Besides carburettors for light-applications such as types 308 and 355 for
Pedal Cycle Motors, Amal historically had three popular carburettor designs: 'Standard' fitted up to 1955, 'Monobloc' fitted from 1955 and 'Concentric' fitted from 1967. The more modern "Concentric", developed and introduced for the 1967 Model Year, was initially designated 600 and 900. The largest bore at 32mm was slightly larger than any previous Monobloc, and all were flange-fitting with stud-holes at two-inch centres. In 1964 a right hand version of the 389 was introduced, designated 689, which was a mirror image of the 389. This allowed twin carb installations with each carb having its own float chamber. Production ended in 1967 when the Concentric was introduced. The Monobloc was reintroduced in 2014, the 375 body being cast in
aluminium alloy. Replacement hard
anodised slides to reduce wear were made available.
Concentric Mark 1 , with the second just visible to the rear To reduce the technical and manufacturing problems of previous carbs and with pending
emission controls requiring more accurate fuel metering, a new carburettor, the Concentric, was introduced for the 1968 model year. A hemispherical float chamber was introduced below and concentric with the main jet to eliminate differences in fuel height during cornering. There was no jet block, the jets being mounted directly into the body. The pilot jet reverted to a drilling instead of a removable jet. Right and left hand carburettors were available with flange mountings only. Three body sizes were initially produced: • 400 series - 12 - 20 mm bore diameters • 600 series - 22 - 27 mm bore diameters • 900 series - 28 - 32 mm bore diameters To satisfy the demand for larger carburettors the 1000 series was introduced in 1970 with bore sizes of 34, 36 and 38mm. The British motorcycle industry was in decline and with the lack of orders for OEM carburettors, production was limited after 1975 for replacement and aftermarket sales. Following the introduction of other models of the Concentric, this model became known as the Mark 1.
Premier An updated version of the Mark 1, the Premier, was introduced in 2012. The castings for the body and float bowl were in aluminium alloy and the slide was made of hard anodised aluminium. A new float was introduced that was resistant to the
ethanol in modern fuels. The pilot jet was removable. and the idle circuit improved.
Mark 1.5 To supply demand for OEM carburettors for Spanish made motorcycles, Amal licensed
Talleres Arreche to produce carburettors in Spain. Initially this production was of 400, 600 and 900 series Concentrics. The Spanish motorcycle manufacturers required a better cold starting system so Amal and Talleres Arreche jointly designed modifications to the concentric to provide a separate enrichment circuit. These carburettors, introduced in 1973, were available in both flange and spigot mountings to suit the Spanish manufacturers requirements. To distinguish this variant from the British made version, they became known as Mark 1.5 or Mark 1 1/2 and a 1 prefix added to the product number, for example the modified 626 became the 1626. A derivative of the 400 series is still in production by Talleres Arreche in 16, 18 and 20 mm bores with Amal embossed on the body.
Mark 2 Introduced in 1974, the Mark 2 used aluminium alloy for the main castings rather than the zinc alloy previously used. The cold starting system from the Mark 1.5 was integrated into the new design. Whilst the float chamber was still concentric with the main jet, it was now a square section. The pilot jet reverted to being removable and could be inserted in an alternate position for higher downdraught applications. Flange mounting was discontinued and the Mark 2 was only available in spigot mounting. Three body sizes were available: • 2600 series - 22, 24, 25, 26 and 27mm bore diameters • 2900 series - 28, 30, 32 and 34mm bore diameters • 2000 series - 34, 36 and 38mm bore diameters
Racing carburettors Type 27 The Type 27 was Amal's first carburettor specifically designed for racing machines, and was based on AMAC's TT carburettor that had been introduced in 1924. A jet block and hollow slide were used. The body, jet block and slide were assembled and then machined as one unit to ensure perfect matching and unobstructed flow through the carburettor. No needle or needle jets were fitted. For starting a pilot jet and bypass were used. To reduce fuel surge when cornering, two float chambers were used with the 27, one mounted on either side of the carb.
TT The TT (Series 27) was introduced in 1932 as a progression of the Type 27. A needle and needle jet were incorporated to provide additional tuning for road machines and when the carb was used with alcohol as a fuel. Pre-war models were suffixed with the year of manufacture but when production resumed after
WW2, all of the TTs had a 9 suffix regardless of year. As well as the spigot mounting as used by the Type 27, flange fixing was introduced. As the spigot mountings were susceptible to air leaks, flange mounting was seen as an improvement. Three body sizes were made: • 25TT - 3/4" and 7/8 bore diameters • 15TT - 15/16", 1" and 11/16" bore diameters • 10TT - 1 1/16", 1 3/32", 1 1/8" and 1 5/32" bore diameters Sales of the 25TT were small and the model was discontinued in 1934. Production of the other two versions continued to 1954.
RN The RN (Series 185) was introduced in 1937 as a development of the TT. The needle and jets were moved to the side of the carburettor bore to give a less obstructed flow through the carburettor, although this caused a complicated attachment of the needle to the slide. Although at the time the less restricted flow was seen as an advantage, later opinion was less positive. Initial the RN was designated as a type of TT, eg 10TT37KN, but soon it was given its own type reference, eg 10RN. It was available in the same sizes as the 10TT and 15TT. This type was less popular than its parent TT. Production ended with the introduction of the GP in 1954.
GP The GP (Series 316) was introduced in 1954 replacing both the TT and RN, and was a further development of the RN. Although the needle and jets were still offset, they were moved away from the side of the bore which simplified the needle mounting. Air for the pilot feed was taken from the outside of the carb through a drilling. The GP was available in four body sizes and in flange mounting only: • 15GP - 7/8, 15/16", 1" and 11/16" bore diameters • 10GP - 1 1/16", 1 3/32", 1 1/8", 1 5/32" and 1 3/8" bore diameters • 10GP - 1 7/32" and 1 3/8" bore diameters • 10GP - 1 3/8", 1 13/32", 1 7/16 and 1 1/2" bore diameters Initially the GP was produced in zinc alloy but in the late 1950s the material was changed to aluminium ally. The GP suffered from pilot systems, partly due to dust and dirt entering through the air drilling. The pilot system also limited the carb from being used with more than 20° downdraft otherwise flooding of the pilot circuit occurred.
GP2 The GP2 series was introduced in 1962 to try to overcome the shortcomings of the GP. The pilot circuit was moved to the inlet side if the slide. The air bleed was taken from the incoming air rather than the outside, reducing the possibility of dirt ingress to the circuit. The moving of the pilot jet allowed far greater downdraught angles without the jet flooding. The GP2 was available in the same sizes as the original GP.
Mark 2 Smoothbore Concentric A smoothbore version of the 2000 series Mark Concentric was made available for racing used. To obtain the "smoothbore", a jet block and hollow slide were fitted. Although the carb was never used as an OEM part, it was popular with smaller specialist racers. ==References==