Depends what batteries you are using. Your motors are running on 800 rpm per Volt applied. So with a 3S LiPo you would get: 3,7V x 3 x 800 = 8880 rpm, with a 5S LiPo you get: 3,7V x 5 x 800 = 14800 rpm...that's a huge difference. Either try a LiPo with 1S less than you have at the moment, or try...