Technical - Carburettors

A quick quote from Phil Irving's 'Tuning For Speed' - 'A 1" (25.4mm) carburettor is optimum for a 500cc twin motorcycle', which is interesting because both the original Amal 276 and the aftermarket replacement Amal Concentric is 24mm which is spot on size considering we are not 'Tuning For Speed' but for an economy, reliability, performance balance and is therefore just a touch on the smaller size. The 276 is actually listed as 15 sixteenths of an inch (24mm).

Replacement Amal 276 D0 Sunbeam carburettors are now available new but you may find it difficult to get the small '3A' remote float chamber. (I may have spare ones if required... E-mail for details). The only problem is that the new carbs have their adjustment screws and jets on the right hand side which on our bikes would be covered by the float chamber, which makes them very difficult to adjust and set up. The original Sunbeam 276 carburettors were left handed to accommodate this.

One alternative is to buy a completely reconditioned carburettor... One source of these I highly recommend is Forest Classics, UK, http://www.forestclassics.com who supply reconditioned carburettors for nearly all machines. These items are acid dipped, resurface finished where necessary, rebored and sleeved and all jets, slides, gaskets, screws ect. replaced. These carbs look brand new and for all intentions and purposes are new. They even give a small exchange price on your original carb! They supply the carburettors configured to the application (i.e., per model) so you will receive a carburettor pretty well suited for purpose out of the box.

Another alternative is to fit an Amal Concentric 624 carburettor as recommended by Stewart Engineering, although unless the carburettor is purchased new from Stewarts or second hand from another Sunbeam you will be hard pressed to get the unit to run correctly as the Stewart Engineering model is highly modified and in fact uses parts from both the 2 stoke and 4 stroke Amal concentric 624 versions. Basic configuration is given below.

Advantages\ Disadvantages

The Amal 276 D0/3A is the original carburettor and therefore will ensure that the looks of the bike are kept completely stock, however once hidden behind the air filter cover it is difficult to spot the difference and the Amal Concentric is generally easier to purchase new.

To fit an Amal Concentric carburettor you will also need to replace the studs that hold the air filter cover as it will sit a little farther from the engine block. You will also need to run a cable for the choke as the original carb has a lever on top or a lever on the air filter and Concentric does not.

I have asked many owners if they have noticed any differences in performance between the Amal 276 and Concentric carburettors in terms of Starting, Economy, Speed and general running and although the vendors selling the Concentric will state that mid range power is boosted, I have received no reports from owners of any performance increase or real world benefit at all, although Stewart Engineering state better starting and tick over and 'No Leaks!'.

The Amal 276 carb is sometime referred to as 'Old Leaky' and other such terms, however with new gaskets, float valve tips and a carb in generally good condition it should not leak.

Sunbeams are known to suffer from 'Pre-Evaporation' because the carburettor is situated very close to the exhaust pipes, however the fact that the Concentric carb sits a little farther away from the cylinder head may help the problem a little.

It's your choice!

E-mail: bsmc@classicglory.co.uk

www.classicglory.co.uk