REPAIR AND SERVICING TIPS

1 Accelerator pump

The fabric-rubber diaphragm can degrade. There might also be a steel or glass ball functioning as a one-way valve in a connecting passageway. This may have escaped during careless dismantling.

2 Worn needle valve

Fuel pump pressure remains for some time after the engine’s stopped, so a leaking valve will cause the carb to slowly flood. Viton rubber types are an excellent upgrade. If you can’t find a new valve, it’s possible to recondition a worn one. Hold it in an electric drill, then use a fine file followed by very fine wet-and-dry paper to turn down the tip.

3 Throttle spindle

Look for play in the throttle spindle(s). If it’s severe, it introduces air leaks that lead to poor idling, plus hesitations and flat spots when setting off from rest. Repair bushes can be fitted by a specialist.

4 Other faults

If you can’t find anything wrong, look for fundamental faults. Are all the lead seals still intact in drillings? Some carburettors are just wrong ‘uns, with porous castings and manufacturing errors.

5 Setting up

Setting up a fixed jet carb is easy. Make sure the float level is correct. Turn the idle mixture screw until the highest idle speed is attained, then adjust the throttle stop screw to the desired rpm.