Ok......i have been scrolling through past topics all the way to 2007 to look for an answer to my question but not really seeing it so i will ask. i bought my first racing mower last week, it is nicely built with a brand new custom front end that is way more rugged than factory. lowered, widened....real nice, and has a great running 12.5hp Briggs motor....fires right up and purrs like a kitten. the guy said he "raced it", but when i got it on the track the first time i was disapointed because it was only going about 9mph  i started researching all kinds of websites and realized it was all in the pulleys, it had a 3.5" on the front and a 6" in the back, and the governor was still connected......boring. i spent a few days trying out different pulley combos and it now has a 4.5" in the front and a 3" in the back. i temporarily disconnected the governor and ran the throttle cable directly to the carb.......it had lots more speed with these few changes. i was so afraid to blow the motor though so i have hooked the throttle back up to the governor now. on the track i am running on, my tach is showing my RPM reaching 3950 at the end of the straights but it seems to be topping out just a bit too soon.......my question is....how many RPM can be safely turned on a stock 12.5hp briggs motor? when i say stock i mean stock, no billet flywheel yet (saving for one now). i also found a 2" rear pulley last night at a nearby shop......do you think if i put the 2" rear pulley in with the 4.5" pulley in the front....will i get just a bit more top end speed while keeping the RPMs down to a safe level? thank you everyone