Crankcase Breather Issue

CaptainBillK

CaptainBillK

Joined
Dec 19, 2019
Messages
6
Country flag
I have a 1971 750 Commando. About 1.5 years ago the engine seized and destroyed the case. Luckily I found a mechanic at Fair Spares New England who specializes in Nortons. He rebuilt the engine using a Combat case. The problem is, that case has a rather robust vent behind the cylinders towards the left side. I am getting significant oil "mist" that is spewing a lot of oil. I have about 30psi of oil pressure. I was able to install a 5/8" rubber hose to it and used a cheap aftermarket breather filter - all as an experiment. It doesn't work. I'm still getting significant oil loss through this vent. I'm looking for possible solutions. It would be nice if I could vent it directly back to the oil tank, but again... that's a big hose.
 
Keep the breather body, its in the right place for good breathing. However it also allows a lot of oil out at high revs which you need to stop. So just above the breather in the rubber line fit an inline reed valve. This will allow the initial breathing on startup but the reduce the amount of air and oil once an equilibrium is reached. I will post an example shortly.
 
This is the one repurposed for the XS650 as a breather valve and used on Commando's, need to be mounted higher to get around the 90 degree turn.


Crankcase Breather Issue


You can get it in the US from

www.mikesxs.net

PCV Reed Valve - Crankcase Breather

Color/Finish: Silver Material: Metal Dimensions: Overall Length: 86.50mm Width (at widest point): 28.85mm Engine Inlet OD: 14.15mm Engine Inlet 11.85mm Filter (Air) Outlet OD: 12.00mm Filter (Air) Outlet ID: 9.45mm Fits: Universal Note: Easy Installation (Linked Below)
www.mikesxs.net www.mikesxs.net

Crankcase Breather Issue



Its from a Chinese scooter.
 
kommando said:
This is the one repurposed for the XS650 as a breather valve and used on Commando's, need to be mounted higher to get around the 90 degree turn.


Crankcase Breather Issue


You can get it in the US from

www.mikesxs.net

PCV Reed Valve - Crankcase Breather

Color/Finish: Silver Material: Metal Dimensions: Overall Length: 86.50mm Width (at widest point): 28.85mm Engine Inlet OD: 14.15mm Engine Inlet 11.85mm Filter (Air) Outlet OD: 12.00mm Filter (Air) Outlet ID: 9.45mm Fits: Universal Note: Easy Installation (Linked Below)
www.mikesxs.net www.mikesxs.net

Crankcase Breather Issue



Its from a Chinese scooter.
Click to expand...
That looks like a great solution. Of course, they're out of stock... but, I'll wait. My only question is... I see that I can put it in the line... but, would it be better leading back to oil tank? I'm getting a lot of oil there even at idle.
 
There are several things you need to do to fix the Combat oiling problem and they are described at the bottom of this page:
https://jsmotorsport.com/reed-valve-crank-case-breather/

The breather in the photo is specifically for a Combat and its a drop in kit. But eventually you still need to address the case oiling problems mentioned in the link in the above paragraph or the case can wet sump at higher RPM. You also have to plug any other case vent if there is one. You want your breather attached directly to the case so it gets warm. If you use a hose to a reed breather it can clog with foam in cold weather. Yes CaptainBillK the hose should return to the oil tank.
Crankcase Breather Issue
 
CaptainBillK said:
I see that I can put it in the line... but, would it be better leading back to oil tank? I'm getting a lot of oil there even at idle.
Click to expand...
It goes into the current line which should be leading to the tank and so remains leading to the tank.

If your line is not currently leading back to the tank then reroute so its is as this is how it left the factory on a 72.

Crankcase Breather Issue


Part 44 goes from the breather up to the oil tank with the reducer 46 joining onto 45 to join the tank at the top. As the pressure builds its let out of the tank via 47 and enters the air filter to be sucked into the engine.
 
Last edited:
CaptainBillK said:
I have a 1971 750 Commando. About 1.5 years ago the engine seized and destroyed the case. Luckily I found a mechanic at Fair Spares New England who specializes in Nortons. He rebuilt the engine using a Combat case. The problem is, that case has a rather robust vent behind the cylinders towards the left side. I am getting significant oil "mist" that is spewing a lot of oil. I have about 30psi of oil pressure. I was able to install a 5/8" rubber hose to it and used a cheap aftermarket breather filter - all as an experiment. It doesn't work. I'm still getting significant oil loss through this vent. I'm looking for possible solutions. It would be nice if I could vent it directly back to the oil tank, but again... that's a big hose.
Click to expand...
Connect the vent to the oil tank neck - that's where it goes. You do that using 06.2617 which is standard.

Crankcase Breather Issue

Better still, install the cNw reed valve for 1972 and it will all be 3/8" hose. https://coloradonortonworks.net/1972-breather-modification.html
 
CaptainBillK said:
Sadly that won't resolve the issue of the vent on the top of the engine case. Additionally, the sump drain is no longer that size... it's a simple bolt. :(

Click to expand...

That will blow oil forever.
As mentioned above, it was designed to be routes to the oil tank on the top fitting.
Will require the reducer as shown.
Reed valve can be fitted inline or on crankcase but is an improvement and not a fix for your issue. Route it to oil tank.
 

Similar threads

E
1970 Commando crankcase breather options
Replies
9
Views
1,248
jseng1
jseng1
cliffa
Crankcase breather / separator
2
Replies
21
Views
2,418
Stu Bodycote
Stu Bodycote
TomU
  • Locked
CNW Crankcase Breather Gasket
2
Replies
22
Views
3,101
cNw
cNw
Mofosheee
  • Locked
Crankcase breather
Replies
5
Views
1,276
Time Warp
Time Warp
javifoto70
  • Locked
Oil valve solution to oil falling to crankcase
10 11 12 13 14 15
Replies
290
Views
34,042
Kip
K
Back
Top