I was initially worried that I might have to try and source a correct carburettor for 2751. This is what came fitted to the bike:
This appeared to be of the correct clip-fitting configuration. The float bowl angle should be 15 degrees. Note the film case cap – photographer Peter Johnson’s touch no doubt!
I needed to confirm that what I had was indeed correct so I removed and photographed it and sent the images through to experts David Dunfey and Bill Hoddinott.
Both David and Bill carefully examined the above photos and agreed that the carb’ appeared correct.
Peter Johnson commented: “The carb came with many other original parts, and I suspect/believe it is the original item. The manifold is cast iron .. it was cracked when I got the bike, so we constructed a jig, and rewelded it with cast iron rods , and remachined the clamp area to the correct diameter.. I left it as plain cast iron but suspect the previous owner has painted it silver to leave his mark .. back to bare metal it should be.” Actually, we now know that these were supplied either polished and chromed or left in the “raw” and cadmium plated, which this one would have been.
So, original carburettor and manifold in fine condition – a great piece of luck!
Grey Flash 2751 
When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls extra fuel through the main metering system to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.
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