The breather is a hose, sometimes with a small box or valve, that runs from the top of the crankcase or rocker cover to the airbox. Some older bikes vent to a catch tube under the bike.
What it does
A little gas always leaks past the rings into the crankcase (blow-by). The breather lets this gas out, so pressure does not push oil past seals and gaskets. Sending it to the airbox means the fumes are burned in the engine. The breather must let gas out but keep oil in; a separator chamber drops the oil mist back into the engine.
Signs it is worn or failing
- Smoke or oil mist from the breather, which often points to worn rings, not a breather fault.
- An oil-soaked air filter or oil in the airbox drain.
- Oil leaks from seals when the hose is kinked or blocked.
- Oil overfilled pushes oil up the breather.
When to replace it
Replace the hose when it is cracked, hard, split or kinked. Clean the separator or airbox drain at each service. Most breathers need no other work.
What to match when buying
- Match the hose inside diameter, length and shape. Use oil-resistant hose, not fuel hose of the wrong grade or garden hose.
- Fit new clips if the old ones are rusty or loose.
- Some models have a one-way valve in the line; match its direction arrow.
Fitting notes
- Home job: Check the crankcase breather.
- Route the hose with no kinks, low points that collect oil, or contact with the exhaust.
- If the breather keeps filling with oil, do a compression test and see Blue smoke.