( MK1 & MK2 parts at bottom of page )
|GP 1 Guides
||GP 1 Tuning
|GP1 Parts Catalog
||GP1 Tuning Page 1
|The GP Carb ( Some Features )
||GP1 Tuning Page 2
|GP1 Float Illustration (2.5mb)
||GP1 Tuning Page 3
|GP1 Float Spare Parts (1.0mb)
||GP1 Tuning Page 4
|GP1 Mixing Chamber Illustration
||GP1 Tuning Page 5
|GP1 Parts List w/302 Chamber
||GP1 Tuning Page 6
|GP1 Parts Specification Page 1
|GP1 Parts Specification Page 2
|GP 2 Guides
||GP 2 Spare Parts
|GP2 Carburetor Guide Page 1
||GP2 Spare Parts Page 1
|GP2 Carburetor Guide Page 2
||GP2 Spare Parts Page 2
|GP2 Carburetor Guide Page 3
||GP2 Spare Parts Page 3 ( 1.8mb )
|GP2 Carburetor Guide Page 4
||GP2 Spare Parts Page 4
|GP2 Carburetor Guide Page 5
The company and trademark "AMAL" was formed from the 'amal'gamation of two or three other organisations (notably Brown & Barlow, Amac?) in the years immediately following the First World War in order to manufacture and market carburetters and associated products.
In the 1960s AMAL became part of the IMI group of industries before being sold in 1993 to Grosvenor Works Ltd in North London: a family run business specialising in supplying components to various fuel systems companies. Grosvenor then began revitalising the AMAL product range by commencing a programme of re-introducing some of the more popular ranges of obsolete product.
In 2003 the business was sold yet again, this time to the current manufacturers, Burlen Fuel Systems Ltd. BFS, who also produce both SU, Solex and Zenith products over recent years gained a high reputation for continuing to produce carburetters and spares keeping a vast range of British classic vehicles alive.
Burlen Fuel Systems have continued to invest in AMAL product by further increasing the range of popular obsolete carburetters back into production.
The 'AMAL' and 'AMAC' trademarks have been in constant use since their initial conception and now cover a range of products covering carburetters (principally, but not exclusively, for motorcycle engines), controls (brake and clutch levers, cables, etc.), fuel lift pumps, gas jets and burner devices as well as gas safety valves.
AMAL component and assembly part numbering sequence usually enables the end product type and design age to be determined, i.e. the earliest of the carburetter type discussed below is the 4, 5 & 6 series; these are generally assembled from piece parts that have the same initial number (for example 4/035 where 4 denotes the carburetter for which the part was initially designed / used and the 4/035 designates the part itself.)
In principle, this numbering system has been used for all AMAL product and piece parts over the years.
Pre Monobloc Carburettors
76 & 276 Series Carburettor Types ("AMAL Standard" series) can be spigot or flange fixing with adjustment screws on the left or right hand sides of the carburetter body. The available bore sizes are 15/16", 1" and 1 1/16" diameters.
The 276 type carburetter is, in virtually all aspects, identical to the 76 version, except that the main emulsion air is drawn from inside the air intake (air filter) whilst the 76 carburetter uses unfiltered air. The 76 types are of diecast zinc construction and were introduced in the early 1930s, replacing the earlier 6 series, which were of bronze construction, although basically the same design.
The 276 type was introduced in 1940 in an attempt to improve the carburetter's durability in dusty and dirty operating environments. In the same design range were the 4 and 5 types with smaller bore sizes and the 29 type with larger bore sizes. These were replaced with the 74, 75 and 89 and then later, by the 274, 275 and 289 carburetter types.
The float chambers for all early carburettors (including these types) were separate assemblies from the carburetters and were generally attached to the carburettors by connections made under the main jet.
Depending upon the physical size and arrangement, these separate float chambers were designated types 1, 2, 3 & 4 in the main carburetter specification numbering and were assembled from components generally in the 64/ component part number sequence.
This high performance carburettor was developed during the 1940s from the earlier pre war type 27. The mixing chamber body is either of the spigot or flange type mounting. The flange type has two options of a long or short delivery choke. The bore sizes can vary between 1 3/32" and 32mm.
The throttle slide, with its centrally mounted needle and jet arrangement, was later repeated on the later designs of concentric smoothbore carburettors.
The enrichment air slide is mounted separately to the throttle slide bore and provides a simple enrichment for cold starting. The unit can be used in conjunction with either a separately mounted float chamber or with a rigid float chamber of either top or bottom feed.
The flange type carburetter has a recently been re-introduced by Burlen Fuel Systems Ltd in both the short and long delivery choke.
Development is currently under way to produce a replacement rigid float chamber utilising components from the 276 series.
Remote mounting float bowl applications can be satisfied using the 510 ("Matchbox") float chamber.
Current production is based on the T3GP model which was made in 1 3/8" and 1½" bores sizes originally designed in the very early 1950s; these are the only sizes produced now.
The original range covered sizes from 1" up to 1½" bores in various body dimensions. The GP design was the last in a series designed specifically for racing engines that started with the type 27 in the 1930s; was developed into the TT type with an enrichment (or emulsion) air slide located outside of the throttle slide bore; and then through to the RN (remote needle) type, where the throttle needle itself was located outside of the throttle bore.
This feature of remote needle was then carried through to the GP design. After the GP design was reintroduced into production by the AMAL Carburetter Company, it was a further development of the range, which became known as the GP2 type, where the pilot circuit air supply was arranged to come from filtered air.
The majority of the carburetters produced were of the flange fixing type, although spigot mounted and remote mounted float chambers were all utilised, and currently both the remote mounted ("matchbox") type 510, and the more traditional and widely recognised round Type 504 remote float chamber are in production in the UK.
This range (generally known as the "Monobloc" type), was produced from 1954 in a range of sizes matching those of the AMAL "Standard" range in body types designated 363, 374, 375, 376 and 389. This carburetter type replaced the "Standard" series with the design intention of lowering production cost / complexity and reducing the major fault of the "Standard" series which was fuel surge due to the remote location of the float chamber.
The monobloc (one piece, hence the name) design incorporates the float chamber into the carburetter body casting, although it is still located to one side of the main fuel metering circuitry.
The bore sizes currently manufactured are 15/16", 1" and 1 1/16" diameter for the 376 body type and 1 1/8", 1 5/32" and 1 3/16" for the 389 type. The range originally supplied included a 389 in 1 5/32", and the 375 in 21/32", 23/32", 25/32", 13/16" and 7/8".
Mk1 Concentric Carburettors
These were developed with the float bowl arranged centrally around (concentric with, hence the name) the main jet in order to remove the sensitivity to fuel surge inherent an all the earlier designs. They were introduced into production in 1967 in two body sizes (600 series and 900 series) covering bore sizes from 22 mm through to 32 mm.
The design catered for both left hand and right hand positioning of adjuster screws, but only flange fixing to the engine. They still utilised the cold starting mechanisms of the "Standard" and "Monobloc" ranges, i.e. enrichment by float bowl flooding (tickler) as well as enrichment by restriction of intake air (air valve within the throttle slide bore).
With its compact design, the "Concentric" design was the simplest manufacturing arrangement of the AMAL range and all the earlier "Standard" and "Monobloc" specifications were replaced by "Concentric" applications, although the timing of its introduction of the product range was such that only two or three years after its introduction the bulk of UK motorcycling manufacturing had disappeared.
It therefore did not have the very large OE volume demands of the carburetter types it replaced. The more common sizes were the 600 in 22mm, 24mm, 26mm and 27mm, and the 900 in 28mm, 30mm and 32mm.
Mk11/2 Concentric Carburettors
Mark 1 1/2 Concentric Series Carburettor Types - were virtually identical to the Mark 1 series, except in that the tickler and air slide cold start enrichments were replaced by a cold start system having its own fuel and air supplies, separate from those of the main carburetter.
Body and bore sizes are as for the Mark 1 range. The main market for the product range was on Spanish motorcycles, although a few were used on very late model Triumph engines.
Mk2 Concentric Carburettors
Introduced into production in 1974 and designed to replace the Mark 1Â½ range, these carburettor types are constructed of aluminium, rather than zinc alloy, diecastings.
They incorporate the design features of the Mark 1Â½ regarding cold starting circuitry, but otherwise have circuitry that is similar to earlier products.
The range is manufactured in three body sizes (2000, 2600 & 2900) and within these body types, bore sizes from 22 mm to 40 mm. Within the 2000 type body types, there is a "smoothbore" (i.e. similar arrangement to the 276, 376 and GP) arrangement with a "jet block" allowing a smooth airflow through the carburetter. The smoothbore arrangement is popular for racing applications.
AMAL Carburetters, Spares and Controls
Pilot circuit - for idle and off-idling fuel control.
The adjustment of fuel flow is by means of variation in the amount of air bleeding through that is allowed into the circuitry; this in turn then allows variation in the metering signal (vacuum) that is applied to the fuel jets.
There are usually two outlets into the main carburetter bore form the pilot circuit; one cited downstream (engine side) of the throttle valve and one sited just upstream of the downstream edge of the throttle valve, such that when the throttle valve is in an 'idling' position, the downstream outlet hole (pilot outlet hole) is subject to inlet manifold vacuum and therefore feeds fuel and air into the main air stream.
In this throttle position, the upstream outlet hole (bypass drilling) acts as an air bleed into the pilot circuitry and its effect is dependent upon the size of the bypass drilling and the pressure drop across the throttle edge.
When the throttle is opened marginally, the vacuum acting across the face of the bypass outlet hole increases and gradually the bypass flow will change from bleeding air into the pilot circuit to feeding fuel into the air stream additionally to that from the pilot hole.
As the throttle valve is further opened, more air flows into the engine, the inlet manifold vacuum falls (inlet manifold pressure increase towards atmospheric) and the flow for the pilot circuitry reduces.
Throttle Slide Cutaway
The forward facing (air inlet edge) edge of the throttle valve is cutaway at an angle and the amount of this cutaway influences the vacuum acting on the main circuit spray tube (i.e. the main circuit outlet into the main air stream).
On most of the more modern AMAL carburetters, the amount of throttle valve cutaway is designated in 1/16", whilst in the older products (GP, 276 and earlier) it is designated in 1/32" amounts.
The amount of throttle valve cutaway has a great influence on fuelling in the area of initial main circuitry fuel feeding (i.e. off-idle) and up to around 1/4 to 1/2 throttle opening.
Needle Jet and Throttle Needle Combination
The needle jet is located at the outlet of the main circuitry, at the point of discharge into the main air stream. The throttle needle is attached to the throttle slide and moves up and down within the needle jet orifice in accord with movement of the slide.
The throttle needle has a parallel section at its lower end (main jet direction). The parallel section is within the metering section of the needle jet under idle and off-idle running conditions and the tapering section moves into the needle jet orifice as the throttle opens beyond the "off-idling" running.
This taper within a fixed diameter orifice provides a variable section orifice, which in turn gives rise to a varying vacuum onto the main jet itself.
At full throttle operation, the needle is out of (or almost out of) the needle jet and there is no restriction to flow from the main jet and into the air stream.
The needle has a number of locating grooves at the throttle valve attachment end, which permit the user to adjust the needle "richer" and "weaker" than the mid position.
The needle jet design is different for 2 stroke or 4 stroke due to their different fuelling requirements. A 2 stroke needle jet has its metering section at the upper (discharge) end, whilst in a 4 stroke jet, the metering section is at the lower (main jet) end.
Most 4 stroke needle jets also have emulsion air drillings located in the needle jet above the metering section.
The same design of main jet is used on all of the current carburetter ranges, excluding the 276 and 400 series. The main jet determines the maximum air flow amount of fuel that can be drawn into the engine and is therefore selected early in the carburetter tuning sequence to meet the engine's maximum demand.
Amal Mk2 Parts
Contact me for part prices
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