I installed a PZ27 on a 125-4 stroke. (Mid Ohio) Before I installed it, I set the air/fuel screw to 1.5 turns out. That's all I did. After an hour of hard running I checked the plug and it was perfect dead on. I also don't have any back fire either from both the carb and the pipe when I come off the throttle. Why 1.5 turns when everyone says 2 turns to start with? Because the intake runner is a match for a 18mm carb, so the intake runner acts like a restrictor plate and being the carb's exhaust side, to the intake, is larger in diameter than the small 18mm carb that came with the bike, so of course I had to limit the fuel to air because the intake runner. The good news is a narrow intake runner (Air flows faster through narrow passages, so the smaller diameter intake runner actually speeds up the flow (Fuel/Air) from the carb. Of course this setting is what I found to be a perfect setting for Ohio. 1.5 turns out from seat and runs with full power and the plug is perfect. I have not messed with it after. Installed and ran.