mine does that m8, no problem.
just have a look though if the vacuum line(s) are perished and split.....
i just replaced the lines on mine cos they were a bit poor and i went down from 173 on 14 1/2L to same as you almost with a 147!
im guessing this is cos the mix may have been artificially weakened slightly & someone else said the same... but also could be because i raised the needles a clip.
still awesome economy imo but a major difference from the day before and i was on more or less the same roads at similar mixed pace.
some are obviously just more fuel efficient than others.
i ride mine high up the rpm though m8..... i think the reason some get poor economy is they are lugging the engine at low rpm in high gear. they seem to be more fuel efficient higher up (imo)