As mentioned on another page, my bike had the wrong carburettor fitted which caused problems. At the time, I didn’t have the correct one, and didn’t feel inclined to spend another £300 plus on a new one just to test a theory. As luck would have it, a very new looking Amal Concentric R300/930 turned up at a VMCC club night auction and I picked it up for £20. Note that the 930 has a 30mm (1 3/16″) bore as per the 389/27 Monobloc, whereas the 932 mentioned later has a 32mm (about 1 1/4″) bore.

AMAL Concentric Carburettor

When I got it home, I noticed that both the float bowl and body had faint ‘X’s on them; I suspected they were problem parts which proved to be true. The float bowl had been over-tightened so I had to lap the mating surface with an oil stone. The body had a very slightly oval slide aperture…I suspect someone had lightly clamped the body in a vice. That was resolved by careful internal polishing and very fine wet and dry paper. I took the opportunity to obtain the recommended jets and other parts for the carburettor from Burlen / Amal.

Once fitted and correctly adjusted, the bike was transformed and is now much more enjoyable to ride, although I do feel that the main jet may be a bit too large.

The carburettor settings for the G80CS over the years, are in Roy Bacon’s excellent restoration book. Most had the Monobloc fitted, but the very last bikes made, were fitted with Concentrics. Here are the factory settings suitable for the 1956 on bikes, according to Roy Bacon, and assuming a GP isn’t fitted:

Monobloc Carburettor

Type 389/27
main jet 440
needle jet 106
pilot jet 30
Slide no 3 cutaway

Concentric Carburettor

Carb 932/7 (as per late G85CS)
main jet 270
needle jet 107
pilot jet 30
slide 928 3 cutaway
needle position 2 from top
bore 32mm

Mine has an R300/930 which I believe has a bore diameter match for the inlet spacer on my cylinder head. Its settings are as follows:

main jet 350 [I think it may be too big]
needle jet 106 [107 might be something to try]
pilot jet 30
slide 928 a 2 1/2 was needed to control surging
needle position middle notch
bore 30mm

Spacers and Manifolds

Here is a quick compilation of the spacers and manifolds used by AMC in their HW singles over the years:

015875   Spacer, tufnol  1-1/8″ x 3/4″
021002   Spacer. tufnol  1-3/16″ x 3/4″
021252   Spacer, tufnol  1-5/32 x 3/4″
022103   Spacer, tufnol  1-1/16″ x 3/4″
023428   Spacer, tufnol  1-3/16″ x 1/4″ (multiple?)
000581   Gasket for carb spacer xxx (all except GP, 1 x)
024532   Manifold, aluminum  1-3/8″ x 2-1/8″ (?) (GP)
023717   Gasket for manifold (GP) (2x)
032016   Spacer, tufnol  32mm x 16 mm (?) (Concentric 932)
032026  Gasket for carb spacer 032016 (1x ) (?)

From a groups.io post:

For the late-type carb spacer used with the AMAL Concentric 932/7 on the 1968–69 Matchless G80CS, the correct details are:

  • Part Number: 032016
    Application: Late G80CS with AMAL 932/7 Concentric
  • Material: Phenolic (heat-resistant Bakelite)
  • Thickness: Typically 5/8″ (16 mm), though some variants were slightly thinner
  • Bolt Centers: Standard AMAL flange—2″ (50.8 mm)
  • Bore: Sized for 32 mm carb throat

This part was used to insulate the carb from engine heat and ensure proper alignment with the intake stub. It’s often paired with two gaskets (head side and carb side) and longer studs to accommodate the spacer’s depth.

The late-model Matchless G80CS with the AMAL 932/7 Concentric carburettor was paired with a 1-3/8″ (35 mm) intake duct..

Spacer Implications:

The phenolic spacer (part no. 03-2016) used in this setup was internally tapered—from the carb flange bore (32 mm) to match the 1-3/8″ (35 mm) intake stub.

This taper ensured a smooth transition and minimized turbulence or fuel dropout at the interface.

Flat-bore spacers would have created a step, disrupting flow, so the factory spacer was precision-matched to the intake geometry.

The information has not been verified. One contributor posted: a spacer thickness of 5/8″ is too thin for a tapered duct. A manifold of 30-32 mm would be preferred.