Paul,
You can try buying a carb rebuild kit, they aren't expensive. I rebuilt mine twice, it didn't seem to help, so I bought a brand new manufacture carb rather than an NOS one. It was around £100 but it made one hell of a difference.
My Lightweight was using a lot of oil and pumping out black smoke the other year. This problem peaked at Beltring, on the way home I was struggling to get up the hills and I almost didn't get allowed through the Dartford tunnel.
I stripped the engine down and found the rings on one of the pistons were knackered. There was also a chunk missing from the piston. I put in new valves, valve guides, gaskets, a new piston, 4 sets of rings and honed the cylinders. All these parts are quite cheap and the manuals are easy to find and easy to follow.
Chris