I have Yamaha Gladiator SS 125 model motorcycle. After repairing the engine there has been one problem in my engine. It is that the mobil from the engine flows towards the air filter and then the same mobil moves towards the piston and a large amount of smoke goes out through the silencer.

What is the cause of this problem? I did not got any suggestion from the Yamaha showrooms available in my location in Nepal...

  • Is the oil level at capacity (or above)?
    – Andrew L
    Jul 21, 2016 at 12:48

2 Answers 2


It seems you are describing an issue with the PCV Valve

I am attempting to interpret what you are stating.

You said

the mobil from the engine flows towards the air filter

When you say mobil, you mean oil I believe. Oil getting into your airbox, which contains your air filter, would come from your PCV (positive crankcase ventilation) valve. This valve sits in your valve cover and has a hose that connects to the airbox. Here are two basic possibilities for you to consider.

  • If you have lots of positive pressure in your crankcase/engine it typically means that you have bad piston rings and exhaust gasses are getting past the rings and positively charging your crankcase. As oil mist builds up in the engine during higher RPM running of the engine this oil mist can get past the PCV and into the airbox. It then builds up and eventually gets sucked back into the the intake where it is burned.

  • The other possibility is that your PCV valve is broken and allowing this oil mist to pass by it and get into the airbox.

Leak Down Test

You can validate if the rings are bad by doing what is called a leak down test. Here are some posts and instructions regarding what the test accomplishes and how to perform it.

If your rings are allowing to much exhaust by then you will need to rebuild the top end of your motor by getting the next size overbore piston (1/4 mm) and rings have a machine shop bore your cylinders to match. If the PCV valve is bad you will need to remove it from the valve cover and replace it.


I believe by Mobil you are meaning engine oil.

  • Check your gaskets and oil seals in the engine which might be causing oil to leak towards other compartments.
  • Some motorcycles have a dedicated oil line running towards the valve for lubrication and a rupture on that line then a similar symptom can be seen.

Note: Its highly unlikely that the oil will flow towards the air filter.

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