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Nederlandse versie

Safety: work outside or in a well-ventilated space, no smoking, no open flames. Petrol and a spark together are dangerous. Never hold a plug cap with bare hands while you turn the engine over: the spark voltage is high.

Step 1: the simple things

Is everything set right?

• Ignition on? Key turned properly and is the light on (if your bike has one)?
• Kill switch on RUN? On some old bikes there is a button on the handlebars that shorts the ignition as long as you press it. Check that it is not stuck.
• Is it in neutral, or are you holding the clutch in? Later bikes sometimes only start that way.
• Fuel tap open? Also try the reserve.
• Is there enough petrol in the tank? Look in it, not just at the gauge.
• Is the hole in the filler cap open? If it is blocked, after a while no more petrol comes through.
• Cold engine? Choke on. Warm engine? Choke off.
• Two-stroke? Is there premix in the tank in the right ratio?

Step 2: is it getting fuel?

Checking the fuel

• Open the fuel tap and disconnect the fuel line at the carburettor, over a tray. Is there a good flow? Then the petrol is reaching the carburettor.
• Briefly undo the float bowl drain plug (if your carburettor has one). Does petrol come out? Then there is petrol in the carburettor.
• Has the bike stood for a long time? Old petrol smells stale and is dark. Drain it and fill up with fresh petrol.
• Do you see droplets or a layer at the bottom of the petrol? That may be water. Drain everything.
• Is no petrol getting into the float bowl? Then the float needle may be stuck or the filter in the inlet blocked. See our page Dell’Orto carburettor.

Quick test with starting spray

• Not sure whether it is the fuel? Spray very briefly (half a second) of starting spray into the air intake or air filter and try to start straight away.
• Does it fire briefly and then die? Then spark and compression are fine and the problem is in the fuel supply or the carburettor.
• Does it do nothing at all? Then the problem is more likely the spark or the compression: carry on with steps 3 to 6.
• Use it sparingly and not repeatedly. Too much starting spray can blow back through the carburettor and is hard on the engine.
• Two-stroke: better not, or only a very little. Starting spray washes away the oil, and a two-stroke gets its lubrication from the petrol.
• Do not use brake cleaner. Many brake cleaners contain chlorinated solvents. When they burn, toxic gases are produced. Use proper starting spray.

Step 3: is the battery good?

Battery and starter motor

• Measure the battery with a multimeter, with nothing switched on. A full 12-volt battery gives about 12.6 to 12.8 volts; a full 6-volt battery about 6.3 volts or more. Preferably measure a few hours after charging. If it is clearly lower, charge it first.
• Do you only hear a click when you press the starter button? Then the battery is often too flat, or there is a bad connection. Clean the battery terminals and the earth lead and tighten them properly.
• Check whether a fuse has blown.
• Note: many small Italian classics have a flywheel magneto. It makes its own current for the spark, even without a battery. If your bike has battery-and-coil ignition, a full battery is needed for the spark.

Step 4: does the plug spark?

Checking the spark

• Unscrew the spark plug and push it back into the plug cap.
• With insulated pliers, hold the plug with its thread against the engine or cylinder head, so it is earthed. Keep it away from the plug hole: petrol vapour can come out of there.
• Ignition on and kick the kick-starter or press the starter button. Look at the tip in the shade.
• A bright blue spark: good. A weak, yellow or no spark: first try a new plug.
• Still nothing? Then look at the plug cap, the HT lead, the contact points (dirty or wrongly gapped), the condenser and the coil. With points, cleaning them with a strip of emery paper and setting the gap often does the trick.
• Is the spark fine but at the wrong moment? Check the ignition timing with our free degree wheel.

Step 5: what does the plug say?

Looking at the plug

• Wet and smells of petrol: the engine is flooded. Choke off, hold the throttle wide open and crank or kick it over a few times. Dry the plug or fit a new one.
• Bone dry: no petrol is getting into the cylinder. Back to step 2.
• Black and sooty: the mixture is too rich, for example from the choke or a dirty carburettor.
• Light brown: that is how it should be.

Step 6: compression

Can you feel resistance?

• Push the kick-starter down slowly. You should feel clear resistance as the piston comes up.
• Do you feel almost nothing? Then the valve clearance may be too tight (the valve does not close), or the piston rings or a valve are worn. A compression gauge gives certainty.
• Valve clearance and settings per make are in our workshop guides.

SourcesRider Magazine: Prep your motorcycle battery for spring (English)Yuasa: frequently asked questions about motorcycle batteries (English)