Here are two plugs, the one showing a tan colour around the electrode is from the front cylinder and the one showing a black sooty colour is from the rear cylinder.  The book states to screw the rear mixture screw out three turns, then screw it in until the motor starts to slow and miss.  At that point it states to screw it out a quarter turn.  My mixture screw seems to go almost all the way in before any noticeable drop in rpm results.  The high speed circuit is fine, it’s just the low idle speed circuit I’m concerned about, as like this, it eventually soots up the plugs and causes problems on the low and high side.  Has anyone got any ideas.  Thanks.  The plug type is NGK B5HS.

Ok thanks ...I have tightened the nipple nuts and also removed the carb and submersed it in an ultrasonic cleaner.  I also ensured the float level was set to a 1/4 inch below the bowl lip.  I did find some gumming/corrosion on the needle and seat, which was cleaned by the ultrasonic cleaner.  On refitting the carb, I now have even mixtures in both cylinders, but both plugs are very ‘sooty’.  On adjusting low idle mixture screw, it screws all the way in (clockwise), with no decrease in engine revs or splutter.  Any suggestions welcome.  I also notice that the temp of the front cylinder/head, is much hotter than the rear, with forced draught ventilation provided at idle.  I am running a M741-1 bowl on my carb, which has 1/8 inch clearance from the front and rear barrels.  Maybe this is too close, where the fuel is boiling and offsetting the vaporisation of the low speed circuit.

Once plugs are soot  Starting will be more difficult.