Question 4
- The sketch in fig. 3 shows a strangler valve of a simple
carburetor.
- Name the parts labelled E, F, G and H.
- Explain how the opening of the part labelled H is controlled.
- Explain how depression takes place in the air tube.
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- What material is used to construct an air-cooled engine block?
- State one reason why the material named in 4(b)(i) is used to construct the air-cooled engine block.
Observation
The attempt on the question was good; however, the respondents could not give adequate responses to 4(a)(ii) and 4(a)(iii). The solution to the poorly answered question is given below.
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How the Opening of the Part Labelled H (Disc Valve) is Controlled
As the engine warms up after starting, the air speed is increased causing the strangler valve to open progressively against its spring.
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How Depression Takes Place in the Air Tube
The piston moves, as produced by the crankshaft, together with the closing of the air tube, and then results in the development of a depression in the air tube.