I have just got a 250cc Rev 3, when I put the bike on side stand the petrol runs out of the carb, people have told me that they all done that and you have to get a mod done to the card, does anyone know what needs done.

 The easiest fix is the Billy T carb mod. Someone will come along and give you a link.

The two tubes that come our the side of the carb are vent tubes for the float bowl. Beta made the tubes so long they end beneath the float bowl so if fuel gets splashed up into the tube it acts like a siphon. Take a pair of wire cutters and nip a hole in the tube just below the bend but above the float bowl. That stops the siphon.

There are other more complex fixes but they work about the same and are more work.

 

Dan the petrol doesn't come out of those 2 breather. It comes out of the breather on the float boal 

Ah, the brass tube on the bottom of the float bowl is the overflow. Disassemble and clean the float valve. Make sure there are no holes in the floats. Set the two tabs as per BillyT instruction. Make sure floats aren't rubbing on bowl gasket and trim gasket if so. Reassemble and report results back here.

All the other mods are to deal with fuel piddling out the vent tubes on the side of the carb.

Dan I'm going on holiday for a week tomorrow night. We're would the best place to get a new float valve. I will be back end of next week could order one now and be hear for me coming home.

http://www.r2wtrials.co.uk/html_files/betacarbsetup.html

just to be sure we are talking a Mikuni VM26-208? 

Parts are readily available from any online carb shop but it's rare to damage a float valve especially on a trials bike. Much more likely to have a fleck of dirt on the seat. I think cleaning and setup should be all that's required. Clean with compressed air. NO WIRES! I've seen it noted by some but i'd run nothing through a jet or valve but air.

Update put a new float valve in and set the float height. Much better fuel does not run out when on side stand. The only thing when bike is running and on side stand when you rev it fuel still come out of the overflow on the bowl.