Adjustable carbie mixtures are being phased out due to EPA regulations naturally effecting most engines produced today.
Fixed jet carbies also ensure users do not fiddle and diddle with the fuel /air ratio .
Some carbies are also fitted with idle restrictor caps to prevent excessive ajustment.
Many carbies have neither idle or main mixture adjustment.
Without a mixture adjustment screw the condition of the air filter is more important because operators can no longer adjust the carbie to compensate dirty or blocked filter .