Oil in my air filter

[Athame57]

Could a badly seated valve be causing my air filter to get oil stained via Blow-by gas circulator that exists on the CB400F ( I gather it was created to deal with US emission laws). There is no pooling of oil in it's housing and the drain system below it isn't blocked but they are both messy as usual. I have just fitted a new air filter last week but whatever the problem it has made short work of my air filter unless the staining you see in the pic is to be expected? There is some black smoke at high revs in a short burst in neutral but not enough to have worried me.

[McCabe-Thiele (Ted)]

Does make you wonder if it might be easier to vent directly to atmosphere.

[Laverda Dave]

That doesn't look good. Do you have the piece of foam between the strainer and filter housing (the air filter sits on it and it's held in position with a large crosshead screw. If you don't have the foam the oil gases will blow straight over your filter. It does appear as though the engine is breathing a bit heavy. Are all the breather hoses correctly routed with no kinks? Make sure the large breather hose that runs down the back of the engine is located correctly in the cast boss of the engine case in front of the swing arm and has not been caught between the swing arm and frame as happened to me many years ago.

[Athame57]

That doesn't look good. Do you have the piece of foam between the strainer and filter housing...

Yes, I got it from Julie actually. The bowl below it I didn't have for months when I first got this bike five years ago, I had to scour the globe for a long time and suddenly there it was staring back at me on the 400Four bits site. I do squeeze out the tip of the breather drain pipe but I see your point about it being snagged, but the lack of pooling suggests it will be ok when I go to look proper, we'll see.

[Athame57]

Does make you wonder if it might be easier to vent directly to atmosphere.

Well, I certainly started like that in the first months, the whole system was missing.... no risk of lost pressure et al?

[Oddjob]

Nope, perfectly fine to do it. Most of the early SOHC models vented to atmosphere.

[Trigger]

If that is oil staining, it shows that you have a problem from the vent system

[Athame57]

If that is oil staining, it shows that you have a problem from the vent system

I think I shall give up with it, stuff net zero, ULEZ and Sadiq Khan! I used to think the thing mattered but it doesn't seem so.

[Trigger]

If that is oil staining, it shows that you have a problem from the vent system

I think I shall give up with it, stuff net zero, ULEZ and Sadiq Khan! I used to think the thing mattered but it doesn't seem so.

It is showing a problem that needs addressing. If you bypass, you are ignoring the problem that could get worse   

[McCabe-Thiele (Ted)]

Out of interest what is the condition of the inside of your vent pipe itself not just the long section but the drain section as well,  it might look good on the outside, it could   be well deteriorated internally.

This can lead to high resistance of air flow with a pressure build up squirting out oil that has collected on the walls of the pipe work.

Might be worth removing the top vent cover to put your mind at rest over any excess venting. Just my thoughts it's often not something serious but something simple. Too easy to think the worst.

[Athame57]

Out of interest what is the condition of the inside of your vent pipe itself not just the long section but the drain section as well,  it might look good on the outside, it could   be well deteriorated internally.

I did look down the air vent tube yes, unblocked and looking ok to me, it is after all only 5 years and about 11,000 miles old. The bottom of the air box, and drain below were messy but unblocked. I'll do what you said when I do the valve clearances, I've also got the carbs off for converting to the Viton O-rings I got off Julie. If things don't improve maybe the top end rebuild I have in mind led by the need for a new cylinder base gasket might show something up.

[Oddjob]

For peace of mind do a compression test, I had an engine in doing something similar and it was because the rings were allowing compression past them and into the crankcase, that forced oil out of the engine via the venting system. A compression test would show if that is the problem in this case.

[Trigger]

Out of interest what is the condition of the inside of your vent pipe itself not just the long section but the drain section as well,  it might look good on the outside, it could   be well deteriorated internally.

I did look down the air vent tube yes, unblocked and looking ok to me, it is after all only 5 years and about 11,000 miles old. The bottom of the air box, and drain below were messy but unblocked. I'll do what you said when I do the valve clearances, I've also got the carbs off for converting to the Viton O-rings I got off Julie. If things don't improve maybe the top end rebuild I have in mind led by the need for a new cylinder base gasket might show something up.

Is the air box foam pad oily ?

[Athame57]

Is the air box foam pad oily ?

Yes, I wash it out now and then with petrol.

[McCabe-Thiele (Ted)]

Out of interest what is the condition of the inside of your vent pipe itself not just the long section but the drain section as well,  it might look good on the outside, it could   be well deteriorated internally.

I did look down the air vent tube yes, unblocked and looking ok to me, it is after all only 5 years and about 11,000 miles old. The bottom of the air box, and drain below were messy but unblocked. I'll do what you said when I do the valve clearances, I've also got the carbs off for converting to the Viton O-rings I got off Julie. If things don't improve maybe the top end rebuild I have in mind led by the need for a new cylinder base gasket might show something up.

I'm curious about the need for a new cylinder base gasket as if you are blowing oil out that might be due to excessive crank case pressure rather than seal & gasket failures.