I have a similar problem with my own set up..i get about 8 miles per litre on a reed valved muggello 225 with a 30mm,i did expect a bit more mpg but it certainly isnt as bad as yours with such a small carb...
firstly id post the jetting,you would think if its massivly over jetted it wouldnt run,im not familiar with a phbl24...altho there will be a few phbl25 set ups you can compare it with in the carb section.
i have seen in the past of a friends scooter at the dyno a worn needle which causes this problem.
How old is the carb?..if you take the filter off is there much spitback?
Im not sure running it dry on one litre is a good way to test mpg...fill your tank to the brim..note the milage....next time you fill up to the brim subtract the milage from your previous fill up and divide it by the amount you have put in..thats how i do mine...youll get a more realistic result over different driving conditions.
I get around 10 miles per litre std (stage 4?) Indian GP 200 Jetex carb 123 main jet. I really expected a lot more. works out at 45 mpg.
I can get more if the speeds are kept below 40 mph on a run.
