Oil In Air Box Fix (another one)

You may recall that I had a very bad oil breathing problem on my 961 Sport 2014.

At least 200 ml of oil in the air box (or over the rear wheel) after a spirited ride of only 100 km or 60 miles.


I got together with a couple of old ex racing mechanics/racers and this is what we have come up with and done so far.

They were reluctant to pull the top end apart to check the tolerances of the rings, piston and bore. This is what I wanted to do. They wanted to check and try other things first.

They came to the conclusion that all the upgrading of the breathing system by Norton was only masking a typical british engine with an inadequate breathing system. Don't get me wrong, they think the bike is fantastic.


The following has been done.

We have taken the breathing/oiling system back to the original workshop design.

Removed the oil air separator.

Re used the banjo bolts in the rocker box.

Left the air box drain hose.


As per the attached photos a breather hose has been installed to the oil tank filler to reduce the pressure in the oil tank. This hose is run into a small bottle to catch oil vapor.


I have run this new system now for about 500 miles and whilst I think it is not 100% perfect, I can report the following.

No oil in the air box.

A little bit of oil from the oil tank is collected under normal riding conditions. About say 2 mils every 100 miles.

It seems the only time the new oil tank breather has expelled oil in any quantity is in steep down hill twisty roads where heavy engine braking is used a lot. About 30 mls in a down hill run of about 20 miles but very hard riding. This mountain is close to me, all country highway. The total round trip is 200 km or 120 miles and I get little or no oil going up but do get oil going down that 20 mile section.


As I said it isn’t perfect but it’s 100% on what it was and I can live with it.


Simon

Hello nopdog,

it is not the nicest solution, but when it works it is more than OK.
Have you measured the pressure in the oiltank ?

I had in the last summer an idea like your to test an "oilblower" and we have seen that sometimes (downhill) a lot of oil was coming out of the oiltank into the catch bottle.
After testrides with clocks to measure the pressure we noticed that the pressure in the tank is very unstable.

Panetone

I don’t quite follow your logic here. A pressurised oil tank might result in the oil being supplied to the pump under increased pressure, but I don’t quite see why that would lead to crank case breather issues?

The only possibility that I can see a pressurised oil tank causing is more oil being forced past / through the pump than the scavenge is able to accommodate. Do you think this was the case?

I understood that the factory now fitted pumps with a greater scavenge capacity. I wonder if you have this pump or not? Your symptoms kinda suggest it may be worth finding out.

Anyway, if your breather works, but occasionally expels oil, why not link the oil tank cap tube to the rocker cover, or tee it into the rocker cover breather?

It doesn’t need to be that big either I’d say, a smaller tube might be routed through the steering stem (if it’s hollow) a lot like sport bikes used to do with fuel tank breather pipes.

Does the 961 breather system have a reed valve type none return valve anywhere? They seem to cure so many issues when fitted to old Brit bikes. Maybe these would bennefit too?

Factory has to comply with all sorts of daft OTT emmissions laws, but we don’t have to keep them like that, so a ‘fresh eyes’ approach to the whole breathing circuit may well be necessary / beneficial.

Hi Fast Eddie,

the different pressures in the oiltank is comming from the the little tube in the back of the rocker cover.
The rocker cover has a direct connection to the oiltank.
Different pressure in the engine-> different pressure in the oiltank.
Nopdog has opened the closed system and this is the reason why nopdogs system works.
The oiltank can not be pressurize with the added tube into the catch bottle.

Panetone

I've been wondering about something similar. The original 961 system isn't really much of a system, in that it has no way to separate oil from the breather fumes and return it to the tank. A normal dry sump system on a race bike or car has a breather hose, or hoses, from the engine into an oil/air separator, either separate from the oil tank or as an integral part of it, and then a vent hose from the oil tank to the atmosphere (usually via some sort of catch can or the intake tract). The separator returns most of the oil to the tank, and the output from the tank vent is mostly just fumes. That's how the "old" Commando oil system works, although the separator is really just a simple baffle plate inside the tank. Having the crankcase vent directly to the atmosphere (airbox in this case) almost guarantees a lot of oil will also escape. The 961 has a breather return to the oil tank, but no way to vent the fumes from the tank, so they also vent the crankcase to the airbox. It would have been better for the 961 to have the only engine breather hose be the one from the rocker cover to the oil tank, and then have a vent from the oil tank to the airbox. That would be what nopdog is experimenting with, except that he still has the direct breather to the airbox. It looks to me like it would be fairly difficult to do it properly with the current size and location oil tank. That sort of system works best if you have enough room for an air space above the oil level to include a separator, as well as a baffled high point for the vent. I'm still thinking on this, but I don't see an easy way to retrofit a proper system here. The factory update kit seems to be an attempt to move in that direction. I'm sure Norton have technical people who understand all this, so I'm speculating that the reason we have this issue is that fixing it would have required more modification to the original Kenny Dreer design than Norton wanted to deal with.


Still, I'm new to the 961, so this us just some rambling speculation until I get more experience with the bike.

Ken

Good thoughts Ken.

When a reed valve is added to an old Commando, the reduction in gas volume being shifted is very noticeable, frankly, hardly anything seems to come out of the breath pipe in the tank!

The 961 has a 270 crank, so there should be considerably less gas volume being shifted anyway. So, I’m thinking that if a reed valve was added to the engine breather hose close to the engine, and then this breathed into the oil tank, the reduced gas volume may allay your fears a little about the space needed in the oil tank. Then a modified filler cap with a smaller hose routed to breather into the airbox might work?

Could be. The Comstock breather on the bottom of my MK3 did wonders for it. It looks like there is room for a in-line reed valve in the breather line from the rocker cover to the oil tank. If you then blocked off all the other breather hoses, and ran a vent from the tank to the airbox, it might work. I still worry that there might not be enough volume in the tank for it all to work properly, and there's still the issue of oil in the tank being forced into the vent on heavy downhill braking. But the only way to find out would be to try it.

Pretty low on my priority scale right now, but food for thought.

Ken