Home mechanic (basic tools) ยท Time: 20 to 30 min (estimate) (estimate)
When the piston moves, the air pressure inside the crankcase rises and falls. Some combustion gas also leaks past the piston rings (blow-by). The breather is a hose, often with a small chamber or separator, that lets this gas out without letting oil out. On most bikes it runs to the air filter box so the fumes are burned. A blocked breather builds pressure and pushes oil out of seals and gaskets. A lot of smoke or oil at the breather means heavy blow-by from engine wear.
Safety
- Work on a cold engine.
- Do not run the engine with the breather pulled off near your face; hot oil mist can come out.
You need
- Torch, rags, pliers for hose clips.
- A new hose of the same inside size if the old one is cracked or soft.
Steps
- Find the breather hose. It usually leaves the top of the crankcase or the head cover and goes to the air filter box.
- Follow the whole hose. Look for kinks, crushed sections, cracks, a missing clip or a hose that has come off.
- Remove the hose. Look through it against the light. Clear sludge with a rag on a wire, then blow through it. Do not use a sharp wire inside the engine fitting.
- Find the drain at the bottom of the air box or breather chamber (often a short clear tube with a plug). Remove the plug and let any oil and water out. Some bikes have a mark on this tube; empty it when liquid reaches the mark.
- Look at the air filter. Oil soaked into it from the breather side is a warning sign. Replace a soaked paper element; see Clean or replace the air filter.
- Check the oil level. An overfilled engine pushes oil into the breather; see Check the engine oil level.
- Refit the hose with clips, with no kinks and away from the exhaust.
- With the engine idling, hold a clean rag a little distance from the breather outlet or the air box drain. A light mist is normal. Steady smoke or puffs that pulse with the engine mean heavy blow-by.
Check your work
- Hose clear, clipped at both ends, no kinks.
- Air box drain empty and plugged.
- After a ride, no fresh oil on the air filter or round seals.
If it did not fix the problem
- Heavy smoke at the breather or oil filler: blow-by from worn rings. Go to Test engine compression. The K-Light smoke chart points to cylinder, piston and ring wear when smoke comes from the filler at high rpm.
- Oil keeps appearing at seals: Trace an oil leak.