gear problem
 

I have a spiderbox with a 155cc gy6 motor. Lots of mods. my main problem is I seem to be not getting into the power band. I am running a 17~37 tooth final gear set with 31t and 13t rear sprocket. I was hitting 51gps on slight decline before i added reverse. Now can't get there. Still using 31t but the sprocket on reverse 16t. motor dose not seem to be hitting power band rpms are in the mid 4000. I have tried 36t sprocket more low end but no gain in speed. This is a street racer not off road. I have Dr Pully 114mm with 12gr sliders running a shorty clutch with 1500 springs and 1500 main spring and Kevlar belt. Head is polished and ported with pass thru exhaust. a12 cam 28mm carb with 140 jet (plug is good color seems to be right jet) My CDI is a knm CDI with coil built in. Running low adv on setting #4 and high adv at #4. 11 pole stator. Also running 21in tires on front and 22in on back with 10in rims. Thinking I need to go to a 15~37 gear set but need some guidance. Anyone have any thoughts?

Your current gearing 17-37 is only a 2:12 ratio. The 15-37 will give you a 2:46 ratio which will help your power band. The stock gearing on the yerf was a 3:08 ratio.

Thanks. I still have stock set did not try them with reverse kit. Did bring it in shop today to check what total to wheels would be. With current setup it looks like 11.5 turns of clutch to 1 revolution of wheel. Was going to see what stock set will do but I think I need the 15 x 37 to achieve what I am after.