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#478616 - 02/26/13 2:51 am 71 Norton Roadster crankcase breather issue  
Joined: Aug 2012
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Dana_twin Offline
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Dana_twin  Offline
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Oregon
I have a 1971 750 Roadster Commando and it runs well and does not smoke, the engine has excellent compression and the top end was rebuilt with new valves, rings etc, apparently without issue. It does have - and had before - the usual crankcase breather issue. I was kind of hoping the top end rebuild would fix it but didn't. This is an early Commando with the timed crankcase breather through the camshaft. With oil weeping here and there I'm pretty sure too much pressure is building up in the crankcase. Right now I don't have the airbox on, so the oil tank vents right to the atmosphere instead of to the air intake box. I don't have a PCV valve on the breather line from the case to the oil tank, someone recommended the XS 650 PCV check valve to fix this. Rather than get into a long technical discussion on crankcase breather issues, can anyone please recommend whether the XS 650 PCV valve will help? I was also told to lower the oil level below low and get the airbox back on. I wonder if doing all three of these things will be beneficial? Finally, is it a good idea to blow out the breather aperture on the crankcase with the disc open of course to unblock any debris? TIA

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#478647 - 02/26/13 9:29 am Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin]  
Joined: Dec 2004
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kommando Online content
kommando  Online Content


Joined: Dec 2004
Posts: 8,886
Scotland
The XS650 breather does help but not added to the timed breather line as you want more breathing, I would add it to a new breather line fitted to the inner side of the old magneto postion as per the 850's and try it with and without the timed breather being blocked off.

#478650 - 02/26/13 10:29 am Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin]  
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L.A.B. Online content
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L.A.B.  Online Content
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Norfolk, UK
Originally Posted By: Dana_twin
I was also told to lower the oil level below low


That is a sensible suggestion, as it may help to reduce the amount of oil blown from the oil tank vent, (some Commando dipsticks have the "H" and "L" marks too high) also make sure the two breather pipes are not fitted to the wrong stub connections on the oil tank. Instead of refitting the airbox you could run the oil tank vent outlet pipe into a small catch bottle if you have not already done so.

http://www.oldbritts.com/oillines.html

#478651 - 02/26/13 10:36 am Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin]  
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Dana_twin Offline
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Oregon
Great idea, ie plumbing up a spigot/hose to the mag blanking plate...? I've seen a suggestion as well to drill three holes inside the timing chest but that sounds like a total strip down due to the swarf created? thanks

#478652 - 02/26/13 10:39 am Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin]  
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Dana_twin Offline
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Oregon
LAB my bike was m'fd June of 1970 and only has one crankcase breather pipe that I am aware of. It has the mag blanking plate, on the timing side crankcase, but no breather line there. Sounds like a good idea to add one in that location? regards Dana

#478654 - 02/26/13 11:21 am Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin]  
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L.A.B. Online content
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Norfolk, UK
Originally Posted By: Dana_twin
LAB my bike was m'fd June of 1970 and only has one crankcase breather pipe that I am aware of.


Yes there is ony one crankcase (to oil tank) breather pipe. But there are (or should be? Edit: unless it has the central oil tank?) two breather connections on the top of oil tank?

One is for the crankcase (engine) breather, the other is the oil tank breather which would normally connect to the airbox (diagram shows a later breather) but can be routed into a catch bottle, so make sure both breathers are connected correctly and not reversed.

As your bike was made in June 1970 does it have the central oil tank, or does it have the later side tank shown below? The central oil tank has the vent lower down on the front, some of these central oil tanks have a stub fitting, and some apparently do not.



Last edited by L.A.B.; 02/26/13 12:00 pm.
#478662 - 02/26/13 1:10 pm Re: 71 Norton Roadster crankcase breather issue [Re: L.A.B.]  
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bill50cal Online content
bill50cal  Online Content


Joined: Nov 2003
Posts: 692
orlando fla/ shady valley Tn
I would NOT do the 850 timing chest breather mod but instead IMHO the better way to do this is the Jim Comstock sump plug breather. this is a bolt piece and is FAR more effective. the only thing you have to look at is where is the frane crossmember in relation to the sump drain as the early frames it can be in the way.

Last edited by bill50cal; 02/26/13 2:59 pm.

windy
72 combat
switchbackcreek.com
#478674 - 02/26/13 2:47 pm Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin]  
Joined: Dec 2004
Posts: 8,886
kommando Online content
kommando  Online Content


Joined: Dec 2004
Posts: 8,886
Scotland
Quote:
Great idea, ie plumbing up a spigot/hose to the mag blanking plate...? I've seen a suggestion as well to drill three holes inside the timing chest but that sounds like a total strip down due to the swarf created? thanks


Yes the 850's have the 3 extra holes but the main bearing has lots of potential for the passing of air, plus Jim Comstock's sump breather is a good option too.

#478686 - 02/26/13 3:57 pm Re: 71 Norton Roadster crankcase breather issue [Re: L.A.B.]  
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Dana_twin Offline
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Oregon
LAB I have the S series with central tank, where the oil tank vents through a pipe fixed inside the oil tank to a hole into the air box. Thanks for the diagram that's helpful too.

#478688 - 02/26/13 4:07 pm Re: 71 Norton Roadster crankcase breather issue [Re: bill50cal]  
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Dana_twin Offline
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Oregon
Bill, thanks for the advice. I have the early frame with cross member in the way. The Comstock plug is interesting and ingenious but won't work on my bike unfortunately. And this is my bad, the bike had wet sumped... so there was no breather issue after all! Sorry. Theory is that the new rings had not seated on first run up with the fresh top end, causing blow-by and exacerbating the issue. Drained the sump and did a good run and all is well now.


Last edited by Dana_twin; 02/28/13 8:05 am. Reason: detail

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