Hi Bill,
The carby on this bike is an AMAC (Aston Motor Accessories Co.). There are a number of varieties fitted to the 2 3/4 over the years, most made specially for Douglas. The double lever on the bars is also AMAC.
On the tyre issue, I've resolved to stop using rickshaw/handcart tyres as I use up the stock that I have. The Ensign beaded edge tyres come with all the proper stuff for a motorcycle tyre, like a speed rating. On the downside, they are selling at $A150-ish I think. My preferred tube is a Michelin Heavy Duty 2.50-3.00 - 21, again expensive at about $A30, but a flat tyre on a bike with beaded-edge tyres can be a disaster as there is nothing to hold the tyre on the rim when it goes flat. (There is also a super heavy duty Michelin tube, but it comes in a box the size of a shoe box!)
Cheers
Leon