Score!

So, in addition to the single-carburetter A50, BSA also made a twin-carb version of the 500cc for competition. It was called the Wasp (but there was also the Cyclone - little difference I think) both in scrambler and in road going trim; not many were made compared with the standard version so it is somewhat of a rarity compared to the twin-carb 650s.

Really, the most important component of this version is the cylinder head, which has the twin port for two carburetters. Visually the biggest difference is in the space between the two combustion chambers, about two inches on the 500, whereas there's hardly any meat in between the 650's head.
A few months ago I was browsing around for spares when I came across a spares dealer (NOS and new) that had one up for sale at a very reasonable price and in good nick. I jumped at the chance and now I have the option to transform the engine into a fire-breathing twin-carburetter beast if I should find that the standard head doesn't have enough go.
What the heck, I just like having it sit on my shelf for now!

A bit of background.

Here's what I know about the engine's provenance: it was bought and sold at least a couple of times, the last of which was at one of the regular classic bike autojumbles where blokes go rummaging in big grimy piles of dirty, rusty, sticky, oily metal sitting in the open air, in the baking sun or under the pouring rain.
As I mentioned before, I have no idea why the engine became separated from the rest of the motorcycle in the first place, but judging by the way it seems to have been severed, I'd say it was probably done at a breaker's yard? Or perhaps the bike was in an accident and they salvaged the engine? Maybe they had a 650 engine and swapped it out?
I doubt I'll ever find out, though I might get in touch with the owners club and see if they can trace any history based on the engine numbers. Then again there are only slim chances of that yielding any results seeing as, from what I gather, BSA engine numbers are more of a rough guide than a precise indication.
What I know for sure is that many people have had this engine, but few of them have actually used it. It just sort of got passed around until it reached me - and even then, it's been sitting in a workshop half dismanteled for a good while now.

'sploded view

Behold! And learn all the secrets of the Amal Mk1 Concentric!
How glorious are vintage drawings?
There is only one small piece not shown (or possibly shown within the air slide spring) which is a small brass tube in the shape of an elongated top hat: this fits into the spring, into the air slide.

Features for 1962.

You really do get the best of everything with a BSA!

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.