15/42 combination sprocket top speed on yamaha ybr 125

SOURCE: Front sprocket

Hi ........... One tooth less will give you more top speed (if you can pull top revs) but will reduce accelleration. If you go for one tooth less you'll need to tighten the chain and/or remove a link. It's a trade off ............ decide what you want the bike to do.

SOURCE: yamaha ybr 125 top speed

Hi les
Not much to start with here unfortunatly mate. A free flow muffler will help top end speed, change the factory airbox to a K&N style pod air filter, other than that, hugging the tank at top speed :) If you do change these parts, you may also need to increase the main jet size to richen up the mixture for better tuning of the motor with the better flowing parts. This bike has a top speed of 100kmp and produces 10 hp, so you may only expect to get at best less than 1 extra neddy, and perhaps another 5 km/hr.... I suggest upgrading to a bigger machine of you feel the need for speed :)
If its 125's you are into, test drive a Honda CBR 125R 6 speed fuel injected 13.4 horsepower, and good for 120km/hr
regards robotek