The usual - engine breather

Surely this pipe should go back to the oil tank? As there is no pipe on the oil tank and there never was a pipe to the rear number plate originally??
Further-IF you fitted a Slimline oil tank, you would get a Froth pot that would take this breather pipe. it would also change the tool tray over to a Slim Line one. The two rear mounts still work, but front mount will need some 'arranging' 
Other wise have it original-what ever that was?

Newly rebuilt? Was the crankcase split and the camshaft breather disc either removed or correctly engaged?  Sounds like your breather pipe routing needs altering. Engine breather to oil tank then oil tank to atmosphere.  If you have fitted a NRV to the oil pump supply, beware as if it sticks shut it will destroy your engine (a better one is the type that have a cutout switch so preventing the engine starting without the oil on)  

Richard hello. None of the wideline Dommies nor the first few years of the slimline versions had the breather pipe routed to the oil tank.  Some had the copper exit tube with a bend expelling the output beneath the engine.  Many people extended the pipe without a bend with a plastic tube along a rear mudguard support bracket to exit beyond the rear wheel.  When the oil-froth tower was introduced a piece of black flexible tube was connected to the crankcase breather and the other connected to the top of the rear chainguard top run.  Regards, Howard

I have a 59 wideline dommie and the original breather pipe went to the chain guard to oil the rear chain.
I re-routed mine to the rear mudguard behind the number plate as I didn't want contaminated oil on the rear chain.
I only get a few light drips from the breather after a spirited run, so I suspect your piston rings haven't fully bedded in yet and you are getting some blow-by.

The few light drips that I get from the breather are always emulsified oil and this would be a very relevant reason for not returning this sludge back to the oil tank froth tower as was used on the later dommies. Who wants water and condensation in their engine oil?

I have no drips from my later 99.  A newly built engine can run in within 500 miles or less , but with the wrong circumstances it could take 1000's or never. In which case the case will be under a lot of pressure .  I would run the crankcase vent pipe up to the oil tank and into a tower , then fit a breather pipe above that and vent to the rear . use 3/8 copper gas pipe fittings  with soldered joints into tank and 3/8 bore rubber hose. . You could make the tower out of 25mm copper pipe .ordinary plumbing solder and acid flux. There will be some condensation in the tank ,but we change our oil dont we.. You can fit a non return valve in the pipe before the tank . If it has good compression then its running in. 

Originally, the engine breather was just a bit of pipe that pointed at the gearbox sprocket. Quite a good way of keeping the rear chain oiled on long runs at the cost of the odd drip on the garage floor.
650s with the double speed oil pump had a different oil tank with the engine breather routed up to it. The oil tank breather was routed to the rear chainguard again with the intent of oiling the chain on long runs.
Norton rear chainguards are pretty rubbish, certainly compared to BSA ones which are designed to keep road filth off the chain so any means of lubricating the chain has to be a good thing. I wouldn't worry about breathing contaminated engine oil onto your rear chain. It has to be better than a spray of road muck off the rear wheel.  

The reason Velocette owners don't have problems with their anti wet sumping valves, is due to their engines having a roller big end. This type of bearing can run with no oil pressure at all, & just rely on any oil splashing about for several minutes. Norton twins have plain bearing big ends which rely on a pressure feed. If there is no oil feed to the crank, even for a minute or perhaps less, the bearing shells will run dry, get hot & pick up on the crank.
I know many people have fitted these valves to Brit twins without a problem, but because I have ridden for years without a puncture doesn't mean I won't get one sooner or later.

Just to be more certain of how much oil comes from the breather pipe, a catch tank will tell.
A half litre plastic bottle fitted between the rear engine plates, secured with a couple of zip ties.
A clear plastic breather pipe going halfway down in the bottle. No cap on bottle.
Done in less than 10 minutes.
On the Manx a few drops after 5 years racing. On the Honda racer a few cubic centimetres with a broken piston ring after one race.

My Dommi always used to blow oil out of its breather - no matter what I did to the pump. When I rebuilt the bike I came up with this system which has worked very well now for 5 years (I made a similar set-up for my ES2 18 years ago). I also added an extra breather (+ NRV) coming from the mount that takes the distributor - it's well hidden - I'm not altogether sure that it's necessary but the engine is oil-tight and it's used quite hard. I think the pics make it fairly self explanatory - just make sure room is left to be able to get at the nut of the top oil tank mounting. The large tube out of the unit just drops behind the gearbox and little or no oil is ejected from it.
I ought to add that the reason that I didn't just go straight to the oil tank with the breathers is that the oil tank itself only has a very small breather tube out and speaking from experience, if the oil tank is pressurised in any way, it tends to eject quite a lot of oil.

Some images of the breather fitted during the assembly of my 650SS.

I hope it works!