THE CARBURETTOR

by Vic Locke (published in the Isetta Gazette October 1982)

The first thing to remember is that the carburettor is of alum alloy and all the jet plugs, etc are aluminium and fine threaded so be very careful not to cross threads and beware of over tightening as aluminium threads are very soft and will only stand so much pressure. Fibre washers must be fitted to all plugs. (numbered 129, 131, 143 & 149 )

The Bing carburettor is basically the same as that fitted to old British 4 stroke motorcycles, except that there is no tickler knob on the float chamber cover. There is nothing difficult in its function.

The Isetta's Bing carburettor has two cables running into it. One operates the choke piston, whilst the other operates the throttle barrel (137), which has a tapered adjustable needle (140) attached to the bottom centre of it. This tapered needle controls the mixture strength at intermediate speeds of the engine. The best setting for this needle is the second notch from the bottom. The tapered needle is held in place by a circular plate (141) on the inside of the throttle barrel and, to change the position of the needle you will have to disconnect the throttle cable. You do this by removing the knurled ring (134) from the top of the carb and then pull the throttle barrel upwards and free of the carburettor. The throttle cable nipple slips into an adjacent larger hole in the barrel and pulls clear. All this is done against the rather strong pressure from the throttle barrel return spring (138). Once you have the throttle barrel assembly dismantled you can then change the tapered needle notch. Having positioned the needle in the circular plate, ease the needle and plate gently inside the throttle barrel, being sure that the round plate engages with the peg at the bottom of the throttle barrel. This round plate is then held in position by the spring pressure of the throttle barrel spring. Place the loose spring into the barrel to hold the circular plate in position, and then enter the nipple of the throttle cable through the spring and into the larger of the two adjacent holes. Then slide it into the smaller hole. It's a fiddley job, but you should soon get the knack of it. You must remember that the throttle barrel has a locating peg for correct location into the carb body and, if all is well, the barrel will slide down into place with the needle engaged into its brass diffuser. Then engage the knurled ring that holds the throttle assembly down and into place. There is a location peg on the top that engages with a notch in the top of the thrrotle barrel cylinder. Be sure that the knurled ring is done up tight to prevent it vibrating loose.

The most important function of the carb. is the choke assembly which is located on the side of the carb, and controlled, as I've said previously, by a cable. This unit must be checked ever so often to be sure that the small seal in the end of the plunger (145) has not disintegrated and disappeared, but is still in place, and that the cable is adjusted and that the choke control plunger is not being held off of its seating.

The seal (145) is usually made of cork (although rubber will do temporarily) and one can be fashioned easily with a pair of nail scissors and pushed into place. It is important that a seal of some sort is present, or you will have difficulty in starting the car, and a high petrol consumption.

Be sure that the gasket between the carb and the cylinder head (109) are good, and tighten the two nuts that hold the carb onto the head evenly and well. Leakage of air at this point causes the mixture to change, and the performance of the car to drop.

Flooding of the carb.can be caused by a worn float chamber needle (121) or, a punctured carb. float (120). Check both, and, if in doubt, replace.

Apart from the notch on the throttle needle, there are two other places on the carb. where adjustment can be made. The idling speed mixture strength screw is positioned horizontally in the carb. body, has a screwdriver slot in the end of it, and a locking nut. The correct slow running mixture strength is about one and a half turns, anticlockwise from being fully screwed in. The slow running speed screw is positioned at an almost vertical position, with 4 claws and a spring under it, at the side of the carb. body. You set this last to give you the slow running speed (idling) you require.

Occasionally the carb. will flood when standing with the petrol tap turned on. If the float needle is sound then this will probably have been caused by a minute scrap of dirt under the needle setting, but for any stop over five minutes it is best to turn the petrol off. Serious flooding all the time needs further investigation.

It is most important that the notch in the float chamber needle engages with the spring clip on top of the float. If the float is pushed down with the finger on the needle, which you support with another finger, then an audible click will be heard when the notch and spring clip on the float engages.

And finally the petrol filter unit (150-160).

Be sure that it is firmly bolted onto the float chamber, and vertical in position. Be sure that you have a good fibre washer each side of the securing alum plug. If you have a wet and leaking petrol filter, it is because the carb. washer seal at the top of the unit probably needs replacing. This is a cork seal, and lasts a long time. The old one will have hardened and compressed, and will need to be picked out of its seating with a large needle, and the new one placed carefully. Then replace the glass bowl and tighten carefully with the fingers. Do not use pliers, or overtighten, otherwise you'll splinter the glass bowl. Keep an eye on it for a few days as the cork washer will settle down, and another twist on the pressure screw may be necessary.

Remember, don't overtighten any screw or plug as aluminium threads are very soft.

A fast tickover speed can arise from a sticking or rusting throttle cable. I advise disconnecting all of the throttle slide assembly from the cable, slipping a tightly fitting piece of rubber tube over the engine end of the outer housing of the throttle cable, and using a pressure oil can on the other end of the rubber tubing, force oil through the entire length of the throttle cable. You can thin down engine oil with a drop of petrol or paraffin. All this is old motorcycle practice.

Be sure your air element is clean (tap the muck out every 2,000 miles) and replace every 7,500 miles.