In general a small diameter carburetor will have high velocity and a good flow characteristic for a low to mid rpm power band. A large diameter carburetor works better for high rpm power bands. For 125 cc engines a 34mm carburetor works well for supercross and enduro and a 36 or 338 mm carburetor works best for fast mx tracks. For 250 cc engines a 36 mm carburetor works best for low to mid power bands and a 39.5 mm carburetor works best for top end power bands. These innovations are designed to improve air flow at low throttle openings.
Some companies sell carb inserts, to change the diameter of a carb.
Typically a set of inserts is sold with a service of over boring the carb. The carb can then be restricted to a diameter of 36 or 38mm. In general, reed valves with six or more petals are used for high rpm engines. Reed valves with four petals are used for dirt bikes that need strong low end and mid range power. There are three other factors to consider when choosing a reed valve.