oil tank ventilation

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Sep 2, 2017
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ON the top of the 67 oil tank there are 2 fittings that appear to be for vent hoses. 1 faces forward and the other faces back. Am I correct in my assumption that the forward facing one connects to the crankcase breather and the rear facing one vents to the atmosphere? If this is correct is there a problem in fitting a pvc valve in the line between the crankcase and oil tank?
 
I am surprised no P11 gurus have chimed in, so I will contribute the little I know.

If the P11 oil tank is similar to an Atlas, in function, not shape, the forward facing tube is for a vent hose to the crankcase breather. I don't know if the '67 model was still using the timed rotary breather, but if so, that is where the other end goes.

The rear facing tube is the oil tank vent. This goes to atmosphere via a tube. On my Atlas, this tube ends at the front end of the rear chain guard. On the P11, it may drop down to vent near the front of the rear tire. I have seen some vent tubes routed along the inside of the rear mudguard, terminating at the end of the mudguard.

I assume you mean a PCV (not pvc), or positive crankcase ventilation valve. There is no problem fitting one of these between the crankcase and oil tank (free flow towards oil tank).

If your engine has the timed breather, and if a PO has not removed the spring and vane, there will be no advantage to fitting one. I have measured the crankcase pressure with an XS650 one way reed valve in series with the rotary breather, and found no difference in crankcase pressure. Otherwise, go for it .... it will not hurt anything.

Slick
 
Slick - Thanks for your reply. As I’m relatively new to Nortons this is very helpful and verified what I thought. not sure about the timed rotary breather so fitted the PCV valve anyways. I did mean PCV - my dyslexia must’ve kicked in. I routed everything tonight but won’t have a chance to start it up as it’s on the lift waiting for a new speedo gearbox. John
 
Slick is correct on the pipes, if you want to see if your Atlas engine is using the timed camshaft breather just unbolt the elbow on the L H side and turn engine over –you should see a flat disc with 2 slots going round
 
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