Tomos no spark problem | Check ignition, CDI and spark plug

Estimated read time: Calculating... | Originally published: October 2, 2025 | Latest update: May 12, 2026

Check your Tomos ignition: no spark, weak spark or poor starting

The ignition is the heart of your Tomos. Without a good spark, your moped will not start, may run unevenly or may cut out as soon as you open the throttle. In this guide we explain step by step how a Tomos ignition works, which parts commonly cause problems and how to trace the fault logically.

This guide is useful for Tomos A3, A35, A52 and A55 riders who want to measure, test and repair their own ignition system. You will learn how to check the spark plug, spark plug cap, spark plug lead, kill switch, wiring harness, CDI/coil, stator and flywheel with basic workshop checks and a multimeter.

Quick answer: Does your Tomos have no spark? Always start with the simple parts: spark plug, spark plug cap, spark plug lead, ground connection and kill switch. If those are good, check the CDI/coil, then the ignition, stator, stator plate, flywheel and flywheel key.

How does a Tomos ignition work?

The ignition path on your Tomos starts with the ignition. The flywheel rotates around the stator coils. The magnets inside the flywheel generate voltage as they pass the coils. That voltage goes to the CDI/coil unit, where the ignition pulse is built.

The coil converts this low voltage into high voltage. That high voltage travels through the spark plug lead and spark plug cap to the spark plug. There the spark jumps across the electrodes. If one part of this chain fails, you can get no spark, a weak spark or an irregular spark.

Part	Function	Typical fault	What should you check?
Flywheel	Spins magnets around the stator coils	Loose flywheel, damaged key or weak magnets	Tightness, key, timing and damage
Stator / ignition coils	Generates voltage for the ignition	No voltage or voltage too low	AC voltage and wiring from the ignition
CDI/coil unit	Controls the pulse and creates high voltage	No spark or irregular spark	Resistance, corrosion, moisture and burn marks
Spark plug lead and cap	Carries high voltage to the spark plug	Leakage, poor connection or broken lead	Resistance, insulation and visual damage
Spark plug	Creates the spark inside the cylinder	Worn, wet, fouled or wrong electrode gap	Colour, condition, gap and spark test
Kill switch and wiring harness	Switches the ignition off by grounding it	Stays grounded or has a broken wire	Continuity, ground and connectors

Quick diagnosis for no spark on your Tomos

Always work from simple to complex. Replace or test the spark plug first, then check the spark plug cap and lead, rule out the kill switch and only then start measuring deeper parts such as the CDI/coil and ignition.

Important: Never hold the spark plug with your bare hand while kickstarting. The coil can create high voltage. Make sure the thread of the spark plug rests firmly against a clean ground point on the engine or frame.

Symptom	Likely cause	First test	Next step
No spark at all	Spark plug, kill switch, ground, CDI/coil or stator	Test with a new or known working spark plug	Rule out the kill switch and measure the ignition
Weak yellow spark	Bad spark plug, plug cap, lead or coil	Check spark plug gap and plug cap	Replace the lead/cap or test the CDI/coil
Spark disappears when moving wires	Broken wire or bad connector	Measure continuity with a multimeter	Repair wiring harness or replace connector
Engine starts sometimes, then cuts out	Poor ground, loose stator plate, faulty CDI or wrong timing	Check ground and flywheel/stator plate mounting	Measure ignition and CDI/coil
Wet spark plug after repeated kicking	No spark or too much fuel	Dry/replace spark plug and perform spark test	Test ignition and check carburettor setup

Check the Tomos kill switch and wiring harness

A faulty kill switch can permanently ground the ignition. When that happens, your Tomos will not make a spark, even if the spark plug, coil and ignition are otherwise good. Loose wires, poor connectors, corrosion and broken wires in the harness can also interrupt the spark.

How do you recognise a faulty kill switch or broken wire?

Does your Tomos have no spark while the kill switch is in the run position? Or does the spark disappear when you move the wires near the handlebar, steering head or frame? Then the problem is often in the kill switch, ground, connectors or wiring harness. Also look for green corrosion, loose plugs, cracked insulation and places where the wires rub against the frame.

How do you test the kill switch with a multimeter?

Set your multimeter to continuity or low ohms.
Hold one probe against a clean ground point on your Tomos.
Hold the other probe against the output wire of the kill switch.
In the stop position, the kill switch should make contact with ground.
In the run position, the kill switch should not make contact with ground.
Then measure the wires from end to end. If you hear no beep or see no continuity, you probably have a broken wire.

Tip: Wire breaks often happen in places with a lot of movement, such as the steering head, handlebar controls and wire routing along the frame.

View all Tomos kill switches and electrical parts for your wiring harness.

Test the Tomos spark plug, spark plug lead and cap

The spark plug, spark plug lead and plug cap are small parts, but they often cause major ignition problems. A worn spark plug, bad cap or broken lead can cause no spark, a weak spark or a spark that jumps to the frame instead of the plug.

Recognising problems from the spark plug

Spark plug appearance	What does it mean?	Possible cause	What should you do?
Black and sooty	The engine is probably running too rich	Main jet too large, too much oil or poor combustion	Check setup and clean/replace the spark plug
White or very light	The engine may be running too lean	Main jet too small, air leak or not enough fuel	Do not keep riding; check carburettor and air leaks
Wet	Fuel is not being ignited properly	No spark, weak spark or flooded engine	Dry/replace the spark plug and perform a spark test
Worn electrode	Spark becomes weaker or irregular	Old spark plug or wrong spark plug	Replace the spark plug and check the gap

How do you test a spark plug for spark?

Remove the spark plug from the cylinder head.
Check the electrode gap. For many Tomos setups, around 0.45 mm is a good starting point.
Put the spark plug back into the spark plug cap.
Hold the thread of the spark plug against a clean ground point on the engine.
Kick the engine over. Do you see a clear blue spark? Then the spark plug is probably working well.
If you see no spark, test with another spark plug to rule out the plug itself.

Check the spark plug lead and cap

A bad spark plug lead or cap can break internally, hold moisture or leak current. Then the spark does not jump cleanly at the plug, but disappears through the lead, cap or frame.

Visually check the spark plug lead for cracks, hard spots and damaged insulation.
Set the multimeter to resistance (Ω).
Measure between the copper core of the spark plug lead and the contact point inside the plug cap.
If you get no normal resistance reading or no connection at all, the cap or lead is suspect.
Also check for unwanted continuity to ground. Continuity to ground often means current leakage or damaged insulation.

View all Tomos spark plugs. Want to know more about plug types, resistance and short or long reach? Also read our blog: explaining different spark plugs for Tomos.

Check the Tomos CDI, coil, stator and flywheel

If the spark plug, plug cap, spark plug lead, ground and kill switch are all good, go deeper into the ignition path. On many Tomos A35 and A55 models, the CDI and coil are combined in one unit. This unit processes the ignition pulse and creates the high voltage for the spark plug.

How do you recognise a faulty CDI/coil unit?

A faulty CDI/coil unit often causes no spark, a weak spark or a spark that drops out irregularly. Sometimes the engine runs reasonably when cold, but cuts out when warm. Always check the connectors, ground, moisture, corrosion and visible burn marks first.

How do you measure the CDI/coil unit?

Disconnect the CDI/coil unit from the wiring harness.
Set your multimeter to Ohms (Ω).
Measure between the CDI/coil input and ground.
Then measure between the coil spark output and ground.
An infinite reading (OL) often points to a broken winding or wire break.
A reading of 0 Ω often points to a short inside the coil.

Note: Resistance values can differ between ignition types and parts. Use the measurement mainly to find obvious faults, such as OL, 0 Ω, corrosion, bad connectors or visible damage.

Measure the stator and ignition

The stator and coils generate voltage as soon as the flywheel rotates. You can use a multimeter to check whether AC voltage is being generated while kickstarting.

Set the multimeter to AC voltage.
Measure between the ignition wire and ground while kicking the engine over.
You should see a fluctuating voltage while the flywheel rotates.
If you see no voltage or extremely low voltage, a coil, wire or ground connection may be faulty.

Check the flywheel, stator plate and flywheel key

The flywheel must be securely fastened. A loose flywheel, shifted stator plate or damaged flywheel key can disturb the ignition timing. This can make your Tomos run poorly, refuse to start or cut out irregularly.

Check that the flywheel nut is tight.
Check that the stator plate is tight and has not shifted.
Check that the stator plate is set to the correct snor or brom position.
Check that the flywheel key is not bent, worn or broken.

View all Tomos ignitions and parts for your ignition system.

FAQ: Tomos ignition and no spark

What should I check first if my Tomos has no spark?

Start with the simple parts: spark plug, spark plug cap, spark plug lead, ground connection and kill switch. First test with a good spark plug. Then rule out the kill switch and wiring harness before measuring the CDI/coil or stator.

How do I test the kill switch on my Tomos?

Set your multimeter to continuity. In the stop position, the kill switch should connect to ground. In the run position, there should be no connection to ground. If it keeps grounding the ignition, your Tomos cannot make a spark.

What spark plug gap does a Tomos need?

For many Tomos setups, around 0.45 mm is a good electrode gap. Always check that the spark plug is clean, dry and not worn. The wrong gap can cause a weak or irregular spark.

How do I know if my Tomos CDI or coil is faulty?

A faulty CDI/coil often gives no spark, a weak spark or a spark that drops out. Check connectors, ground, spark plug lead and plug cap first. If you measure OL, 0 Ω, a clear short, corrosion or visible damage, the CDI/coil unit is suspect.

Can a loose flywheel stop my Tomos from starting?

Yes. A loose flywheel, shifted stator plate or damaged flywheel key can disturb the ignition timing. This can make your Tomos start poorly, run unevenly or fail to produce a reliable spark.