Fuel seeping from pilot air screws.

The downdraught  carbs  have always put the idle system a bit too sensitive.  I have had 10 thou machined off the bottom of a spare needle assembly body  (as recommended by Anna!)  and in cases when the fuel level seems high tried it out. If that does not work try 20 thou. Before doing this check the floats are not fouling casting ribs  and the float and brass bush are not binding on the cover.( if so file the bush or fit a thicker gasket)

I abandoned the new "stay up " plastic floats and went back to the brass ones.

A single concentric (930) , as per Mercury would solve your problems (280 main jet) What I did with mine and no leaks or starting issues. Like you though, a bit lumpy on tick over. But I do get reliability and over 70 mpg.   

Another alternative  for  a performance boost would be to fit a remote matchbox float. This will provide an accurate  and adjustable fuel level  and eliminate the vibration frothing that can inhibit power at high revs. Made a considerable difference to the early Bonneville. Not much room with the Norton set up . 

Is it right to assume fuel level is the problem here? I also have a new pair of monoblocs (from Burlen via Surrey Cycles) and not seen leaks from pilot jet screws. I think the slow running screw has an o ring, but the pilot doesn't, so the designer didn't expect a problem...and they must have mass produced hundreds if thousands of them, with inevitable variations.

Sometimes my carbs have leaked when standing still when fuel level slowly goes past the float needle and creeps up.  But it sorts itself out on the road.  The earlier carbs with metal to metal float needles are almost bound to leak a little if left with fuel on and engine off.

While I can agree about the less maintenance, lighter twistgrip etc, etc, I've often wondered where the  claims of better economy come from. If the carbs are set up properly, the engine will get the correct air-fuel mixture with one or two carbs, and arguably better prepared with one carb per cylinder. 

To get improved economy means  either less fuel being used per cycle, or a dramatic decrease in pumping loss friction. Why is either going to be dramatically different? I'm not too sure if the 'difference' in air speed over the jets is giving improved mixture preparation as a reason because our carbs have slides to keep that reasonably constant for a given speed/load.

So, is the economy effect because most owners have carbs set up way too rich at lower throttle openings or are owners simply riding slower?

Just wondering.

George 

If you have brass float needles try swapping them for nylon which are lighter and easier for the floats to lift. The steep carb angle also makes for less leverage by the float action. Making the design less efficient.  I would keep trying with the twin mono carbs , They absolutely "make" the bike an SS  and are dead cool !.  And I want one.

Thats the sort of problem I expect to happen to my  289 side float carb. The taps HAVE to seal off otherwise the garage always smells of evaporating petrol. I found with monos the jet base nut often needs an extra tweek a few days after working on the bike , a hand passed under the base nuts often shows a seep is starting.Fibre washers are not what they were it seems.