Weber carb problem

Wonder if any Weber carb expert can help with this little problem. Rough tickover (carbs are balanced ok) and slight stumble off idle but rest of the performance fine. Three of the idle mixture screws (no's one, two and four from the front) are effective and adjusting them makes a difference. Idle screw number three seems to have no effect. Whether it is screwed right in or right out it does not effect engine speed. Could it be that that idle circuit is blocked? Any ideas would be welcome.

Yes, no. 3 idle jet and/or main jet can be blocked. Try to take HT lead to no. 3 plug of and see if this makes a difference.
Next, take out all plugs and see the colour of the tip, if all carbs are ajusted correctly they are all light brownish.
Erik

Are you sure the plug is firing? Try sticking a colourtune on that cylinder and see if you can see the spark. If you can, that suggests the ignition is o.k. and it's got to be the carbs. Idle circuit or idle jet, as Erik says. Try switching the idle jet + holder over with another cylinder and see if the problem moves.

will

All the above are good suggestions. To check plug is sparking just remove and put an old plug ‘in plug hole’ (I just had to type that) and lay original on top of engine still connected to HT lead. Wedge if necessary to make good contact with threaded part of plug. Start engine and you should see if it sparks. If yes, plug, HT lead ok. If no, try another spark plug, lead etc.

To check idle jet, when engine is COLD. Remove all HT leads of plugs DO NOT TOUCH THE THROTTAL, crank engine several times. Remove spark plugs. If plugs are wet, petrol is coming through idle jet. If plug is dry then idle jet is blocked.
Both simple ways to test.