So kid has a 2002 kx125 we have been running it for about 7-8 races with no issues. Only recent changes were returning to stock exhaust because the old one essentially disintegrated. We are running Amsoil the red I believe it’s the Dominator 32:1, 91 ethanol free petrol. Have run about 2 motos with this type oil and exhaust with no issues. Saturday at practice first lap kid came off track complaining of no power, I knew the clutch needed replacing anyway so did that no change. While at race quickly cleaned carb replaced spark plug, which didn’t look to bad. Bike ran ok but not great for about 2 laps then bogs down at low RPM’s. Tonight removed air filter (thought might be over oiled and restricting air flow) rode in grass same issue. Essentially bike has no power until it hits the power band just bogs down and now fouling plugs quickly. I checked the reeds and they look tight. My next step is jetting and power valves but not sure why this would have changed since was running well with same combo of fuel and exhaust. I am not terribly familiar with 2 strokes but learning anyone have any suggestions??