Excess oil through crankcase breather

**saiko** · 8th September 2005, 12:40

Hi,

Can anyone tell me please how oil comes through the crankcase breather? I've lost the bung on the end of the hose, and am getting huge amounts of oil coming out. Perhaps it needs the bung to keep pressurised?

Anyway I note there appears to be petrol in with the oil. If someone can tell me how all this is supposed to work (what's the breather for), and why mine is leaking so much oil, it would be gratefully appreciated).

Thanks.

**Ixion** · 8th September 2005, 12:46

Hmm. Not wanting to alarm you, but blowing oil out the breather generally means that you have excessive piston blowby. Worn bores, scored bores, broken rings or a holed piston.

However , there can be other reasons and maybe that bike is just a blower. Depends a bit on what you mean by "huge amounts". The old Meridan triumphs used to blow a fair bit of oil, so much so that they pointed the primary chaincase breather over the drive chain and claimed it as a "feature" .

Don't like the sound of "petrol in with the oil". Sounds a bit ominous. Does the engine smoke at all.

I suspect that if you put a bung in the pipe it will just blow it out.

EDIT. the breather is just that , it's to let any pressure differential escape. Otherwise the bottom of the engine, being in theory a sealed compartment, when the motor heated up, the air would expand and blow a joint somewhere. And even on a sound engine there's always a little bit of blowby, expecially when cold.

**Sniper** · 8th September 2005, 12:46

Is that your GSXR mate?

**vifferman** · 8th September 2005, 12:47

Originally Posted by saiko

Can anyone tell me please how oil comes through the crankcase breather? I've lost the bung on the end of the hose, and am getting huge amounts of oil coming out. Perhaps it needs the bung to keep pressurised?

Usually (on modernish bikes at least), the breather feeds into the airbox, usually via a litle foam-filled box thingo (tech term). Otherwse, it just breathes oily fumes directly onto the road. If you position the breather in front of your rear tyre, you could have some good dirty stunting fun.

The pistons going up and down may cause some oil to travel out the breather, especially if the crankcase is too full, or the piston rings and/or bore are worn, as the compression 'blows by' the rings, pressurising the crankcase, pushing the oil out.

Originally Posted by saiko

Anyway I note there appears to be petrol in with the oil.

Ooops. That's not good. It usuallly means your carb float(s) are stuck and/or holed, and/or the needle valves in the carb bowls are worn, so they're not shutting off petrol flow to the carbs. When the bike's sitting there, petrol is running into the cylinders and leaking past the rings into the crankcase.

**Paul in NZ** · 8th September 2005, 12:47

A bit of info on age / mileage / year / capacity of bike might help a bit?

Regardless, I don't think having petrol in the oil is a good move. If you really think this is the case DON"T try and start it.

Take it to a shop and get it checked. Could be something simple like your fuel tap stuck on and fuel is dribbling into the engine, past the rings and into the crankcase. (check the oil level)

But I would NOT run the engine until I'm sure. A little explosion down there could really ruin your day!

**saiko** · 8th September 2005, 13:07

Originally Posted by Sniper

Is that your GSXR mate?

G'day, no a Honda VF500, 1986 but good condition with 50,000 km on the clock. Cheers.

**saiko** · 8th September 2005, 13:10

Originally Posted by Paul in NZ

A bit of info on age / mileage / year / capacity of bike might help a bit?

Regardless, I don't think having petrol in the oil is a good move. If you really think this is the case DON"T try and start it.

A little explosion down there could really ruin your day!

Probably not an issue now, as the carbs have had new kits installed, and I noticed the petrol in there before then. Thanks.