Carburettor

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The original Amal carburettors had brass bodies and separate float chambers, and were known as types 4, 5, 6 and 29. They were fitted from 1929 until 1933 when the body material was changed to zincdie cast alloy. These were known as types 74, 75, 76, and 89, and can be distinguished from the later types because these have four air holes around the base of the body which the next type do not. In 1939 types 274, 275, 276 and 289 were introduced; these have two internal primary air passages in the base of the jet block. A later version with an ‘R’ suffix denotes that the primary air passage hole was moved to the air intake.

1955 saw a completely new carburettor being manufactured, which had the float chamber, cast in one piece with the mixing chamber and called the ‘Monobloc’, these are types 375, 376, and 389. This was fitted to the Colt on and after LA 966.

Each type number mentioned above indicates a different size of casting. Each casting was to any one of several bore sizes. Therefor when searching for a replacement carburettor, the bore or choke sizeis needed in addition to the type. The suffix shown after the type on the flange indicates which jets, float chamber and to which machine it was originally fitted, it is irrelevant providing the body is of the correct size and type and the correct jets etc are used.

Poor starting and erratic tickover is usually caused by wear between the throttle valve and the body. Fitting a new slide in a worn body is like fitting a new chain to old sprockets, a waste of time. Wear is likely to be apparent as early as 5,000 miles, but can be rectified by boring the body oversize and fitting a sleeve to the slide. The materials used in this process will last a lot longer than the original. This is preferable to fitting a non-original later type of concentric carburettor. The new Monobloc carburettors that are now available have improved materials. The prices for re-sleeving are given in the price list by RES170 for the type 6 and RES169 for the Monobloc.

 

Carburettor Settings List

A standard needle jet is used on the type 6 models unless a size is given. Standard needle jet on the Monobloc is .106. Standard needle jets are not usually marked. The needle position is the number of grooves from the top.

 

This was fitted to the Colt on and after LA 966.

Colt carburettor

Years

Carb. Type

Bore

Main Jet

Slide

Needle Position

Needle Jet

Pilot Jet

1954-55 up to LA 966

275BU/1EM

13/16

75

3

3

.106

1955-56 after LA 965

375/5 Monobloc

13/16

110

3 ½

2

.105

25

275 Type 6 Carburettor 1954

Part No.

Description

4/031

Mixing chamber top ring

4/032

Mixing chamber top, 2 adjuster type

4/035

Cable adjuster (lock nut is 5/077)

4/040

Jet block washer

4/042/75

Main jet

4/043

Float chamber holding bolt

4/053

Float chamber holding bolt washer, 2 per

4/060

Split pin to stop cable falling out

4/061/106

Needle jet, standard .106

4/062

Union nut

4/063

Throttle stop screw

4/137A

Blanking plug on top of mixing chamber

4/148

Air screw spring

4/230

Needle clip

4/235

Cap retaining spring clip, for top with adjuster

4/241

Cap retaining spring clip screw

4/263

Throttle spring

5/052/3

Throttle slide, No. 3 cutaway

5/065

Taper needle

13/129

Pilot air screw

14/015

Float

14/021

Float chamber lid lock bolt

14/024

Float needle

14/032

Tickler spring

14/036

Union nut, holds banjo to base of float chamber

14/037

Union nut washer, 2 per

14/208

Float chamber lid with fittings

14/209

Tickler button

14/210

Tickler bush

14/289

Tickler split pin

14/301

Tickler complete

14/345

Fuel banjo

16/010

Idle screw lock nut for 4/063

23/110

Float chamber blanking screw washer

64/353

Float chamber blanking screw
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