You’ll be able to simply get a free auto repair guide for whichever restore need you might need. Usually a small diameter carburetor can have high velocity and a superb flow characteristic for a low to mid rpm energy band. A big diameter carburetor works higher for prime rpm energy bands. For 125 cc engines a 34mm carburetor works nicely for supercross and enduro and a 36 or 338 mm carburetor works finest for quick mx tracks. For 250 cc engines a 36 mm carburetor works best for low to mid energy bands and a 39.5 mm carburetor works best for high end power bands. Lately there was a trend in the usage of air-foils and rifle-boring for carbs. These innovations are designed to enhance air movement at low throttle openings. Some companies promote carb inserts, to change the diameter of a carb. Typically a set of inserts is offered with a service of over boring the carb. For instance; a carb for a 250cc bike (38mm) can be bored to 39.5mm and two inserts might be supplied. The carb can then be restricted to a diameter of 36 or 38mm.