I want to put a M21 cranckshaft (with M21 conrod and piston) in my M20. I wonder if I have to change the carburettor as well. And if so what type do I need?
The M20 carries a 1 inch (25mm) bore, the M21 uses a 1 1/16 inch (27mm) bore.
It would cause no harm to use the smaller M20 carb with the larger displacement engine, however apparently BSA felt it necessary to use the larger bore Amal 276 on the M21 for optimal performance.
The larger 276 carburretor can be found if you decide to install one, or you could go with the more “modern” Amal Monobloc style, or even the Amal Concentric in 27mm. It will depend on how “correct” in appearance you wish to keep you machine.
A 626 concentric fits nicely if you space it out far enough to clear the mangeto drive.
A 928 concentric will also fit but you will need a right angle guide on the throttle cable or it will foul on the tank.
The smaller carbs tend to favour lower revs and a nice clean idle.
The bigger carb will allow the engine to rev way past the maximum torque range, so gobble fuel and idle poorley
