umm unless some 4908 models are different mine came with a 22 tooth bell and a 58 tooth spur and when i put my bigger tires on i lost wheelie power off the line so i went down to a 20 tooth bell which brought my wheelie power back, so in my opinion going down teeth on the bell should give u more bottom end power which is opposite of what u want....loosening the slipper will help but i also find its too loose it gives u less throttle response.......i would check for sure what ur running for gears,but if it is the same as mine 22/58 i would change ur spur to a 54 tooth(less wheelie power and more top speed) i am a newbe so all i can really tell u is what i have experienced......think about a 21speed mountain bike,if u want it ez to peddle, ur chain is on the smallest sprocket in the front(clutch bell)......ur chain is on the largest sprocket in the back(spur gear) maybe that example will help ur decision......hope this helps and goodluck!!!