Turn the cutaway model around, let fuel run into the float chamber, open the throttle and watch what happens inside. Then switch on a fault and see why an engine runs badly with it.
The float rides on the fuel. When the level is right, it presses the float needle shut.
Most carburettors want about 0,25 bar. Above that, the pump pushes the float needle off its seat.
A running engine draws in air. In the narrowing (venturi) a vacuum forms that draws fuel along with it.
The choke flap restricts the air. Less blue air with the same fuel gives a richer mixture.
Screw in is leaner, screw out is richer. Only noticeable with the throttle (almost) closed.
Switch one on and try at idle and on throttle to see what changes.
The round float chamber sits beside the carburettor. A hose underneath feeds the jet, so the level in the jet is the same as in the float chamber.
Most carburettors want about 0,25 bar. Above that, the pump pushes the float needle off its seat.
The vacuum behind the piston lifts it. The more air the engine asks for, the higher the piston and the further the needle comes out of the jet.
An SU does not restrict the air. The choke pulls the jet down, so more fuel can pass around the needle.
Nut up is leaner, nut down is richer. You notice it mostly at idle.
Switch one on and try at idle and on throttle to see what changes.
The float bowl hangs under the carburettor. Fuel runs from the tank by gravity, so there is a fuel tap here and no pump. You are looking into the front one of four carburettors. The other three work in exactly the same way. Link pipes pass the fuel from float bowl to float bowl.
Your twist grip only works the throttle butterfly. Through the diaphragm, the vacuum lifts the slide, and the needle rises with it. Each body has its own throttle spindle. Between the bodies are link arms with a synchronising screw.
The idle jet and this screw set the mixture with the throttle closed. The cold start is not a flap but a plunger that opens its own passage.
Switch one on and try at idle and on throttle to see what changes.
You see the same Weber DCOE twice. On the left the complete carburettor, cut away at one barrel. On the right cut deeper, through the middle, so you look into the float chamber. It sits in the body itself, between the two barrels.
The twin brass float hangs by the float pin from two posts under the cover. With the needle just closed it sits 7,5 mm below the gasket. If the chamber runs empty, it drops to 15 mm and stays there.
The venturi sets the air speed in the bore. Each barrel has its own jets, but the throttle butterflies sit on one spindle and so always open together. Snap the throttle open and the accelerator pump gives an extra shot of fuel.
The cold start is a separate starting circuit. It does not restrict the air, but supplies an extra rich mixture through its own passage behind the throttle butterfly.
The idle jet looks after idle and the transition through the progression holes just above the throttle butterfly.
Switch one on and try at idle and on throttle to see what changes.
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