cords behind your computer desk, as well as the wires in your moped, can, in principle, be laid out in nice straight parallel lines in order to visualize the “scheme”. This is called a “schematic” diagram. Being able to visualize or draw the schematic is required to understand any circuit.

Figure 1: Schematic diagrams of a moped and a house are the same. Every device is in a “parallel” loop or circuit.

     When two devices are connected one after another, they are said to be “in series”. The only things on a moped or in your house that are in series with other things are the switches. From figure 2 below, you can see from the house wiring diagram that the TV switch turns on the TV and not the computer. You can see that the breaker turns off everything. From the moped wiring diagram you can see that the light switch turns off the lights but not the horn. This is basic.   

Figure 2: The devices in a moped and a house are in parallel, while their on/off switches are in series.

        You can usually tell how a moped or a house is wired from observing the behavior of all the switches and devices. Any switches must be in series with the devices they turn on and off. Each device and its (optional) switch must be in parallel so that they can be turned on and off independently of one another. This is predicting the wiring diagram from the observed behavior. Conversely, you can also predict the behavior from observing the wiring diagram.

Shorts and opens

     Not every electrical problem is a “short”. The term “short” is unknowingly misused by the general public to mean two opposite things at once, a “short circuit” and an “open circuit”. It can mean a true short, where a wire (or anything that carries electricity) touches another, forming a shorter loop that redirects the electricity through the short loop and not through the original loop. Or the term “short” is misused to mean an open, where a wire (or anything that carries electricity) does not touch another, interrupting the loop and causing the electricity to not flow through the loop. Both a short and an open can cause electricity to not flow through a device, but for completely opposite reasons.

Figure 3: A “short” is a shortcut that steals most of the current from the device. An “open” interrupts the current.

Volts and amps

     Electricity flowing through a wire is analogous to water flowing through a pipe. It is measured in two ways. The “voltage” (in volts), is like the water pressure (in pounds per square inch). The “amperage” (in amperes or “amps”) is like the water flow (in gallons per minute). When you’re taking a shower and someone turns on the garden hose, your water pressure drops. Less water pressure results in less water flow and your shower gets weaker. Likewise, when starting your car, all your lights get dim. This is because your 12 volt battery momentarily drops down to 6 or 8 volts when the starter motor is using up most of the available current.

Batteries and generators

      There are two main sources of electricity. Batteries (and fuel cells) produce steady current, called “DC”, or direct current.  Generators (and alternators, magnetos, and dynamos), make “AC”, or alternating current. The current made by a generator alternates direction back and forth, in sync with the rotation. The rotation is what causes the electric current to be AC. All power plant generators in the USA are set to rotate at a speed that always makes 60 cycles per second AC, or 3600 cycles per minute. On a moped, the generator rotates with the engine, which varies in speed from about 1000 cycles (revolutions) per minute “rpm” at idle to above 8000 cycles per minute going fast downhill. So the “frequency” (how fast it alternates) of the AC is variable in a moped, but not in a house. Many devices such as lights, behave the same no matter what the AC frequency is. In fact an incandescent light bulb works exactly the same on AC or DC current. In figure 4 below, graphs of voltage versus time show the main types of electricity. 60 wave crests happen in about one second, so the time scale is too short for the human eye to see. Only an oscilloscope can show the “wave form”. 

Figure 4: AC is from generators. DC is from batteries. In between is rectified AC or “rippled” DC.

Magnetos

     Mopeds and small motorcycles have magnetos. A “magneto” is a generator plus a no-battery ignition. A magneto ignition will operate with a dead battery. Automobiles and large motorcycles have battery ignitions. They require a battery, both to operate and to start up. Dirt bikes and power equipment without lights have magnetos with a single wire output. That is the ignition wire that goes from the source coil (and points and condenser) inside the engine to the transformer coil outside the engine. All street bikes with lights have two or more wires coming out of their magnetos. One is for ignition, and the other(s) is for lights. We now focus on the lighting wires, which are normally separate from the ignition. If you turn over a moped engine, that is, cause it to rotate, then some electricity will be generated and the lights should come on while it’s being rotated. Just because the lights work does not mean the ignition works. They are like two separate generators tuning on the same shaft. As the engine rotates faster, the lights get brighter, indicating more voltage is being produced.  The magneto/generator will reach a speed where the voltage will level off. Above that speed the lights won’t get any brighter. So a magneto is self-limiting with regard to engine speed (and thus vehicle speed). The bad thing about a magneto is when you are stopped idling your lights are dim. With a battery, your lights are always almost the same brightness all the time. The good thing about a magneto is you don’t have a heavy expensive battery that goes bad if you leave it sit for over a few months.

Magneto versus battery

     The most important difference between a battery and a magneto is a battery will deliver an almost unlimited amount of amps, if it is allowed to, while a magneto will limit the maximum amps by dropping in voltage. Because of this, a battery system needs fuses to protect its wires from melting from too many amps. A magneto does not ever have fuses, nor does it need them. This difference makes moped electrical troubleshooting different from automobile troubleshooting. A magneto almost never burns out or causes wires to melt. You can short a magneto’s lighting wire directly to ground, and it will only get warm. Short a battery’s positive terminal to the negative and the wire will suddenly glow red and splatter molten copper and sparks. That’s quite a difference. A battery is always “live” even when the engine is not running. A magneto, however, is only “live” when the engine is running. So when troubleshooting a magneto system, instead of starting and stopping the engine to perform voltage tests, it is more convenient to leave the engine off and perform continuity (ohms) tests on “non-live” wires. When the leads on an ohm meter touch each other, the resistance (ohms) is essentially zero. When the pair of test leads does not touch each other the resistance (ohms) is essentially infinite. Instead of watching the needle swing on a meter, a continuity tester that makes sound is better. With a tester that beeps when the leads touch, and with the leads connected to the wire being tested, you can then focus your eyes on the suspicious area and wiggle or move the wires around listening for the beep to change. When your suspicious wire touches the bad spot, the tester will beep. Or when your suspicious wire goes open, the beep will stop. This is the usual way of checking bulbs, besides looking visually for a broken filament. A good bulb will have continuity (very low ohms), while a bad bulb will have no continuity (very high ohms). You can often check a bulb without removing it from the socket, by connecting the test leads to the wires that lead to the bulb. If there are branch points along that wire that lead to other bulbs, then those other bulbs must be removed from their sockets in order to isolate the suspicious one.

Testing a without a tester

     You don’t need a weatherman to know which way the wind blows. Likewise it is possible to perform useful tests with just swapping things around, unplugging things, or using paper clips or clip leads as temporary wires. When your toaster stops making toast, the first thing you do is go plug it into a wall socket that is known to work. Then if the toaster works there, you know the problem is in the wall socket. If it still does not work in the good wall socket, then you know something’s wrong with the toaster. Right there you just applied mathematical logic to perform troubleshooting. People who are good at math and logic puzzles also have an easier time solving electrical problems. The most important tool needed for electrical troubleshooting is the mushy grey stuff between your ears. Each test, when performed properly, establishes a new truth. The newly established truth forms the basis for a further test. Each successive test narrows down the possibilities, until finally there is only one possible explanation for the results of all the tests. There is almost always more than one way of troubleshooting. Instead of moving the suspicious toaster to a known good wall socket, an alternative way of troubleshooting it would be to take a known good lamp and plug it into the suspicious wall socket. If the known good lamp works there, then you know the wall socket is good and the toaster is bad. If the known good lamp does not work there, then you know the wall socket is bad but you still don’t know about the toaster.  That is the kind of precise mathematical thinking required to solve electrical problems (puzzles to some).

Example 1: Prince of Darkness

      Here is a real life moped troubleshooting example. Let’s say your 2003 Tomos Sprint head light does not work. You first learn about how it’s wired from the wiring diagram. That tells you there is a head light on/off switch on the right handlebar, and a high beam/low beam switch on the left handlebar. Right away you look to see if the switch is on. If it is, then you see if switching from high to low beam matters.  On an already running bike, you would start the engine, flip the switches, and observe. If the head light worked on high beam but not low, that would mean the problem must be after the high/low switch, in the low beam (white) wire, low beam bulb socket prong, or most likely the low beam filament inside the bulb. If the head light did not work on either high or low, then nothing new is learned. Looking at the wiring diagram you can see that all of the lights work of one wire. So you would test to see if the tail light, brake light, speedometer light, and horn work. Again you would start the engine, press buttons, and observe. If none of the devices work, regardless of what position the switches are in, then it’s possible that all of the devices (head light bulb, speedometer light bulb, tail light bulb, brake light bulb, and horn) could be bad all at once. However, it’s far more likely to be a single fault in the yellow wire that supplies all the devices. There’s two ways the yellow wire can be bad. It can be touching ground anywhere along its length, from the speedometer bulb to the magneto, causing a short circuit, or it can be an open circuit somewhere before the first branching point, possibly in the magneto. Understanding that is the hardest part. The next test would be to unplug the yellow wire from the engine, start the engine, touch the yellow wire from the engine directly to ground and see if it sparks. If the generator is working, it will spark. Let’s say it does spark. Then you would look to see if it sparked when you plugged it back in, maybe using a paper clip to see sparks that might be hidden by the connector cover. If it does spark when it’s plugged in, yet none of the lights come on, it can only mean that the yellow supply wire is touching ground somewhere, allowing the current to return without going through the lights. Next step is to start eliminating the various possibilities. The AC voltage regulator is a good place to start. You would unplug it and then start the engine and see if the lights work. On a 1992-later Tomos moped you need a voltage regulator only when the engine is revved up to keep the bulbs from burning out from too much voltage (like 18 VAC max without regulator, 13 VAC max with). The lights will work fine without a regulator, but the engine must not be revved up too much. Let’s say that the test is performed, and the lights still don’t work, even with the regulator disconnected. Next possibility might be to look at the yellow wires going to each brake light switch, since they are easy to get to. Pull the rubber hoods off and let the wires hang loose, a yellow and a green on each side. Now if the brake light green wire was shorted to ground, and at the same time the yellow and green on either side were unplugged and touching each other , then the brake light would be always on, always stealing the juice from the other lights. You would start the engine with the brake light switch wires loose. Bingo! Suddenly the head light, tail light, speedometer light, and horn all work. So you would put the regulator back and then go look for the green wire touching ground somewhere. You know it must be true. You would look first under the back fender, where the tire sometimes rubs against the tail light and brake light wires. Eureka! The green wire is all shredded and touching the brown ground wire. You went from one end of the bike to the other, without any test equipment, just plugging and unplugging wires in a logical progression.   

Example 2: The Flasher

     In this example, a 2008 Tomos LX, turning on the right turn signal makes all the lights flash. You go to the wiring diagram and notice that all the devices run off the yellow AC power wire. You reason that if the purple wire, which is for right side turn signals, is shorted to ground somewhere, then that would explain the bad behavior. Whenever the right side turn signals would try to go on, the short would steal the current away and make all lights go off. The turn signals are plastic and almost never get shorted internally. It can happen from someone bending the prong that touches the bulb, so that it touches the bulb socket. But more likely, the purple wire is touching ground somewhere under the back fender. After looking under the rear fender and seeing no signs of wire damage from rubbing on the tire, you would start performing tests to isolate the problem. You would unplug both right turn signals. The problem would go away if it was in one of the turn signals. But it doesn’t. You study the wiring diagram and notice there are many ways you can unplug things on a 2008-later model. You might choose to unplug the main plug. From looking at the wiring diagram you would know that the front turn signals and indicator light would be eliminated, but the rear turn signals would still try to function. You perform the test, and lo and behold, the problem goes away. This means that the purple wire is touching ground somewhere from the main plug forward. You would then maybe look at the front harness where it goes up under the gas tank. A normal 2008-later Tomos Sprint, ST, or LX has the three electrical bundles and the two control cables (throttle and rear brake) distributed evenly on the right and left sides of the steering tube part of the frame. Some 2008-09 bikes came from the factory with all of the wires and cables on one side. This makes it very difficult to get the cable tray on and very likely to pinch or puncture the wires with one of the cable tray screws. You notice that your bike is like that, with the cables and wires all on one side. You loosen the cable tray screws, letting the wire bundles dangle. You plug back in the main plug and start the engine. The problem goes away. Then you notice the snake bite puncture wound the cable tray screw made in the front wiring harness. The permanent fix is to unplug the main plug, and the right switch plug, and relocate the front harness and right switch harness to the empty left side. So the right side should have the throttle cable, rear brake cable, and the left and switch harness. The left side has the hefty front harness and the slim right switch harness. With this proper routing the cables and wires almost never get damaged.   

Make predictions

     The troubleshooting process involves making predictions. Look at any of the Tomos moped wiring diagrams in the collection. Imagine cutting a wire or two, here or there, or touching one or more to ground. In each case a good technician should be able to predict the misbehavior of all the lights and devices for all possible cases, just by looking at the wiring diagram or visualizing it in their head. If you study each of the eighteen Tomos moped wiring diagrams in this collection and imagine what would happen when any given wire becomes open or shorted, then when you do see the real life misbehavior, you will often know right away what the possible causes are, since that scenario has already been rehearsed. For example, ask “what if the blue (high beam) wire touches ground”. Answer “all the AC lights would go off, but only when the high beam is turned on”.

AC lights and DC lights

      The deluxe Tomos “battery” models, 2001 and later, including Revival and Streetmate, have both 12 volt AC and 12V DC electrical systems. Like the regular models (Sprint, ST, LX), the “battery” models use the 12V AC generator/magneto for the head light, speedometer light, and tail light. The other lights and devices, which are the brake light, turn signals, horn, oil light, and electric starter, are powered by a 12V DC battery.

 Example 3: The Double Crosser

 <Under construction>

Ignition

     The ignition is like the electric pulses of a heart beat. The voltage pulses are in sync with the rotating crankshaft. When the crankshaft rotates faster (or slower), the spikes are closer together (or farther apart) in time. See figure 5.  

Figure 5: Ignition is neither AC nor DC, but instead a train of narrow pulses, each happening at a precise time.

What makes the gasoline ignite is the spark plug sparking, in the right way, at the right time. What makes the spark jump the gap of the spark plug is a sudden very steep rise in voltage, to over 10,000 volts in under a few thousandths of a second. What makes that high voltage spike is a transformer, usually called “the ignition coil”. A transformer is like a car jack. It steps up (or steps down) voltage while stepping down (or stepping up) current. A sudden change in current in the (thicker but fewer) primary windings induces a sudden large change in voltage in the (thinner but many more) secondary windings. Supplying the transformer with a sudden change in current is either a CDI unit (Capacitor Discharge Ignition), on 1994 and later, or a mechanical points and condenser, on 1993 and earlier models. On late model Tomos mopeds with CDI ignition, the electronics are molded into the coil (unlike most Asian types). The “control unit” or “CDI unit” is a small 1.5 inch square circuit board, hidden inside the black plastic of the coil. Early models before 1994 had plain coils, without any electronics.

How to test a coil

     Any ignition coil (coil only, not coil-with-CDI like Tomos uses) will produce a spark on a spark plug, at the instant the input wire is touched to a 12 volt battery (+), with the battery (-), coil, and plug all grounded (with clip leads). If you touch it slowly to the battery post, it will produce a weak spark. If you swipe the coil wire fast and hard against the battery post, it will produce a strong spark. This demonstrates that it’s the shorter “rise time”, much faster than the blink of an eye, that makes a better spark. Only perform the above spark test on plain coils. CDI/coils might get damaged from the high current possible. Never connect a 12V battery to a coil for longer than one second. It turns out that moped coils are never bad. Only occasionally the CDI/coils go bad. “Bad” means it produces no spark at all, or it produces weak or intermittent spark. Intermittent spark is like turning a light switch off and on, often in an unpredictable way. The only way to test a CDI/Coil is to put it on a known good running bike and test ride it, listening and feeling for any stumbling or misfiring. If no test bike is available, then the only way to test the CDI/Coil is to replace it with a known good one, and hope that fixes the problem. If it’s not the CDI/Coil, and not the bike’s wiring, then it must be the magneto. The CDI magneto costs way more than the CDI/Coil, and is more difficult to change, and so it’s last.

How to check for spark

      CDI solid state ignitions have shorter rise and fall times than mechanical point ignitions. That makes them start with a weaker kick, or even with your hand. CDI ignitions have a faint spark that is sometimes invisible in bright sun. Always check for spark in the shade. “Checking for spark” means removing the spark plug, connecting the plug wire to it, laying it on the engine so that it’s metal shell is touching ground (the head fins), and then turning the engine over (by pedaling backward or kick starting) while watching the gap between the electrodes of the spark plug. Spark color indicates temperature. Blue spark is hotter than pink or yellow. Faint blue spark is good. Bright white spark is bad, usually because of a semi-conductor-coated “fouled” spark plug.  The white porcelain insulator must not be coated with black or shiny carbon. If it is not dull white, tan, or brown, then use a new spark plug to check for spark. A fouled plug will spark badly or not at all, even when the ignition is working fine. You can also hear the snap sound of the spark. Loud is good.

Grounding the coil wire stops the engine 

     In both CDI and points type magneto ignitions, the “heart beat” is intentionally stopped when you turn the engine stop switch off. When the “kill” switch is “on” the ignition wire is not grounded. When the kill switch (engine stop) is “off” the ignition wire is grounded. This is the opposite of a (battery ignition) car. So to hot wire a magneto ignition bike you only need to disconnect or cut the kill switch wire so that it cannot be grounded. When troubleshooting a late model Tomos that has bad or no spark, the way to eliminate all the wires in the bike is to plug the black (or red) magneto ignition wire (which has a female blade connector) into the male blade of the coil (or it’s black or red wire with male blade). This isolates the circuit to just the engine and coil. Just remember you can’t turn the engine off.  

Tomos A35 and A55 CDI ignitions

     There are two types of Tomos CDI magneto ignitions, the A35 type (1994-2006) and the A55 type (2001-later). They consist of two or more coils mounted on a stationary plate, surrounded by a rotating flywheel with four strong magnets molded into it. The A35 ignition has no pulser coil to tell it when to spark. It triggers when the source coil voltage reaches a pre-set level. So changing the location of the source coil (by rotating the stator plate) changes the ignition timing. Tomos provides adjustment slots in the stator plate, and timing marks (thin angled black lines) on the crankcase (minimum and maximum ignition timing), at about the 10 o’clock position. An ignition timing strobe light is the only way to check the ignition timing. The strobe light illuminates the fast moving flywheel mark only for an instant. So it appears, magically, to be not moving. The fire mark on the flywheel should lie between the black lines on the case. It should not jump around. The light should not flicker. Once the stator is set in the desired position and the screws are tightened, the timing is set forever, and never needs adjusting (that is assuming the flywheel locating key is in its proper place and the flywheel nut is fully tight). Unlike the A35, the A55 ignition does have a pulser coil that signals the control unit when it’s time to spark. It’s an external pulser (or exciter or trigger) coil that senses a raised strip on the outer edge of the steel flywheel/rotor. So changing the stator position does not change the ignition timing. The A55 external pulser is fixed and mounted in a way that prevents it from being adjusted.   

Figure 6a: A55 “no battery” ignition exploded view       Figure 6b: Same ignition shown in a semi-schematic diagram

         There are about ten different Tomos magnetos for the A35 and A55 engine series. They are all covered in detail in the wiring diagrams collection that this written text supplements. There are also a few more for the earlier A3 models. Before 1986, Tomos A3 mopeds had an external ignition ground that powered the brake light. On those early models if you unplugged the brake light, and then squeezed either brake, the engine would lose spark and cut out. Those early A3’s, with 3-wire magnetos, yellow,  black, and blue, often need the blue magneto wire to be grounded in order to have spark.  After ‘86, all Tomos magnetos are 2-wire magnetos, yellow and black, with no external ground, one less worry.        

Ignition timing

      The spark occurs a little before the piston reaches top. That explains ignition timing in one simple sentence. Spark happens before top because the gasoline and air take a tiny bit of time to burn. If the spark happened right at the top of the piston stroke (“top dead center” or TDC), the engine would run, but be weak. If the spark happened twice as early as it should, the engine would run, but be weak and run hot. There’s a preferred range where the “porridge is just right”. Expressed in terms of crankshaft angle, the preferred ignition timing is between 10 and 20 degrees before top. Imagine the flywheel is a pie. Cut the pie in six pieces and you have 60 degree slices. Cut one of those pie slices in three thin slices. That would be a 20 degree angle. That thin slice is a little less than an inch wide on the edge of the flywheel. The time taken for the wheel to move that much is about the time taken for the gas to ignite. At 6000 rpm the piston rises every 0.01 second. Expressed in actual seconds, at 6000 rpm, the spark occurs .01*20/360 = 0.00055 seconds before the piston reaches top. That’s half of a thousandth of a second. You don’t need to know that to fix mopeds, but it makes the lesson interesting and amazing. The faster the engine turns, the less time the fuel has to burn. Because of this, most gasoline engines have variable ignition timing, rather than fixed timing. Four stroke engines like the timing to be about 10 degrees at idle, advancing to about 35-40 degrees at high rpms. They have vacuum advancers, centrifugal advancers, and electronic control units (ECU), to vary the ignition timing automatically. Two stroke engines like the timing to be about 20 degrees at idle, retarding to 10 degrees at high rpms. The explanation is better atomization at higher engine speeds makes smaller fuel droplets which take less time to burn. Before 1994, Tomos A35 mopeds had points, with fixed ignition timing, 20 degrees BTDC. From 1996 on, they had Iskra CDI, with variable ignition timing, 20 deg at idle changing to 10 degrees above about 6000 rpm. The only way to see the timing is with a strobe light made for automobile ignition timing checking. To actually measure the degrees, cars use a thing called a “degree wheel” that attaches to the crankshaft. Any piece of paper with accurate degree lines can be attached to the flywheel, but putting it in the exact correct position is difficult. There’s a clever and simple procedure for finding top. A tool called a piston stop is screwed into the spark plug hole. The crankshaft is rotated by hand until the piston is parked up against the stop. A line is made on the flywheel, adjacent to any chosen case mark. Then the crankshaft is turned by hand the other way until the piston is again parked up against the piston stop. A second line is made on the flywheel adjacent to the case mark. Then a tape measure is laid around part of the flywheel to find the midpoint between the two marks. That is “top dead center”. Fortunately the Tomos flywheels already have inscribed timing marks, one for top, one for fire, spaced 20 degrees apart. The F mark should be between the crankcase marks, when viewed with the strobe. The newer A55 engine mopeds have the same ignition timing but it is not adjustable like the A35 is. That’s Tomos saying don’t mess with a good thing.

            Normally, ignition timing is never an issue with CDI ignition 1994-later Tomos mopeds. That is because those magnetos are maintenance free. It is, however, often an issue on the earlier points-ignition models. This is because the ignition timing gets retarded as the points rubbing block wears down. So points need adjustment (spread apart to have more “gap”), and lubing with high melting point grease, every few thousand miles. Besides that, they can be adjusted wrong. The points are designed to open at the right moment in a running engine, when at fully open in a stationary engine they have a gap of .014 to .017 inch (.35 to .45mm). In a magneto with points ignition, the spark happens when the points open, not close. That is essential to know. You can spot check the ignition timing of a points model with nothing but a pen and a flashlight. Find TDC with the pen in the spark plug hole. Rotate backward 20 degrees and see if the points are just starting to open there. In both points-ignition and CDI ignition, the flywheel can be installed in the wrong position on the crankshaft by leaving out the little locating pin. That’s not a normal thing. Neither is running with a loose flywheel nut until the pin shears off and ruins the crankshaft and flywheel. Ouch. It’s also not normal to have some other flywheel, like from a Puch, Sachs, or aftermarket one that has no timing marks or is sparking at the wrong time. A bunch of different moped flywheels will fit the Tomos crankshaft, but they might have the wrong ignition timing because of a different point cam angle. Some stator plates can be installed upside down, making the timing 180 degrees off. Only in these not-normal circumstances you might need to mark the flywheel and check the ignition timing.

Ignition symptoms

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