Friday 12 February 2021

Carburettors and Inlet Manifolds

I had already acquired a couple of carburettors for this project some time previously and had decided to use early AMAC instruments. These bronze carburettors were in production in the mid 1920s – a few years before the original AJS 1000cc V-Twin was designed but it is not clear from the pictures which carburettors were actually used on the original bike. Whatever they are they have massive float bowls

 The carburettors that I have are an AMAC 15 MDX and an AMAC 15 MDY

According to the period AMAC catalogue and spares list, these were produced in 1928

The only difference between them is that the MDX would have been supplied with a top-feed and the MDY with a bottom-feed float chamber – the carburettor bodies are the same. Here, I have fitted them both with bottom feed float chambers and they have been refurbished and converted to use a needle (by Martyn Bratby) rather than the needleless carburettors that left the factory. They are both 1” choke and 1 1/8” stub fitting.

Inlet manifolds are required for the front and rear cylinders. For the front cylinder, the carburettor needs to be located between the “V” of the timing cases and is a stub fitting onto the cylinder head. The fitting on the rear cylinder head is a 2-stud flange (the KTP cylinder head has a different carburettor fixing to the other Mk 1 OHC engines) and the carburettor positioning needs to avoid a frame bracket. This bracket could be removed if necessary but it would be advantageous to keep it if possible might come in use for something later.

Although I have a decent pipe bender for small pipes and tubes up to 1/2" diameter, I do not have a bender that can accommodate a pipe of 1" ID. Luckily, pre-formed bends in stainless steel are available on ebay at a reasonable price and in a variety of sizes and angles.

 They are sold under the banner


Two 900 bends were ordered and these turned out to be high quality and very well made (in Italy). Stub inlets for the carburettor ends and flanges for the connections to the cylinder heads were machined and, after reducing the angle of the bend a few degrees in both cases, the pieces were silver soldered together to make 2x one-piece manifolds.


A flange was silver soldered onto the front cylinder head stub to provide attachment and, by putting the float chamber on the right side of the carburettor, the front carburettor could be attached.

and it “sits” snuggly between the timing case castings.

By carefully positioning the bend on the rear cylinder it was possible to fit the carburettor without removing the adjacent bracket.

Overall, this turned out to be a fairly elegant solution ….although it does give the bike a bit of a “Mad Max” look… like the original....


1 comment:

  1. Hi Richard....I 'm loving your work on restoring the AJS.....I myself am in the midst of completing my 1929 Norton M18 Sprinter....a brief spec'.......knife edged Crank, 9.5:1 BSA piston, 1 7/8" exhaust valve.....Std inlet....ITG profile Cams.....all I need to finish off is a AMAC 15 MDY carburettor which it had originally when first modified back in the day to replace the modified AMAL 76 it has today.....would you have or know of anyone who has one they might be willing to sell....I'd be very grateful if you could ask your contacts....Kind Regards.....Russ Jackson