If the slow running mixture wont 'lean off' as the air screw is wound out, it would suggest that the pilot air drilling in the main body is blocked. If this is the case, try completely unscrewing the air screw. As the screw comes out, it will provide a 2nd supply of air to the jet, leaning off the mixture, causing the engine speed to momentarily pick up, and then die as the mixture becomes excessively weak. If you get these symptoms, you will need to check that the pilot air drillings are clear of any obstructions - use small drills carefully to remove any blockage - do not rely on just poking out any debris with a pin - if the blockage is caused by the sticky goo left behind by modern fuel, the pin will push it out and probably draw it back in as you remove the pin! Another thing to check is that you have the correct hand choke block fitted - although they are rare, Monoblocs were available as LH units - in which case the drillings are reversed in the choke block.
You say you have fitted a new Viton needle and washer under the valve body - there should NOT be a washer under the valve body - it screws down direct onto the top of the float chamber! Because of the float geometry, the washer will raise the fuel level considerably. Also, the slow running adjustment is an air screw, so winding it in will richen the mixture - the standard way of getting an initial setting is to wind the screw fully in, then back it off 1½ - 2 turns. One further point - the slide cutaway should not affect the actual slow running - the cutaway should only affect the pickup as you open the throttle.
Hope some of this helps,
Regards,
Eddie.
