the old '626'

I took apart the A50's original carburetter to see what was inside and get an idea of jetting and needle position.
When BSA originally produced the first 500 unit twin, they fitted a 1 inch Monobloc, which was then superseded by the Mk1 Concentric when the Monobloc line was discontinued. Switching from inches to millimeters, the 26mm was the closest to the 1 inch measure.
I'll fit another carburetter altogether but I kept this one for the original air filter and as a source of float-bowl screws (you can never have too many of those, trust me.)

Look at the petrol pipe feed and the throttle cable sticking out at the top: my guess is this was chopped off the bike with a pair of wirecutters when the engine was removed. Now, why the engine was removed from the original bike, we will never know. 



All the bits are there, further proof that this thing was just ripped off the original bike together with the engine - to which it was still attached, air filter and all, when I got it.

But what's this? The float needle is of the modern viton-tipped variety as opposed to the original all plastic kind. This would suggest that at some point, someone opened this up and changed it.

The stampings at the bottom of the slide are "622" and "3 1/2"

The needle is marked with two thin grooves on the head (these are not the ones where the clip goes, Amal needles come in several different types, identified by the markings on the blunt end of the needle) - the clip is in the 1st groove (or 3rd, depending which way you look at it) meaning that with the throttle off, the needle sits high in the jets assembly, allowing more fuel through.
This is one of several things you can tune on the Mk1 and they each make a difference. Do not be fooled by the apparent simplicity of the Concentric, for therein lies the brilliance of this design. You don't need anything more (pointlessly) complex than this, and you can tune them to suit your engine perfectly.

The main jet is a 200 and I don't even want to guess what that freaky residue is inside it. Probably something organic left by insects. I had just ran out of WD-40 so couldn't get the jet holder off to get to the needle jet, which I'm guessing is probably a 104 or a 105.

Speaking of insects, what the frak is that?

Definitely not in the Amal parts list.