I have a 66cc 2 stroke that was getting top speeds of 25mph and now is topping out at only 15mph. I replaced the spark plug and cleaned the carb with no change. I then noticed that if I took the air filter off, so it's just open, the bike will get back up to 25mph. This told me that it was running to rich, so I thought is was a main jet problem. I replaced the stock 0.70 main jet with a 0.64 and I am still having the same problem. Is it something in the intake maybe? I am new to small motors and am unsure how to proceed. I don't want to screw it up as this in my main commute vehicle. Thank you for anyone that can help this newbie.