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Parts to tune up with

 (a), This fig. 4 is two diagrammatic sections of the carburetter to show :-
1. The throttle adjusting screw
2. The pilot air adjusting screw

Set this screw to hold the throttle open sufficiently to keep the engine running when the twist grip is shut off.

This screw regulates the strength of the mixture for idling and for the initial opening of the throttle. The screw controls the depression on the pilot petrol jet by metering the amount of air that mixes with the petrol.

(d), MAIN JET. The main jet controls the petrol supply when the throttle is more than three-quarters open, but at smaller throttle openings although the supply of fuel goes through the main jet, the amount is diminished by the metering effect of the needle in the needle jet.

Each jet is calibrated and numbered so that its exact discharge is known and two jets of the same number are alike. NEVER REAMER A JET OUT, GET ANOTHER OF THE RIGHT SIZE. The bigger the number the bigger the jet. To remove the main jet unscrew the main jet cover, the exposed main jet can then be unscrewed from the jet holder.

(e), NEEDLE AND NEEDLE JET. The needle is attached to the throttle and being a taper, either allows more or less petrol to pass through the needle jet as the throttle is opened or closed throughout the range, except when idling or nearly full throttle.
The taper needle position in relation to the throttle opening, can be set according to the mixture required, by fixing it to the throttle with the needle clip spring in a certain groove (see above illustration), thus either raising or lowering it. Raising the needle richens the mixture and lowering it weakens the mixture at throttles openings from quarter to three-quarters open (see illustration sheet 7 ).
The needles are marked with the letters: B type are fitted in the 375 carburetter
  C type are fitted in the 376 carburetter
  D type are fitted in the 389 carburetter

(f), THROTTLE VALVE CUTAWAY. The atmospheric side of the throttle is cut away to influence the depression on the main fuel supply and thus gives a means of tuning between the pilot and needle jet range of throttle opening. The amount of cutaway is recorded by a number marked on the throttle, eg: 376/3 means throttle type 376 with number 3 cutaway; Larger cutaways, say 4 and 5, give weaker mixtures, and 2 would give a richer mixture.

(g), AIR VALVE, is only used for starting and running when cold, and for experimenting with, otherwise run with it wide open.

(h), TICKLER, a small plunger spring loaded in the float chamber wall. When pressed down on the float the needle valve is pushed off its seat and so "flooding" is achieved. Flooding temporarily enriches the mixture until the level of the petrol subsides to normal.

Opening times: Monday - Friday 9am - 6pm | Closed: Saturday and Sunday
Units 7 & 8, Rosemary Court, Oldwich Lane West, Chadwick End, Solihull, B93 0EY, UK

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