Marc:
I don't have any experience with alternate carburettors for Douglas bikes but in the May/June issue of the New Conrod (LDMCC Newsletter) there was an article by a Mr. Doug Cross who seems to have made some interesting modifications to his Mark V engine. He uses it as an alternate (test) engine in his T35.
From what I can gather he has fitted Suzuki GS650 pistons, raised the compresssion ration to 9:1, fitted a hotter cam (something referred to as No. 3 cams?), bored the inlet tracts to 24 mm (from 19mm), and using a tapered adaptor, fitted 28mm Amal MK II Concentrics. He fitted 150 main jets.
I know that's not much help with selecting a direct replacement, but it does imply to me that the Douglas motors are relatively tolerant of major modification, suggesting that:
1. if you want to fit another carburettor, the trick is not so much picking one, but attaching and fettling it:
2. the motor is quite tolerant of modification, especially with respect to the inlet tract.
I kind of like the look of the original 274s, despite incessant leaking and poor performance, but if I was looking for a modern replacement, I'd go for something more adjustable, like a Mikuni.
Best of luck.
David A
