Motorcycle Repair: yamaha xv125, traffic lights, air filter

Question
my girlfriend has just bought a yamaha xv125, when the bike is started it ticks over fine, but when riding and you change down gears the bike will stall,so have to keep the engine reving at traffic lights, was wondering if you had an idea what might cause this

Answer
Make sure the choke lever is in the "off" position once the motor has warmed up, as this will cause problems similar to what you have described. You might want to check the air filter to see if it is clogged and needs replacing (I'm assuming this is a used bike, not a new one). It might also need a carb adjustment and tuneup, too.