Overhauling a Zenith carburettor hot-Air Choke

Zenith Carburettor Service guide for hot air top choke see other guides for different types

The spring that controls the choke flap is operated by air from the exhaust hot-box unit. First remove the carburettor to allow access to parts if space is tight insuring fuel pipes are disconnected safely.

Allow it to cool down if it has been used before dismantling the choke.

1. Clean the choke unit and check that the white line on the serrated cover coincides with the casting mark on the choke body. If no marks can be seen, scribe a line across both surfaces to establish their positions with marker that will not be removed by fuel or cleaning.

2. Hold one spanner on the centre nut of the assembly and, using another spanner, carefully undo the union nut securing the hot-air pipe. Remove the screws that hold the clamp plate and lift out the thermostat cover, heat mass and coil spring. Remove the heat insulating plate.

3. Check the fast-idle cam and choke flap spindle for free operation. If they are tight, clean them with a lint-free cloth and lubricate them first.

4. Check the operation of the piston in the choke housing. It should move freely inside its cylinder. If it is tight, remove and clean it. It should be level with the top of the cylinder on 42 WIAT carburettors, and •040 in. below the top of the piston on 42 V NT carburettors.

5. Thoroughly clean the inside of the choke housing with methylated spirit or clean petrol insure all is cold and away from heat. Remove any dirt or carbon deposits found. Dry the housing and oil the piston lightly.

6. Some automatic chokes have a diaphragm-mounted unit attached to the side of the choke housing. Undo the three screws holding the diaphragm cover and remove the diaphragm from the housing. this may be worn so with care stretch the diaphragm to see if it is holed or damaged or hardend from age. If the diaphragm is faulty, fit a new one kits may be diffrent so insure this is corect befor purchase .

7. Check the choke spindle for play by moving in all directions there should be no play. If it is worn, remove the choke flap and fit a new spindle. Make sure the choke linkage is refitted the correct way and clean all parts.

8. Reassemble the unit. Fit the heat insulating plate and the coil spring. Make sure that the spring engages the operating peg on the mechanism inside the choke housing. Fit the heat mass and thermostat cover. Replace the retaining plate. Make sure that the marks on the plate and choke housing line up. Do up the securing screws and refit the pipe from the exhaust hot-box. Make sure that all the carburettor linkage moves freely and no lose or worn parts of fuel lines before starting the engine.