6a Carburettor 

The carburettor fitted on  my engine # 2173. 

The carburettor is a top feed Brown & Barlow Plain Jet Type with a bore of .78”, a throttle valve diameter of 15/16” and a jet diameter of .029” 

48 Dust Cap  (Brown & Barlow calls this sometimes Back Cap), which controls the amount of air to be taken in, has 10 holes with a Ø of 7/32”. 

11 Jet  is fitted in 30 Spraying Chamber, which  has 2 x 3  circumferentially placed holes, with a diameter of 3/16”. The air flows through these holes, upward  around the jet, where it is mixed with the atomised petrol.
The air/petrol mixture now flows through C6  Division Plate, which  has a cup-shaped horizontal part with 9 holes, placed at an inner circle and 16 holes placed at an outer circle.  At the circumference there are 3 rows of 9 holes placed in the cylindrical part of C6 Division Plate over an area with an angle of about 160º at the engine side of the air flow. All holes are approximately .050” in diameter.

After the air/petrol mixture has flown upward through the Division Plate, it meets the horizontal air flow which has entered through 48 Dust Cap.  

The notch for guiding the throttle valve was missing and has been replaced by a 3/32” brass round headed screw. 

Summary, derived from Ner-A-car and Brown & Barlow documentation. 

The carburettor is a top feed Brown & Barlow Plain Jet Type.
Model:            Type 122-2 or T.O.D. (both are Two-Stroke Plain Jet  Type carbs.)
Bore:                          .78”
Outlet:                        1 1/8”
Jet:                 70 cc or .029” diameter
Dust Cap:      10 holes, dia 7/32”
Petrol level:   .2” (5 mm) below the top of the Jet. 

Starting: C5 Air Lever closed, C4 Throttle Lever ¼  open.
Under normal conditions, C5 Air Lever has to be fully open; the speed is controlled with C4 Throttle Lever.
At low speed, when climbing a hill or in case of popping back through the carburettor, C5 Air Lever has to be partially closed.
When the engine is “four stroking”, C5 Air Lever has to be opened more.

Selected quotes from the referenced documentation: 

Ref. 1:
Always run with the air lever as wide open as possible, without having the engine “pop back” in the carburettor. …  To correct this, shut off some of the air supply.
Ample air gives a cooler running [this seems a false statement to me (B.G.)] and a high speed with low fuel consumption.
On the other hand, to secure maximum power at low speeds ( as when finishing the climbing of a hill) retard the air and so give engine nearly pure petrol vapour.
End of Ref. #1.

 Ref. 2:
Type 122-2  .78 ”bore 1 1/8” outl, no Pilot jet, two stroke.
It will be seen from the sectional view that the Carburettor is simple in construction, and is designed to be practically a one-lever instrument under all ordinary conditions. The second lever is simply provided as a means whereby the mixture can be temporarily enriched when circumstances demand, but for all ordinary purposes it is kept in the fully open position.

 The tuning of the carburettor consists in having a Back Cap of such a size that will allow both the air and throttle valve to be open at maximum speed on the level (the Back Cap will usually be found to be correct as sent out) and fitting a jet in the carburettor of such a size that under normal conditions on the level the air valve can be kept in the fully open position, and under these conditions giving a mixture a little on the thin side. ….. To keep the same conditions in the extreme maximum summer and the maximum winter cold the jets should be changed with the seasons. 

All jets are known by their actual flow when measured by Air Board standards, and for the sake of clearness for those who are used to think of them in sized holes, the approximate equivalent sizes are given below:

The following is the average setting required for an ordinary engine under average conditions:
Two Stroke Engines 250 cc Carburettor Type 122 Back Cap 10 holes 7/32 in. Jet size 65.
End of Ref. 2.

Ref 5.:
Hints and Tips from the “Barnstormers”
The fuel level should be .2 in. (5 mm) below the top of the Jet.
End of Ref. 5:

 Ref. 4:
The “tuning” of the carburettor consists of fitting a jet of such a size that that full air can just be used with full throttle at very high speeds (not on hills) with the engine and weather warm. … The carburettor then becomes practically automatic (except at very low speeds) and the air lever is used from one-half to full open according to the temperature of the atmosphere, and will only require slightly shutting for slow running, or when slowed up on a hill. 

The control levers should be set for starting as follows: Air Lever shut. Throttle Lever ¼ open. 

The Air Lever on a two stroke machine requires a certain amount of judgement in handling as the result of its movement is not instantly indicated by the engine. There is always a certain amount of “lag” between its operation and the result.
The Model described is “The 1923 Two-Stroke Carburettor Type T.O.D.”
End of Ref: 4

My own experience:
A good practice seems to be not entirely opening the Air Valve, to prevent overheating of the engine, as a poor mixture will result in higher engine temperatures and popping back through the carburettor. In case the engine starts “four-stroking”,  the Air Lever has to be opened further.

References:

1. Ner-A-car, Motoring on two wheels and list of spare parts. Intercontinental Engineering Co. Ltd.  20 Conduit Street, London, W.1.
2. Motor Cycle Carburettors for 1924, manufactured by Brown & Barlow Ltd., Carburet Works, Witton, Birmingham.
3. B&B Motorcycle carburettors 1922, Hints & Tips to Motor Cyclists.
4. All about Two Strokes, B&B Two Stroke Booklet, 1923.
5. Barnstormers (New Zealand) http://www.barnstormers.co.nz/?p=147

 #3 and #4 can be obtained from:
Photocopy manuals and illustrated parts lists
BMS
Bruce Main-Smith & Company Ltd.
5 Lincoln Drive
Wigston
Leicestershire,
LE18 4XU, England
Tel:  +44 (0)116 277 7669
Fax: +44 (0) 116 277 7669
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