NB: These instructions are given for guidance only and cannot engage the author's liability in the event of misinterpretation, failure to assess the vehicle's actual condition and/or non-conformity, unsuitable mechanical practice or the use of inappropriate tools.
The mixture screws of the Dell'Orto VHB and PHF carburettors comprise a spring / washer / O-ring set.
Following numerous assembly errors, note that the O-ring goes at the very end of the stack of parts, i.e. between the washer and the carburettor body, thus ensuring optimal sealing and preventing air leaks at this point.