Wet sumping again.

If it wet sumps that quickly, the oil pump needs servicing and the clearances closed up. Probably why the pump can't prime itself. (even though you say you have primed the feed pipe) Why fit one of these things? You will be forever wondering if it has stuck shut.
The only form of 'oil tap' i would condone is one of the magneto/ignition cutout variety. I know Norman White had a rider on his invoices that said that if any form of oil tap was fitted to the engines he had done for customers his guarantee was invalidated. 

Wow, chill out Howard. Doesn't get around the fact your oil pump is worn. Will it open that valve just kicking it? No. Service the pump properly, then maybe.  

Hi Howard,
When my 650 began wet sumping at a more than acceptable rate, I fitted an RGM supplied 'Velo' type valve on the inlet line. This worked 'perfectly', in that I no longer had a wet sumping issue. 
However,  the nagging doubts of the naysayers made me change to a switch with an ignition cut out on it instead. I kept it fitted even after the oil pump was "serviced".
Since fitted, I have a lot more confidence that all is ok on the oil supply side.
So, yes the oil pump on a twin will operate a velo valve quite happily, but as Peter says, it is better to fix the root cause of the problem first.
Regards, 
George 

You say you have primed the feed pipe: but upstream of the valve or downstream? Downstream ( between valve and engine, obviously) is vital, as on instructions with the valve I got from Mick Hemmings in 2002 for my Commando 850.
    When I overhauled the bike in 2024, after a 21 year lay up, I took the valve apart to check it, and it was perfect and  oily and not stuck etc. 

The commando pump runs at twice the speed of the earlier models. It also has wider pump gears so it will pump over twice as much oil for the same engine speed. That means it has over twice as much 'suck' as the earlier pumps. The result is any comparisons between the ability of these pumps to open a sprung loaded ball valve is not relevant, especially when worn.   

Have you tested it to make sure it does flow?  A small magnet should probably pull the ball away and let oil flow.
It does take time for oil to return of course

I'd temporarily replace the tube below the valve with clear plastic.   Then you'll see for sure if it's working.  It probably won't do for a long term fix because it's likely to be too soft and might get sucked flat.

Hi Howard, 
I'm running 3 start gears on mine. I have the original scavenge fed rockers, removing much (all?) of the need for 6-start cogs. 
My engine is a mongrel, so engine number isn't any use as a guide as to what should be inside it!
The pump is one of those with wider gears inside it though.
Hope this helps,
George 
 

A couple of points re causes of falling oil levels.
Dominator engine parts made from Aluminium alloy were well know for being porous. The main problem being the coarse molecular blend of metals used for these castings.. 
Any sections that were badly cast or machined too thin were prone to leaking. In cylinder heads this was primarily near the inlet tunnels and in the crankcases the walls of the oil pump and associated tunnels.
The face used to mate the oil pump face was also prone to distortion thanks to the cast iron pump and flexing of the nearly main bearing housing.  This being one of the reasons, on later twin engines,  a paper gasket behind the oil pump became standard.

A pressure gauge would tell you if the oil is getting to the pump.
You could fit one in place of the pressure relief valve temporarily and just run it at tickover? 
 

...while researching my Roadholder article about stopping wet-sumping on the lightweight.

From an American site, I think.  Intended for heavyweight engine.   What caught my eye was the ball valve in the timing cover modification, looks like a neat, straightforward job.  It puts the valve exactly where it should be, downstream of the pump.

Anyone tried it?

Not so sure about the oil pump modifications, should not be needed if the pump is in good condition.

Thanks Peter, edited accordingly to reflect what I meant to say.

I disagree that fitting a ball valve either before or after the pump will cause a restriction.  I say this with one proviso; the valve must be designed properly.   Looking at the Velocette NRV, as that seems to be the inspiration for owners using this type of valve, the cross-sectional area through the valve is (much) greater than that of the various fittings elsewhere on the pipe.  Also important is the ratio of ball diameter to seat diameter.  If the design is done properly there will be no restriction to flow and the oil pump will not notice the presence of the valve.

Point taken on losing the "prime".  But, does that really stop the pump sucking oil from the tank?  My experience with Velo's says no, as long as the last bit of the  oil way to the pump is "uphill" and doesn't drain, the pump will have some oil to work with and get the ball off the seat.

Getting this thread back on track, I would advise the OP, if he has the resources, to fit a valve as described in the attachment  in my first post.   If not, use one of the proprietary "fit and forget" ball valves.  I would not recommend a Velo valve in this instance, because the ball is not properly guided.  It is lifted on to its seating by a feeble spring that relies on the valve being (near) vertical to work properly.

Going back to Velo's, a Yamaha NRV for, I think, an outboard motor fuel system, is much favoured as a replacement to the original NRV.  I'll try and find the details if anyone is interested,

Howard, here are the details of the Yamaha NRV, I think they cost around £25.

I have never used one so cannot recommend it.  But, I know Velo owners who have used them with success.  If I could not get my Velo to stop wet-sumping with the standard arrangement I would use it!  Having said that, I have made and fitted a valve downstream (thanks Peter!) from the pump, like they originally were.

I would mount it vertically to make priming easier.  Some Velo owners like to mount them horizontally under the gearbox to hide it, must be difficult to prime in that position?

.. Yamaha valves on my ES2 and it worked very well. Of course the advice re ensuring that the oil line downstream of the valve is full of oil is essential. The bike came to me with a manual valve but I was never sure I'd opened it.....