A two-stroke burns oil with its fuel. Over time the oil leaves carbon in the exhaust port, the expansion chamber and the silencer, and wet oil sludge builds up if the mix is too rich or the bike is ridden slowly for long periods. A choked exhaust makes the engine lose power, run hot and four-stroke (fire every other turn). Decarbonising (decoking) removes this carbon. The silencer and port are workshop jobs on many bikes because the cylinder may need to come off; the K-Light schedule also lists decarbonising for four-strokes that lose power, as an inspection item.
Safety
- Exhaust parts are hot for a long time; start cold.
- Burning carbon out of a silencer with a flame or a fire produces toxic smoke and can warp the pipe. It is not recommended.
- Strong caustic soda solutions burn skin and eyes and attack aluminium. Use only where the shop has the right protection.
What you can check safely
- Remove the spark plug. Heavy wet black oil on it points to too much oil or a blocked exhaust.
- Remove the silencer end cap or baffle if your model has a removable one, and look inside with a torch.
- With the engine cold, look into the exhaust port from the header side (after removing the header pipe nuts) with a torch. A narrowed port with thick carbon needs cleaning.
- Check the two-stroke oil supply first: Check the two-stroke oil pump and lines. Decoking without fixing a rich oil setting only buys time.
What the workshop does
- Removes the exhaust and the removable baffle, and soaks or scrapes the carbon out. Packed silencers (with glass wool) have the packing replaced.
- Removes the cylinder if the port is badly choked. Turns the piston so the exhaust port is closed, or removes the cylinder completely, so no carbon falls into the crankcase.
- Scrapes the exhaust port only. Transfer and intake ports may have special coatings and are left alone.
- Cleans the piston crown and head chamber with soft scrapers, without scratching the aluminium.
- Checks the piston, rings and bore while it is apart: see Assess the top end (cylinder, piston, valves).
- Refits with new gaskets and torques to the manual.
What to ask for
- New exhaust gasket and silencer packing.
- Check of the oil pump setting or premix ratio for your model.
Do not attempt at home
- Drilling or cutting into a sealed silencer or expansion chamber.
- Removing the power valve: deferred to a specialist with the model manual.
Check your work
- Engine revs freely, pulls cleanly and smokes only lightly.
If it did not fix the problem
- Low compression after decoking: Low compression.
- Still oily: Oil dripping from the exhaust (two-stroke).