Although many people consider carburettors to be simpler than fuel injection, a lot of things have to operate correctly and in sequence for a carburetted vehicle to even run, let alone run well. When fuel enters a carburettor, it flows into a small cavity called a float chamber or bowl. The flow of fuel is controlled by a valve known as a needle and seat, which is attached to a float. As gas rises or falls in the float chamber, the float moves with it, opening and closing this valve. A number of things can go wrong with the process, causing all sorts of problems, including poor idle, stalling or not starting at all.
Remove the top of the carburettor. Keep track of all screws and their positions. Depending on type, the float or floats will either come off with the top and dangle, or are mounted in the carburettor body. Carefully turn the carburettor top upside down on the top-mounted variety so as not to damage the delicate float mechanisms.
Examine the floats for any obvious damage and the float chambers for signs that the floats have been rubbing on the sides. Check the float operation by gently lifting with a finger to make sure nothing sticks or binds.
Remove the pins that hold the floats. Carefully remove the floats. The needle valves will usually come out as well, so check them for wear at the tips, then be sure to slip them off the float tangs and put them back into the seats for safekeeping.
Hold a float next to your ear and shake it. If you hear gas inside, the float has an obvious leak. Reveal less-obvious leaks by grasping the float by the tang with a pair of pliers and submerging it into a pan of very hot water. A stream of bubbles will also indicate a leak. Leaking and gas-saturated floats must be replaced or repaired.
Install a new carburettor top gasket. Reinstall the floats, making sure you engage the wire hangers on the needle valves. Check the float action again to make sure they move freely.
Consult the specifications for your carburettor type. Check and adjust the float height using the adjustment tool or a small steel rule. Reassemble the carburettor and check for proper operation.
Brass floats can be repaired with a low wattage soldering iron. Plastic floats are less expensive to replace, but can also be repaired using a fuel resistant glue. Some floats are made of a foam material, which doesn't leak, but will absorb gas over time. If you suspect a foam float is saturated, replace it.