A7 Amal 275 equivalent

[TassieChris]

Hi All
I have a BSA A7 Star Twin which the previous owner has converted to a single monobloc carb.
I would like to revert back to two carbies as the bike had originally and doing some research I understand that originally it had 2 x AMAL Type 275 carbies.

Now as these are hard to source and are no longer made I was wondering if there is a currently available bolt on equivalent I could install.

I know it wouldn't be strictly correct but would at least look (and possibly perform) like the bike was when original.

I'm guessing performance may not be significantly different but I don't like the look of the single carb and there is no room to run an air cleaner due to the 1 into 2 adaptor between the carby and inlet.

If anyone can shed some light on a possible replacement it would be much appreciated.

Thanks
Chris.

[BSA_54A10]

A 626 concentric will drop right on and is a much better carb to boot ( actually I think it should be a 624 for a 275 ).
They are a bit taller but the trick is to get a right angle adjuster on the end of the throttle cables because you can not get a smooth entry angle under the tank going into the carb.

[TassieChris]

Thanks Trevor
I think it would be 624 as the info I can find tends to suggest a bore size of 15/16 for the Star Twin which is about 24mm.
I did find another post that claims a 7/8 bore size though which is only 22mm.

Is it as simple as measuring the bore size of the inlet on my bike or is there more to it than that.

Also do I need another manifold for the carbs or do they just screw straight on to the head? - The existing monoblock is connected via a two into one manifold so it looks like I just take this off and screw the new carbs straight on?

Appreciate your help.

[trevinoz]

Chris, you will need the twin carb manifold, good luck finding one!
Also, if your head isn't really a Star Twin one, you will have to grind the ports to accept the manifold.

[TassieChris]

Thanks
Pretty sure it is a star twin head as the engine number matches and the original owner did have twin carbs on it many years ago - He just took them off to make tuning simpler (aaargh).
will start searching for a manifold.............

[BSA_54A10]

You won't find one or more over you won't find 2.
They are an easy fab job for any alloy welder.
The last ones I saw came out of one of the railway workshops.

[bikerjohndavies]

Here is a picture of the correct manifold with carbs attached. They do sometimes come up at auto jumbles and on EBay. Which year is your bike as the carbs numbers were different for 1949 (275AR/1A, 275AS/1A)and 1950 (275BA/1F, 275BB/1F) though I think the only difference were the length of the arm on the float chamber where the 1950 ones were longer to move it further out from the carb.

[duTch]

 The only thing I can suggest is go back to the guy you bought it from, and ask him what he did with the originals...?

[bikerjohndavies]

And here is a picture of the correct head. As you can see the inlet ports have been machined to allow the lip on the manifold to recess into it. If your head does not have these then it is not a twin carb Star Twin head.

[TassieChris]

Thanks Guys
It all looks do-able now I've done some research.
My head appears to be correct for a twin carb (see pic below) and I did find repro manifolds on line but they look pretty easy to fab up my own as BSA_54A10 said so I'll probably go that route.
I emailed AMAL and they suggest a pair of 22mm Mark 1 Concentric carbs as a replacement for the 275's so that's the latest thinking.
Still need to do a bit more research but looks like it wont be too hard to do.

Appreciate all your help so far.

[BSA_54A10]

And just to prod the memory.
The concentric will sit higher than the old carb and you may need the curved ends on the throttle cables to get them to work properly as your tank will sit very close to the top of the mixing chambers.

[TassieChris]

Thanks Trevor
Yes I found those on line after your initial comment plus I need a 2 into 1 cable adapter as my throttle control is only a single cable type but this is pretty readily available too so all coming together.

I think the single carb would be better with a right angle cable entry as well as its pretty close to the tank and bends tighter than I really like - seems to work ok and admittedly it is under the saddle cutout in the tank but I still think it would be an improvement.

[bikerjohndavies]

Further to my comment that the carbs and manifold do some times turn up at auto jumbles, I saw a complete set of 1949 version carbs and manifold yesterday at Founders Day though they had gone buy the time I went past the stall for the second time so somebody is a happy bunny.

[terryk]

Hi Chris send me an email I'm in Nth QLD I have a 49 ST in pieces and Im fairly sure I still have a few of those twin manifolds spare at least if you email me I will start looking.

No need to put email address in Terry, members can see it, visitors & spamers can't.

[TassieChris]

Thanks Terry - have emailed you - much appreciated.