150km with 24l of fuel thats horrible for any A series engine i would think the carby is jetted extremely rich.

i have a weber 34 adm and get more than 200km from 18 litres.

as for your other problem of stalling it seems that your float level could be to high especially if you start it and you have to keep the revs up a bit for few seconds because its flooded.

if the floats high and you take it on a hill you may see the idle drop and engine sputter.

but you said it was after putting in 95 octane but worked before so maybe totally different problem.

thanks