I'm used to the carburetors on Kirby-Lauson engines having a mixture screw. With those, all you need to do is rotate the mixture screw clockwise until they aren't rich any more. I'm sure you'd have done that if yours had such a screw, so I'll summarise the problem and possible solution as if there were no mixture screw on your carburetor.
The carburetor needle-and-seat can contribute to rich running if there is leakage between the two, and the float is unable to shut off the supply of fuel to the float bowl. You can test whether this is happening very easily, just by leaving the fuel turned on for a couple of minutes when the engine is not running. If the float bowl floods and petrol drips onto the ground when you do that test, you need to check for leaky needle/seat, or sticking float pivot mechanism, or a leaky float (which no longer floats properly due to having fuel inside it). Conversely if the carburetor does not flood when you do that test, you need to look for another source of rich mixture. The obvious candidates would be choke not opening fully, obstructed air cleaner, float level set too high, main jet not seated (or fibre washer omitted from under head of flat-seating jet) so fuel can by-pass it, or oversized main jet. Oversized main jets are not all that unusual - sometimes previous tenants have drilled them out because it seemed like a good idea at the time.
Last edited by grumpy; 04/11/13 09:40 PM. Reason: Add comment on mixture screw
