An engine that misfires, chokes when you open the throttle or stalls when you stop at a light is both annoying and dangerous in traffic. These symptoms have dozens of possible causes, but all of them come down to one of three basic needs: the engine needs the right amount of fuel, clean air and a strong spark at the right time. In this guide we list the possible causes according to when the symptom appears and give an order of checks starting with the simplest. It is a roadmap for diagnosis; for a definitive finding you will often need a workshop.
First: is a warning light on?
On fuel-injected engines, the malfunction indicator light (MIL or FI light) on the instrument panel is the first place to look. The Turkish manual for Honda's CB125F (SC74 type) says that if this light comes on while riding, there may be a serious problem in the injection system, and tells you to slow down and have the motorcycle looked at by a workshop as soon as possible. If the light is on, go to a workshop instead of working through the list below; the fault code is read with a diagnostic tool.
Quick order of checks
- Fuel: Is there enough petrol in the tank? On carburettor engines, is the fuel tap open, and do you need to switch to reserve?
- Engine stop switch and side stand: The stop switch on the handlebar may have been accidentally moved to the off position. On most motorcycles, if you select a gear with the side stand down, the safety switch stops the engine; if the stand switch is faulty, it may cut the engine while riding too.
- Battery and connections: When the engine will not run, the Honda manual asks for a loose battery connection, corrosion on the terminals and the battery's state of charge to be checked. A weak battery can also cause erratic running on a fuel-injected engine. See battery care.
- Fuse: If nothing happens when you press the starter, there may be a blown fuse.
- Spark plug: A fouled or worn plug is a well-known cause of misfiring and hard starting. See the spark plug guide.
- Air filter: A blocked filter starves the engine of air; it gets dirty faster, especially on dusty roads.
- The fuel itself: Petrol goes stale in an engine left standing for a long time; if water has got into the tank, the engine misfires.
Misfiring or stalling when cold
- Choke (carburettor engines): A cold engine wants a rich mixture. If you pull away without using the choke the engine chokes or stalls; if you leave the choke on, it also misfires and uses too much fuel after it warms up.
- Weak battery: In cold weather the battery's efficiency falls; the starter turns heavily and a fuel-injected engine is hard to start.
- Spark plug: A fouled plug shows itself even more in the cold.
- Loading before warm-up: Taking a cold engine to high revs as soon as you start it can cause misfiring. Wait briefly at idle and ride the first kilometres gently.
We explained how to prepare the motorcycle for winter in the winter guide.
Running unevenly or stalling at idle
- Low idle speed: If the idle speed has fallen below the value in the manual, the engine stalls at a light. For example, in the CB125F manual the idle speed is 1,500 ± 100 rpm. Honda puts a check of this setting in the maintenance schedule at 1,000 km and then every 4,000 km.
- Dirty air filter or spark plug: The mixture balance is delicate at idle; even slight fouling causes vibration and stalling.
- Blockage in the carburettor: On carburettor engines that have stood for a long time, the fine passages in the idle circuit can clog with deposits; they need cleaning.
- Air leak: A leak on the intake side upsets the mixture and makes the idle hunt. A workshop will find this.
Misfiring, choking or not picking up when you open the throttle
- Fuel supply: A blocked fuel filter or a weakened fuel pump falls short just when the engine demands more fuel. The Honda manual also says that when the fuel pump filter is blocked the engine may often run erratically while riding; if it runs erratically despite enough fuel, take it to a workshop.
- Air filter: A blocked filter is felt most when the throttle is opened; the engine is left breathless.
- Spark plug and ignition: If the spark weakens under load, the engine misfires especially while accelerating.
- Throttle cable: Too much slack or sticking in the cable disrupts throttle response. The operation of the throttle grip is also among the items regularly checked in the maintenance schedule.
- Misfiring at full throttle: If it appears only at high revs, suspect the fuel supply or the performance of the ignition system under load.
Stalling once warm
If the engine runs properly when cold and stalls after it warms up, the possible causes are a little more technical: the idle setting being low for a hot engine, a fault in the systems that regulate the mixture, or an ignition part that drops out when warm. On liquid-cooled engines, overheating also needs to be checked; if the temperature gauge or warning light comes on, stop the engine and wait for it to cool. We explained how the cooling system works in the cooling guide. Rather than dealing with this symptom on your own, go to a workshop.
It suddenly stalled while riding
- Pull over somewhere safe and switch on the hazard lights.
- Look at the fuel level and the reserve tap.
- Check the position of the engine stop switch and the side stand.
- Look at the instrument panel for a fault or battery light.
- When you press the starter, does the starter motor turn? If it does not, the problem is most likely on the electrical side (battery, fuse, connection).
- If the starter turns but the engine does not start, it is on the fuel or ignition side; this is hard to fix at the roadside, so call a recovery truck or help.
Before setting off on a long trip, take basic tools and fuses with you. For a list, see the long journey preparation guide, and for tool sets look here.
What should you tell the workshop?
If, instead of taking a fault in as "the engine misfires", you go armed with these notes, diagnosis becomes both faster and cheaper:
- When does the symptom appear: when cold, when warm, at idle, when opening the throttle, at a certain speed?
- Since when has it been there; did it start suddenly or increase gradually?
- Where did you last buy fuel, and when were the last service and plug change?
- Did it start after rain, after washing or after a long period standing?
- Has the warning light come on, is smoke of a different colour coming from the exhaust?
Prevention is cheaper than cure
- Change the spark plug, air filter and oil at the intervals in the manual. For the schedule, see the maintenance guide.
- Buy fuel from stations you trust, and prepare the motorcycle before leaving it standing for a long time.
- Support the battery with a trickle charger; see battery chargers.
- Look at the air filter after dusty roads; filters are here.
Conclusion
Misfiring, not picking up and stalling show that the engine is not getting enough of one of fuel, air or spark. Look at the warning light first; if it is on, go straight to a workshop. If it is not, start with the simple things: fuel, stop switch and side stand, battery and fuse, spark plug, air filter. When the symptom appears, whether cold, at idle, on opening the throttle or when warm, is the most valuable information you can give the workshop.
Frequently asked questions
Why does a motorcycle not pick up?
Possible causes include a blocked air or fuel filter, a weak fuel pump, a fouled spark plug and throttle cable adjustment problems. If the warning light is on in a fuel-injected engine, a workshop finds the cause by reading the fault code.
Why does the engine stall at lights?
Most often the idle speed has dropped, or the air filter, spark plug or the carburettor's idle circuit is dirty. Compare the idle speed with the value in the manual; on the CB125F this value is 1,500 ± 100 rpm.
Can I keep riding while the warning light is on?
Honda's advice is to slow down and have the motorcycle looked at by a workshop as soon as possible. The light may point to a serious problem in the injection system.
My new motorcycle misfires; is it covered by warranty?
Warranty terms vary by brand and are written in the warranty document. Take it to an authorised service centre without interfering yourself and describe the symptom in detail.