GY6 stock carb jetting for 171cc BBK

[Snowmanhat]

Installing a 171cc bbk on my Tao Tao 150cc scooter with stock exhaust and carb. What is the recommended jet size for stock carb with upgrade to 171cc bbk for typical conditions in NJ (not high altitude). I purchased a small kit of main jets with many different sized but don't know where to start? Thank you for your help

[krat]

You did not indicate what you have now. Most 150cc scoots will come with an 88-95 main.

A 20cc jump is not that big a change in air/fuel processing. You will get a microscopic torque increase but any well tuned 150 GY6 is already pushing red line at WOT acceleration in traffic on a regular basis.

If you have not taken your engine apart already you might surprise yourself by trying the carb work first. you may decide the "big bore kit" makes 20cc not worth the effort. The big increase in the BBK is when applied to a 125cc scoot. That gives you a whole moped worth of improvement!

If you have stripped the cat I would start at a 105 jet and move up or down as needed. 105 was where backfiring stopped my upward progress. 105 worked, 110 was too rich.

If a 105 is too much you will know it immediately. Over fueling a GY6 gives instant backfiring due to excess unburned fuel being ignited in the muffler.

Most important thing will be tuning the carb after the jet change. Get your fastest idle on the A/F screw and back the idle down with the throttle stop screw. When the back wheel is just barely moving at idle you are about right.