The carburettor prepares the fuel and air mixture for the engine, and how clean it is decides how the bike starts, holds its idle and responds to the throttle. Fuel deposits, varnish after a long lay-up and dust that slipped past the filter clog the fine passages and jets. The symptoms are familiar: the engine stalls at idle, stumbles when you open the throttle and uses more fuel.
It happens most often in spring, after winter storage with fuel left in the float bowl. Clean the carburettor at the manufacturer's service interval or as soon as the engine starts running unevenly. Carburettors differ in design from model to model, so read the motorcycle and carburettor manuals before you start and do not change the factory settings.
What you will need
Important: Petrol is highly flammable. Work on a cold engine in a well-ventilated place, away from naked flames and smoking, and wear safety glasses. Turn off the fuel tap before disconnecting the hose and have a container ready for spillage.
Step 1
Prepare the bike
Wash the motorcycle thoroughly and let it dry completely: sand, mud and bits of asphalt getting into the carburettor can seriously damage the engine. Remove the side covers or bodywork so you can reach the carburettor easily. Photograph the routing of cables, hoses and linkages so you can put everything back correctly.
Step 2
Disconnect the air filter
Loosen the screws or clamps and remove the air filter housing. Check the filter itself: some types can be washed or blown out, others can only be replaced. A clogged filter must be replaced, or the clean carburettor will soon be dirty again.
Step 3
Disconnect the fuel hose and throttle cable
Turn off the fuel tap at the tank outlet. Squeeze the clip tabs with pliers and pull the hose off the carburettor inlet, holding a container under it for leftover fuel. Disconnect the throttle cable and, if fitted, the choke mechanism from the carburettor top. There is often a mesh filter at the fuel inlet: inspect and clean it.
Step 4
Remove the carburettor
Most carburettors are attached to the cylinder head by a rubber intake sleeve with a metal clamp: loosen the clamp and lift the carburettor off. If it is held on studs with seized nuts, spray the threads with WD-40 Multi-Use, wait a few minutes and undo them steadily without jerking.
Step 5
Check the gaskets and open the float bowl
Inspect the gasket between the carburettor and cylinder and the one on the air filter side: tears, swelling or splits upset the mixture and can even stop the engine. Over a container, unscrew the float bowl. Its bottom is usually covered in sediment and deposits. Carefully remove the float pivot pin, the float and the needle valve, and check the bowl gasket, which often needs replacing.
Step 6
Clean the body and passages
Remove rubber seals, diaphragms and plastic parts that do not need cleaning, then treat the metal body, float bowl and passages with WD-40 Specialist Carb Cleaner, directing the spray into every opening. It dissolves fuel deposits and varnish; give it time to work and repeat on dirty areas.
If you are not experienced, leave the jets, mixture screws and slide with needle in place and simply blow compressed air through the holes after cleaning. Experienced mechanics can remove the jets and clear them with thin soft wire, taking care not to enlarge the calibrated holes.
Step 7
Reassemble and lubricate the throttle linkage
Make sure every part is clean and dry, reassemble the carburettor and refit it in reverse order, checking against your photos. Treat the throttle linkage pivots and the slide mechanism with WD-40 Specialist Dry PTFE spray: it lubricates without being sticky or attracting dust. Open the fuel tap, check for leaks and start the engine.
Tip: When laying the bike up for winter, turn off the fuel tap and let the engine run until it stops, or drain the float bowl. That way no varnish forms in the carburettor and you may not need to clean it in spring at all.
What next
After reassembly, warm up the engine and check the idle. If it still runs unevenly, leave mixture adjustment to a mechanic. For ongoing care, keep WD-40 Multi-Use to protect metal parts from moisture and dry PTFE spray for cables and pivots.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
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