Norton Mk3 crankcase reed valve -June 2019

Both the sump plug NYC breather and the bolt on cNw breather are excellent IMHO. They work at the very source of pressure. And they’re operating in oil all the time, thus no condensation / gunge to worry about.

And frankly, I don’t even agree that they’re expensive, not in real terms, they’re fit and forget and will in all likelihood last the lifetime of the bike.

Yer pays yer money and makes yer choice…

p400 said:

Never worried about this install direction and reed config.
Found more recommendations for this install direction tho.
found somewhere on the interweb.


View attachment 83120

This breather system also includes a vented plastic small soda bottle to capture any liquid coming from the oil tank vent hose.
Catch bottle hidden in the air cleaner space after converting big Mk3 plastic airbox to K&N air filter.
I have thought about increasing the diameter of this "catch" hose to reduce back pressure, but not made chg yet.

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With this venting change is your MK3 oil tank now at atmospheric pressure?

Glen

yes, hose 32 goes to a vented catch bottle,


oil tank out for broken brackets.
plastic bottle somewhat visible at back ground.
Mk3 is Canadian roadster, headlight on all times.

So the cap seal likely isn't critical on your tank now, or on any of the pre MK2a bikes?
My understanding of how the MK3 and MK2a breather and tank vent assembly works is this-
The breather moves crankcase mist into the top of the oil tank, well above the oil level. This mist is drawn away by the vent hose which connects to an oil separator.
The oil separator is connected to and receives vacuum from the carb manifolds. It also has a large diameter out flow hose that feeds into the airbox. Any oil separated drains here. I guess some of it is burned, some serves to keep the airfilter oily. I'm not certain on exactly how that part works.
A tight cap seal would be needed here to make the system work.
It seems to work a bit too well on my Mk 3 . If the oil tank level is kept on the H mark the carb vacuum constantly skims oil off the top of the oil tank.
This results in an oily mess around the air box and rear of the engine in general.
If the oil tank level is left on L this skimming doesn't seem to occur, or is so minimal that it's not a problem, things stay dry and the oil level doesn't drop from L.

With earlier bikes that do not have this oil tank vent to carb manifold connection , the tank is at atmospheric pressure and the cap seal isn't critical.
My statement is more of a question than a statement of fact, I'm hoping the above is the correct understanding.

Glen

Checked Norman White's book but he doesn't provide much info on the MK2a/MK3 breather system.
The big White book does. I have it generally correct except for the outflows from the separator. The big tube going to the airbox carries mist that has had most of the oil removed.
The lines going to the carb provide vacuum and move any droplets of oil formed in the Separator. The droplets are drawn into the manifolds and burned as part of combustion.

Glen

Thinking more about this, I don't think the (stock) MK2a and MK3 should have reed valves added, either in line or elsewhere.
The reed valve could interfere with the breather system, which seems to work really well as it was designed.
Oil leaks have not been a problem on this MK3.

Glen

worntorn said:

Thinking more about this, I don't think the (stock) MK2a and MK3 should have reed valves added, either in line or elsewhere.
The reed valve could interfere with the breather system, which seems to work really well as it was designed.
Oil leaks have not been a problem on this MK3.

Glen

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I had evidence of pressure on my Mk3. Head gasket was blowing and I needed to keep my oil level below halfway between H & L marks and keep the tank cap very tight, or it puked. I put the XS type breather into the breather to tank pipe. I did tighten the head, but not only do I now have no leaks, the bike sounds different. Crisper exhaust note. I have no doubt about this, not a placebo effect. But whether it was tightening the head down to spec, or the breather, or both, I don't know. But the breather doesn'tseem to be having any negative effect.

I don't have the stock airbox. My breather vent from the oil tank, is piped to the rear of the chain guard. No sign of deposits or milkiness at the end of the pipe, so far.

Mart UK said:

I had evidence of pressure on my Mk3. Head gasket was blowing and I needed to keep my oil level below halfway between H & L marks and keep the tank cap very tight, or it puked. I put the XS type breather into the breather to tank pipe. I did tighten the head, but not only do I now have no leaks, the bike sounds different. Crisper exhaust note. I have no doubt about this, not a placebo effect. But whether it was tightening the head down to spec, or the breather, or both, I don't know. But the breather doesn'tseem to be having any negative effect.

I don't have the stock airbox. My breather vent from the oil tank, is piped to the rear of the chain guard. No sign of deposits or milkiness at the end of the pipe, so far.

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Crankcase pressure causes parasitic loss as does running an engine oil of higher viscosity than necessary. It all adds up, especially at higher RPMs.

Dan1950 said:

Crankcase pressure causes parasitic loss as does running an engine oil of higher viscosity than necessary. It all adds up, especially at higher RPMs.

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Yes, but I think @worntorn was talking about th e stock Mk3 system, which I think might be boosted by the vacuum in the airbox, to support a pressure drop in the crankcase. Not sure that how it works, may be.

Mart UK said:

Yes, but I think @worntorn was talking about th e stock Mk3 system, which I think might be boosted by the vacuum in the airbox, to support a pressure drop in the crankcase. Not sure that how it works, may be.

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Given Mart UK's post it might seem that this modification may produce more of a negative pressure then the OEM MK III breather system. That can have an affect at high RPM. Lust my observation. YMMV.

I have a MK II so I think my machine will benefit from this mod. I don't really see a down side in my case. I'm going try venting into the oil tank with the tank at atmospheric pressure.

Mart UK said:

I had evidence of pressure on my Mk3. Head gasket was blowing and I needed to keep my oil level below halfway between H & L marks and keep the tank cap very tight, or it puked. I put the XS type breather into the breather to tank pipe. I did tighten the head, but not only do I now have no leaks, the bike sounds different. Crisper exhaust note. I have no doubt about this, not a placebo effect. But whether it was tightening the head down to spec, or the breather, or both, I don't know. But the breather doesn'tseem to be having any negative effect.

I don't have the stock airbox. My breather vent from the oil tank, is piped to the rear of the chain guard. No sign of deposits or milkiness at the end of the pipe, so far.

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i wrote a post some weeks back asking why the dipstick level lines are lower on later models .... my suspicion at the time was of blowing oil out of the breather when the oils chip fat hot

My MK2a does not have the oil separator and the parts supplement does not have one either, a tube comes out the top of the oil tank and vents into the top of the plastic airbox and keeps the air filter foam element oiled.

kommando said:

My MK2a does not have the oil separator and the parts supplement does not have one either,

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Supplement, Group 16, Not Illustrated, 065573, Oil Separator. Mk2A.

I have been using Comstock's sump/reed breather on my MKIII since 2016 and have been very happy with it. I re-routed the vent tube way before installing the breather. I found that using the "T" fitting on the manifolds lead to erratic idle issues in my case. I removed the small diameter hose and also removed the fitting from the bottom of the oil separator. I installed a barb fitting instead and ran a hose down to the lower frame rail to let the separator drain. One thing I did get was an oil film inside the black air box from the oil vapors being sucked into the filter area. That disappeared along with the slight weeping on the lower case seal after I installed the breather. Every once in a while I blow out the hose to clean out the residual oil in the vent/separator loop. Glad I put it in.
Pete

Once that carb connection is removed the oil separator & breather lose their vacuum, so the system, as designed, wouldn't work.
On the other hand, adding the Comstock reed valve or some other valve at that point could work as well or better than the stock MK3 system.
The stock system does seem to create substantial negative pressure, enough to skim oil off the tank if the level is too high.
On my own bike, the stock system is in place, the idle is good and the engine is oil tight.
Nonetheless, like many here, I'm always trying to make things better. Sometimes it goes the other way!
I have a Mike's XS breather here, was musing about adding it to the MK3.
The BSA is going to get it instead.

P400 makes a good point re the oil tank cap seal. The factory oil separator system needs that cap to seal in order to work properly. I wonder how many of these 56 year old bikes are losing vacuum at the cap?
If it was a modern vehicle, an engine warning light would come on when that seal wasn't there.

Glen

worntorn said:

Once that carb connection is removed the oil separator & breather lose their vacuum, so the system, as designed, wouldn't work.
On the other hand, adding the Comstock reed valve or some other valve at that point could work as well or better than the stock MK3 system.
The stock system does seem to create substantial negative pressure, enough to skim oil off the tank if the level is too high.
On my own bike, the stock system is in place, the idle is good and the engine is oil tight.
Nonetheless, like many here, I'm always trying to make things better. Sometimes it goes the other way!
I have a Mike's XS breather here, was musing about adding it to the MK3.
The BSA is going to get it instead.

Glen

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My take on that system was the Norton was just trying to get the vapors out of the oil tank and not introduce them into the air. Don’t forget there are two outlets on that separator. One goes to the black air box, the other to the “T” on the intake manifolds. The vacuum from the intake probably pulls the vapor into the separator and directs it into the air box with no precision at all. I didn’t like the fact that unregulated , constantly changing mixture of air, vapor and oil got dumped into the intake tract after spending time getting the A/F mixture set with jets and slides.
With the Comstock breather there is always a constant positive pressure going into the oil tank there by pushing the vapors into the separator, and in my case into the breather box.
As like many things about these bike, probably insignificant except for the “tinkering factor”, although I believe the sump breather is a pretty good device to have.
Pete

The Comstock breather also has that advantage of emptying an overly full sump quickly.
It's not so critical with this MK3 as it holds the oil in the tank quite well....but there are those times when that feature would be helpful.

Glen