exactly that...the carbs have a 9mm venturi diameter..the inserts are usually 8mm (standard and usually fitted in the carb at the factory) if you ahve 4 more I would guess you have 7.5mm,7mm,6.5mmand 6mm..so smaller is less power but longer runtimes...

the following is how i do my setups..feel free to disagree everyone)

to decide which is best for you try the following setup..make sure you or preferably your pitman has a stopwatch.fit the 8mm insert..warm the engine up properly then fill the tank and race around your track with the stopwatch running till you run out of fuel.(this is assuming you have already set up your mixtures properly)..OK check time.should be at least 7.5 minutes.

..my club run 5 minutes for qualifying so well under a tank..1/4 finals are 15 minutes so to avoid having to refuel twice you need AT LEAST 7.5 minutes runtime..preferably nearer 8 minutes so you have a bit of leeway..

the real test comes in longer heats..semi finals are 30 minutes..3 fuel stops with approx 8 minute runtimes..if you can stretch your runtimes to 10 minutes the you only need to stop twice in a semi final..at this point its handy to have a stopwatch that has split times so you can measure your individual laptimes as well as total runtime..try a smaller insert to extend your runtime (you will probably need to tweak your needle setting just a touch) but check your laptimes aren't getting longer..it's not much use to have longer runtime if the power drop reduces your laptimes so much that the one less fuel stop is wasted..this will depend a lot on your track,clutch setup etc..there are many variables..

try various setups,also work out about how long you take to stop and refuel,then strain your brain with some basic maths,,hehe.and work out which setup gives you the best overall times..

eg 8mm=[ 52sec laps x 8 ]+ 3 fuel stops @ say 12secs =7minutes32 secs

smaller sizes will probably give you slower laps but less fuel stops BUT one last thing..if the higher power is making you wheelspin and slide more because you have control issues you might just find faster laptimes AS WELL AS fewer fuel stops..

long post..hope this gives you some ideas