Unraveling the 961 part two (oil in the air box)

There are two shortcomings with the 961 that have annoyed me the most. One is the idle which I've been investigating HERE

The other is oil in places it shouldn't be! Neither unfortunately can be fixed with a cable tie!

So this thread is going to deal with understanding the oil in the airbox issue and what can be done to mitigate it. The factory has tried a number of fixes, culminating in an ugly plastic box, which might have been acceptable if the problem had been solved. But the catch bottle serves as a reminder all is still not well!

This is the original MKI oil breather system (taken from the factory manual).



With a sealed oil tank the cylinder head to oil tank pipe seems somewhat redundant, so there's no where for the gas to go but to the airbox via the rocker cover. Without an oil separator the airbox performed that role. Current fix is to block of the rocker cover and run the crankcase breather directly to the airbox with a large oil separator in between. Results vary...

So wanting to understand what goes where, I've removed all the breather pipes and replaced them with high temperature clear silicone tubing. To start with I've gone back to the MKI configuration but have inserted a small clear oil separator (to test) between the rocker box cover and what should be the airbox (vid to follow will make it clear).

My goal is to simplify the crankcase breathing system as much as possible and get rid of the catch bottle if possible. So I've shot a little video to show you all the current situation.

As you'll see the engine wants to breathe via the cylinder head to oil tank tube, but as it's sealed it can't.



Nothing new here nopdog and his mates figured this out a while back in this THREAD. But a video speaks a thousand words. The oil separator btw has virtually no resistance, it's very clear the engine would prefer to breath via the oil tank when given a choice. This is the preferable solution since the best place to collect the oil is surely the oil tank! But as the oil tank hasn't been designed to operate in this way oil will escape if you just try and vent the cap. But I have some ideas and a Fitter and Turner brother...

I haven't run the engine under load yet so a lot may change but I always enjoy these kind of threads. More importantly most of you know far more than me so maybe collectively we can come up with someone better than the current situation. There's no going back for me, I've removed and sealed off the airbox drain hole.

But I'll also be exploring other solutions that don't involve using the oil tank.

I know it's not a 360 degree crank but would the breathing benefit from a reed valve as used on the old Commando, so once the initial excess air is blown out and a partial vacuum created, only a smaller amount of oil carrying air is breathed. Less air less oil.

kommando said:

I know it's not a 360 degree crank but would the breathing benefit from a reed valve as used on the old Commando, so once the initial excess air is blown out and a partial vacuum created, only a smaller amount of oil carrying air is breathed. Less air less oil.

Click to expand...

Yes, you're right.
And I remember a forum member sometime ago hinted that Norton thought about this, but it would require redesign of the crankcases to provide a built-in boss or flange in the case to bolt on a reed valve assemble.

But the best way to solve the problem is to have a different oil tank into which the breather vent can terminate.
The frame mounted tank is too high up.
If you try to terminate the breather into it, you risk tank oil flowing back down to the breather.
The best arrangement is a low mounted tank like the classic 750/850's have.
Breather flows into the oil tank, vapor vent from tank to airbox.
It's not as pretty as the 961 design allowing a thin waist appearance, but it will work better than the high in-frame tank.

I put my reed valves in the breather pipe on the B44, seems to work nearly as well as the direct crankcase mounted ones.

I agree with Kommando, I run ‘in line’ reed valves on two Triumph twins and they work great.

I’m also of the opinion that one would work on the 961, I can’t see any reason why it wouldn’t.

I haven’t tried it though.

I'm sure connecting a read valve to the hose exiting the breather will improve the situation.
However, anyone who has the oil in airbox fix kit knows, there is a whole lot of plumbing behind the cylinder as is.
Looks like a rat's nest of wires and tubing already.

The air/oil separator could be modified to include a reed value in it's bottom where the breather hose enters.
Aesthetically, that would be the least intrusive.

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I’d like to try a reed valve INSTEAD of the separator....

Does your anniversary MK2 CR have a separator installed?

BritTwit said:

Does your anniversary MK2 CR have a separator installed?

Click to expand...

I assume that’s what it is (hopefully you can zoom in and see it)...

Sure enough, there it is.
The only reason I asked is because I remember an owner on this forum said that he had a Mk2 and his did not have the kit installed.
The kit was originally designed for owners who explicitly complained to Norton of having oil in their airbox.

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As far as I’m aware it is now standard fitment.

The Norton fix seems to work ok in terms of venting the crankcase. But the actual separator is not that great hence the need for a catch bottle. The fact you need a catch bottle means a reasonable quantity of oil vapour must be passing into the throttle bodies which is not where oil should be.

So how about a better oil separator - one that would require no maintenance and send the oil back into the engine where it belongs.



The idea here is that when the engine is running the crankcase vents into the oil separator and stores the collected oil until the engine is switched off. At that point without crankcase pressure to keep the valve closed the collected oil can flow back into the crankcase. As long as the separator can hold more oil than can be collected in a single tank of fuel it's maintenance free. I've got a transparent separator currently mounted just to see what's going on. But have another unit coming with a drain plug in the bottom that could work. The problem with Norton's design is that the pipe from the crankcase serves both as the vent and the drain meaning the separator is not particularly efficient and allows to much oil to get pass it.

According to Norton the reason for all the oil is too much pressure getting past the pistons/rings. This is why FE and others get no or little oil in their air box. The tolerances on the oily bikes are on the looser side. This is the crux of it . I like the drain back idea ! I am going to try a primary oil screen filter before the separator and see how it goes. This I hope will help start the separation process and give some capacity to the breather system also . I will send photos when I finish and if it helps . Also , remember the only vent to the frame oil tank is through the engine and out the breather .. Here :

I think to much thought is going into this.
My car is a 2018 GT350R Shelby Mustang and it has catch cans on both banks.
The catch cans collects oil before it enters the intake and does a great job.
How much oil it collects depends on my driving.
If I do a lot of 8400 rpm runs the catch can will collect more oil then if I just normal street drive.
I’m also glad the can catches the oil and not goes back into the engines oil system. It’s not a big deal to clean or empty the cans either. Plus it has a stainless steel screen and baffle. Other brands / models of aftermarket versions are available but I prefer the ford version.
Here’s a video. (Not my video)

Some people even use shop compressed air filter / water separators.

I've got no problem pre and post checking my track car, it gets caned and the extra maintenance comes with the territory. But I've grown used to barely checking the oil and tyres on my road vehicles - because on all my vehicles other than the Norton I don't need to. I just want to get it to a point where I can throw a leg over it and go for a blast without worrying about something falling off, refusing to idle and needing a little baby bottle for whenever it burbs!

I've deleted the Norton oil separator tidied up the hoses, wiring and removed the oil catch bottle and hose to get the bike back to the clean look of the original. I think I'm pretty much there now.

You deleted the oil separator and catch bottle?

That’s good, keep an eye on the oil in the airbox and see what’s what.then you can start adding stuff if / as required.

If the oily bikes are thus becasue of excess pressure getting past the rings, this again supports the idea of a reed valve breather as the pressure will escape and not be allowed to build.

Whilst your experimenting it may have been a good idea to route the open breather directly into a catch bottle as you can easily monitor what’s coming out of it.

iwilson said:

Click to expand...

I think that in this diagram, gasses will vent to the airbox, and becasue of this, there will be nothing providing pressure to open the reed valve and allow oil back to the cases.

I’d say a restriction might have to be added after the separator, but that may (at least partially) defeat the entire objective.

I’ve not seen inside a Norton oil separator, but if it separates the oil it has to go somewhere. I would think it goes back down the breather pipe back into the cases.

I've changed the design already, using a very simple back flow preventer instead. Very small amount of pressure from the crankcase will seal it. With the engine off, gravity and the weight of the oil above it resets the valve to the open position (well that's the plan)!

The idea is that oil from the separator will end up back in the crankcase rather than the airbox. Currently the Norton separator allows to much oil to get through it and into the airbox, this is because it's a very simple design as it must allow the oil collected to flow back into the crankcase when the engine is switched off. I'm trying to come up with a very efficient separator that will also allow the collected oil to return to the crankcase without user intervention. This will mean less oil being burnt in the engine and no need for the catch bottle.

Fast Eddie said:

You deleted the oil separator and catch bottle?

That’s good, keep an eye on the oil in the airbox and see what’s what.then you can start adding stuff if / as required.

If the oily bikes are thus becasue of excess pressure getting past the rings, this again supports the idea of a reed valve breather as the pressure will escape and not be allowed to build.

Whilst your experimenting it may have been a good idea to route the open breather directly into a catch bottle as you can easily monitor what’s coming out of it.

Click to expand...

I don't run the pipe back to the airbox it just vents to the air. The current separator I'm using only has a push button to drain it (or unscrew the canister). I've found a really inconspicuous spot to mount it and subject to the amount of oil it collects might not go any further.

Ahh, you only said you removed the Norton stuff, not added anything!

That oil separator you’ve added looks good and it’s easy to keep an eye on with the transparent bowl.

Every engine ever built has a breather (apart from the first one I built as a kid... learning curve) so this should be solvable.. shouldn’t it...?