Now and then when I check the oil the stick is dry, and the oil has escaped through the crankcase ventilation into the airbox. As far as I know it can be either worn piston rings or a blocked air intake. The rings shouldn't be a problem, they are pretty new and as long as the oil stays out of the airbox the oil consumption is hardly noticeable. When the problems started I did have a pretty clogged air filter, but though I cleaned up the box, put in a fresh filter and even removed the snorkel the oilspitting continued. Not all the time and under random circumstances.

I'm no mechanic wiz so I hope someone here have a clue what's going on. Could the crankcase ventilation have been messed up somehow because of the first oil spills? Could the AIS, that sucks air out of the air box, somehow make it worse? What should I check?

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are you sure you are checking the oil level at the correct time from having the bike running ? cos this is normally signs of to much oil

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Yamaha xt660x 2009 blue/gold mods= iridium plug, stage1+2, removed snorkel, blocked ais, akrapovric pipes, power commander pc3usb (pre made map), adjusted tps, dynojet 02 optimiser 30amp aux relay, smoked puig screen, oxford heated grips and brembo carbon ceramic pads, rfx 30mm bar raisers, puig long adjustable leavers, plus much more and a cheep �30 alarm which stopped it been stolen

I'm sure, and it's not a bit of overflow. When it happens There's a litre or more that pours out of the airbox drain pipe. And it happens in an instant. Went to see a friend just a hundred kilometers away, when I left home oil level was fine and when I arrived I had 1,5 litres of oil in the airbox.

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If it was an Enfield Bullet (or any other old British bike for that matter) then I would say it was a classic case of a dry sump engine "wet-sumping". Basically the oil pump isn't scavenging enough oil for whatever reason, the oil level in the tank falls, the oil level in the crankcase builds up too high and the excess gets blown out the breather (or if the oil control ring is weak and can't wipe the excess, it goes out the exhaust.)

Never heard of it on an XT though? Usual cause on Brit bikes is worn scavenge pump rotors (or plungers), blocked oil screen or a randomly sticky or permanently stuck open check valve on the oil feed line. You often found it an issue on bikes that had been stood for a while.

For 1.5 litres of oil to go AWOL and into the crankcase and out the breather I'd say it could well be something similar?

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Could be. The bike have been "offline" for longer times lately (like me, explaining my slow posting here..), but not for years. Just saw a comment in a totally unrelated thread (this) where a guy named ernibert commented a pic where you could see oil in the tube coming from the air box and said:
"as far as i see - the little tube is full of oil (pic 2, just where the main stand is). There is a little part which is to be exchanged by Yamaha which solves this problem."
That tube is constantly filled with oil on mine. So what does he mean with this "little part", and have I missed something? Should my Yamaha service guy know something about this?

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That little part is modified oil trap in the crankcase breather. On 2008 built bikes the baffling inside wasn't sufficient to catch the oil mist when high engine speeds were maintained for longer periods (long hauls on the motorway). Some bikes had the later oil trap fitted by dealers under warranty, some didn't.

Mine's an 08 bike and it hasn't had the modified part fitted. My dealer fudged the issue, then it ran out of warranty! Never bothered since.

The air box drain tube only ever gets full on a long fast ride 200 miles plus on the motorway. In normal use I find about a 10-15ml every month. Even when it's full there's only about 35ml in it (I measured it once). You can't even notice the loss on the dip stick. No where near what your getting? Do you ride a lot at high engine speeds for extends periods?

The updated part is cheap, but a bit of an arse to fit. So it might be worth a try? But with the amount of oil your losing, I doubt is the fault of the crankcase breather?

I fitted a 30cm longer drain pipe and tucked the extra length in the bash plate so it has more capacity and you see what's in it on longer trips without it backing up onto the air box.

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