1972 Oil Breather Problems

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JimNorton

JimNorton

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I have a very late 1972 750 Commando Fastback. Over the last few years it has started pushing out oil (it seems every where). It has be suggested it might be an oil breather problem pressurizing the crank case. It has the oil breather coming off the back of the crank case and it is clear.
My questions: How does it work? Is it just as a clear shot breather back to the oil tank. 2. Should I put a PC valve in the line? 3. It has the small drain hole so I can't put the Jim Comstock breather on. 4 I have considered replacing the original breather with CNW reed breather but It looks like having to either remove the transmission or engine.
Bottom line I would like to get the breather working as it is designed "if that is my problem."
 
The ‘72-‘73 breather is attached at the bottom of the drive side crankcase. Remove the breather hose and you can unscrew the top of the breather. There is an oil strainer inside. Clean this and make sure air is getting through when you reassemble.
 
illf8ed said:
The ‘72-‘73 breather is attached at the bottom of the drive side crankcase. Remove the breather hose and you can unscrew the top of the breather. There is an oil strainer inside. Clean this and make sure air is getting through when you reassemble.
Click to expand...

Thanks David, I'll try that. It looks difficult to get to but I'll try. I poured some kerosene back down the breather and it appeared clean but I'll try our suggestion.
 
JimNorton said:
I have a very late 1972 750 Commando Fastback. Over the last few years it has started pushing out oil (it seems every where). It has be suggested it might be an oil breather problem pressurizing the crank case. It has the oil breather coming off the back of the crank case and it is clear.
My questions: How does it work? Is it just as a clear shot breather back to the oil tank.
Click to expand...

There is no one-way valve in the standard rear crankcase breather (only perforated discs and piece of foam) so unless the breather is physically blocked then it's probably not the cause of the problem.
https://andover-norton.co.uk/en/sho...ankcases-crankcase-breather-tachometer-drive#


JimNorton said:
2. Should I put a PC valve in the line?
Click to expand...

That might help but it's possible the piston rings are worn or the cylinders are in need of a rebore which could be the actual reason for the apparent crankcase pressure.

Edit: The oil tank must also have a vent to atmosphere.
 
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Another way to know if the breather is clear....if the engine sits long enough to wet sump there will be oil returning to the tank via the breather hose. Watch out if you take the oil cap off to look as the oil blows out rapidly...messy.
 
A reed valve breather is a great idea but if you are getting lots of blow by in the cylinders it will be a marginal improvement.

I'd probably do a compression test and a leak down test to see what kind of information the might provide.

The valve that screws into the sump is actually the second generation of Comnoz breather. The original (as sold by CNW) attaches the back of the case without modification to the case but its true that it easiest done while you have it torn apart for other tasks.
 
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the 72/73 750 case does not have the large sump plug so you cannot use the sump plug reed breather instead you use the rear case unit for that model. you can put it on with everything in place BUT it is a tight fit. there is another type that goes on the rear of the early 750 and 850 cases BUT you have to machine the case for it to work.
rvich said:
The valve that screws into the sump is actually the second generation of Comnoz breather. The original (as sold by CNW) attaches the back of the case without modification to the case but its true that it easiest done while you have it torn apart for other tasks.
Click to expand...
 
Thanks for all your suggestions and help. Looking again at the rear breather it is impossible to remove without taking the gear box out and it I did that I would probably put the CNW breather in.
I see people over and over say "possible the piston rings are worn or the cylinders are in need of a rebore which could be the actual reason for the apparent crankcase pressure." I guess I don't understand that theory, shouldn't the crank case breather address that? I doubt that is my problem as I have what appears to be strong compression. How does one do a leak down test on a Norton. I do one as required on my airplane every year so I am familiar how that goes. What is the procedure for a Norton?

Right now I am going to try and determine where the oil is coming from with a UV test. Anyone done this, any advice? The engine is so clean now I hate to mess it up again but I need to determine the source of any leak.
 
One more question: What is the purpose of a PCV valve in the breather line? If this is my problem would it help? Right now it appears to be a clear shot back to the oil tank.
 
The logic is that IF your rings are allowing excessive blow by then the crank cases may build up pressure as the gases are greater than the breather was designed to handle.

PCV or reed valve breather addresses a lot of crank cases pressure related issues by simple virtue of the fact it allows gasses out, and not back in. It therefore helps prevent the crank cases being under positive pressure.

And positive crank case pressure does an excellent job of finding (or creating) weak spots in gaskets and seals etc.
 
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bill said:
the 72/73 750 case does not have the large sump plug so you cannot use the sump plug reed breather instead you use the rear case unit for that model. you can put it on with everything in place BUT it is a tight fit. there is another type that goes on the rear of the early 750 and 850 cases BUT you have to machine the case for it to work.
Click to expand...

I think we are in agreement here.

From CNW:

"There is one Commando engine that was built that will allow the cNw breather to be installed without taking the engine apart. This is the engine built in 1972 that originally has a large tubular breather installed on the backside of the cases. So it’s located in between the transmission and the engine. By removing this original breather and installing our model you will get all the benefits these well engineered breathers offer".

They list the manufacturer as CNW/Comstock Engineering. It is the generation of breather developed by Comnoz prior to the sump plug breather that is now sold thru NYC Norton.

https://coloradonortonworks.net/1972-breather-modification.html
 
JimNorton said:
Thanks David, I'll try that. It looks difficult to get to but I'll try. I poured some kerosene back down the breather and it appeared clean but I'll try our suggestion.
Click to expand...

I took the screen and foam out of my breather housing and put a reed valve just above it that has a hose to the oil tank. I feel that works extremely well.

It's not easy getting to that breather.

If you're pouring kerosene into the breather it's going into your crankcase and mixing with your oil, that sounds like a "bad idea". The only way I'd do that is to open the drain for the crankcase and see if kerosene comes out.
 
The oil was well drained out and the drain plug open. The kerosene came right out immediately and was actually very clear. I just wanted to make sure the breather into the engine was not clogged with slug.

Fast Eddie: Thanks for your excellent explanation of the PCV valve. I never knew exactly the theory now I and others hopefully do. People who have put in A PCV have report success.
 
JimNorton said:
The oil was well drained out and the drain plug open. The kerosene came right out immediately and was actually very clear. I just wanted to make sure the breather into the engine was not clogged with slug.

Fast Eddie: Thanks for your excellent explanation of the PCV valve. I never knew exactly the theory now I and others hopefully do. People who have put in A PCV have report success.
Click to expand...

There are two camps in regards to PCV and reed valves. Some feel the PCV is cheap and effective although they do wear out. The reed valves tend to last, well, forever, although can be more costly.
 
Reed valves can be bought cheaply by searching for egr valves in motorcycle parts on ebay.
 
No one has the XS reed valves right now... Since you guys have stirred me up on something I deleted 40 years ago I've begun thinking I'm missing out on something I could maybe want/need/or ?. Anyway I've searched everyplace I can and no find.... so I just installed a nice unobtrusive piston type which fit right in place of the conred at the oil tank. No longer shall this trouble me until the XS ones are in stock.... Might not help anything, but it makes me feel better at this moment in time because I now have a PCV. Who knows I may toss it again.

It makes an interesting sound idling with the cap off the tank also. So that's a big +.... It did something.
 
I just cleaned my XS reed valve I put on the 850 last year. Surprised to see so much clag, cream coloured sludge accumulated in valve and in connection hoses. Ive repositioned it to top of front corner of oil tank with pressured hose running nearly straight down then into timinig cover pipe. Previously it was just 3"away from that pipe, basically making valve lowest point in the line. Now its nearly highest. Drawback is the now long hose run to it from case, so maybe small loss of pressure pulse amplitude.
 
yes BUT there was another that you had to machine the rear of the drive side case to except the breather and it was different than the 72/73 750 type.

rvich said:
I think we are in agreement here.

From CNW:

"There is one Commando engine that was built that will allow the cNw breather to be installed without taking the engine apart. This is the engine built in 1972 that originally has a large tubular breather installed on the backside of the cases. So it’s located in between the transmission and the engine. By removing this original breather and installing our model you will get all the benefits these well engineered breathers offer".

They list the manufacturer as CNW/Comstock Engineering. It is the generation of breather developed by Comnoz prior to the sump plug breather that is now sold thru NYC Norton.

https://coloradonortonworks.net/1972-breather-modification.html
Click to expand...
 
Try Kenny @ NYC Norton I recently got the XS breather kit from him,I also run the sump reed valve from CNW on my 71 750 they are both excellent....Depending on your application you can not go wrong with either of these modifications
 
I have an 850 crankcase with PCV reed valve issuing from the rear timing side (standard position) into a small catch bottle.
I have fitted the barrel which has been lined to 750 (for racing purposes), with new Omega pistons, rings and big end shells, along with a new oil pump. The engine was rigorously cleaned including the centre of the crankshaft, before re-assembly.
Fired her up, and all ran sweet as, no oil issuing into the catch bottle.
Having got the engine warmed up, I revved it sporadically to 4500 - 5000 rpm, and when I turned round, the catch bottle was full of foam, and oil pouring out the breather hole in the bottle. I re-routed the main breather pipe after the PCV valve directly into the swirl pot oil tank, and managed to race without further issues.
The unit has an oil cooler, and I heard someone say that this can cause issues with the breather pumping oil out. It certainly was OK as an 850 with no cooler!
Any ideas?
 
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