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71 Norton Roadster crankcase breather issue #478616
02/26/13 2:51 am
02/26/13 2:51 am
Joined: Aug 2012
Posts: 620
Oregon
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Dana_twin Offline OP
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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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Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin] #478647
02/26/13 9:29 am
02/26/13 9:29 am
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Scotland
kommando Offline
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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.

Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin] #478650
02/26/13 10:29 am
02/26/13 10:29 am
Joined: Mar 2006
Posts: 3,678
Norfolk, UK
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

Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin] #478651
02/26/13 10:36 am
02/26/13 10:36 am
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Oregon
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Dana_twin Offline OP
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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

Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin] #478652
02/26/13 10:39 am
02/26/13 10:39 am
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Oregon
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Dana_twin Offline OP
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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

Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin] #478654
02/26/13 11:21 am
02/26/13 11:21 am
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Norfolk, UK
L.A.B. Online content
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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.
Re: 71 Norton Roadster crankcase breather issue [Re: L.A.B.] #478662
02/26/13 1:10 pm
02/26/13 1:10 pm
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Orlando fla/ shady valley Tn
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bill50cal Online content
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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.

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Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin] #478674
02/26/13 2:47 pm
02/26/13 2:47 pm
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Offline
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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.

Re: 71 Norton Roadster crankcase breather issue [Re: L.A.B.] #478686
02/26/13 3:57 pm
02/26/13 3:57 pm
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Posts: 620
Oregon
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Dana_twin Offline OP
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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.

Re: 71 Norton Roadster crankcase breather issue [Re: bill50cal] #478688
02/26/13 4:07 pm
02/26/13 4:07 pm
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Posts: 620
Oregon
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Dana_twin Offline OP
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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
Re: 71 Norton Roadster crankcase breather issue [Re: Dana_twin] #738934
06/16/18 8:01 pm
06/16/18 8:01 pm
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Posts: 107
Alaska
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linker48x Offline
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Alaska
There are lots of good one way breather valve options both cheap and expensive, fancy machined aluminum or plain old plastic, but I thought I would mention one other thing I just discovered to my embarrassment, that is really important to not getting oil flowing out of the breather. Don’t overfill the tank. The test is not where the marks on the dipstick are, it is that your oil level when the engine is running hard and fully returning must be below the top surface of the tank, and not run up into the filler spigot, since that headspace between the top of the oil and the roof of the tank is what allows returning breather air to reach the breather tower and its outlet. I filled the tank out of a gallon jug and while the oil level looked good sitting still the oil level running was too high and I got a mess, because the breather pressure couldn’t easily move from the filler spigot breather inlet to the breather tower outlet. Doesn’t matter what breather valve you use, if the pressure can’t vent to atmosphere you will have a problem.

Last edited by linker48x; 06/16/18 9:16 pm.

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