Model A - carburetter needs some bits...

 The Model A uses an Amal 6/112 carburettor, often known as a pre-monobloc. The Type 6 carburettor has the pilot air inlet in holes around the body and is similar to the Type 76 - the difference is that the 76 is cast alloy and the 6 is cast brass; the later Type 276 has the pilot air inlet inside the bell mouth. The float chamber design associated with each of these carburettors is also quite different in that the earlier float chamber has the extension pipe banjo retained to the float chamber itself with a screwed joint - for the 276, the extension pipe banjo is a part of the float chamber casting.

Whilst the carburettor that arrived with the Model A is complete, it is also pretty tired. The slide is very bad, the main jet is missing and we know from experience that the needle jet and needle will be very worn as well. 

The specifications are listed here:

• Size: 1"
• Main Jet: 4/042-160 (still available new)
• Pilot Jet: none
• Needle Jet: 4/061-106 (still available new)
• Needle Position: 6/065, position not recorded
• Slide: 6/0524 - that's a #4 cutaway, or 1/4"
• Float chamber: 14/004

Folk with some experience of Amal carburettors will realize that these part numbers are used for the Type 76 and Type 276 as well - and are still available from Burlen Services, or Hitchcock's Motorcycles.

I put in an order to Hitchcock's for some bits and sent the body, jet block and jet block nut to Joe the Carb on the 23rd of July:

It came back from Joe on the 8th August with a cylindrical bore, a slide to match, and he has refaced the mounting flange so that it is nice and flat. It looks like it has been vapour blasted too:

Until I make up the cables there's not much else I can do with this carburettor (other than replace all the gaskets and fibre washers), so we'll fit it back on the bike and leave it for now.

Model A - brake plates

 There hasn't been much action on the Model A lately – I have been rebuilding the top end of the Huntmaster and doing a bit of travelling. However, here is a little interlude: some while back we painted the brake plates from the Model A ready for rebuild and last year I acquired a new set of brake cams from the Slovak Klub.

You can see from the picture how bad the originals were - there was an awful lot of movement in the bush on the brake plate. Karol at the Slovak Klub supplies his replacement cams with a diameter of 13 millimeters which I thought was a problem until I realized what he was doing – if you drill out the brake cam bush on the brake plate, 13 millimeters is enough to take out most of the wear and you can fit new brake cams without replacing the bush, which saves a bit of time.

They work very well, although the square section is a bit oversize too - probably intentionally!

I had also noticed that whilst Karol's cams were identical, the old ones were not. You can see from the picture of the rear brake plate as the bike arrived that the position of the brake lever is a long way forward - too far forward:

The new cam puts the brake lever in a more sensible position.

FH - reassembly

 Having checked the pistons, looked at the big ends, camshaft and followers I was ready to put the barrel back in place.

First step is to Wellseal the barrel flange and stick a new gasket in place. I decided it will be easier to have the gasket on the barrel with well seal both sides before introducing the barrel to the crankcase.

Attempting to refit the barrel over the pistons revealed something interesting and that I had not spotted before - there is no lead-in chamfer on the bottom of these barrels making it very difficult to get the rings into the barrels. I used a half round file to make a chamfer of maybe 1 mm, much smaller than is commonly found on cylinders but it made all the difference.

The cylinder head went back on with Wellseal on both sides of the head gasket. I put the bolts in with Copperease and tightened them down as tightly as I could with a 12 in ratchet drive.

I've fitted and removed the rocker box several times now, and I know the optimum way to do it. Fit the gaskets to the head and apply your chosen sealant to both sides, then fit the rocker box from the front right direction remembering to trap the two exhaust push rods either side of the web in the middle of the inlet side.

I hope no one sees this as an act of wanton vandalism but I have removed one of the vestigial fins from below the rocker box, because they are an absolute pain. When they are in position, you have to put these two nuts on before the rockerbox sits down on its gaskets, because the shape of the casting means that you can't get the half nut on with rocker box fully down. One of these things was missing when I got the bike and I have just removed the other one.

I must remember to clean up that area next time the head is off.

With a bit more spannering, the head steadies are in place and the oil lines are connected. We will end this blog post here as this concludes the work on the replacement of the cylinder head.

Model A - painting more bits

 A rewarding but seemingly never ending job is painting bits for the Model A. The wheels are off and the brakes are currently in pieces and here we are with two brake arms and the torque arm for the front brake.

These are the two brake plates, with the paint cleaned off with a Scotchbrite wheel and coated with primer:

These bits had their second coat of black on the 31st of July 2026.

FH - carburation

 For the record, at the 500 mile service I have changed the carburetter slide on the FH for one with a #4 cutaway.

A #3 1/2 was previously fitted, and the plugs were quite dark.