I bought a TY80 a week ago for the kids to learn on, everything was running fine until the other day when it ran out of petrol. Since then it has not fired up at all. I have checked the obvious things: the plugs sparking, fuel's getting to the carb although both jets were blocked, these have now been cleaned and carb seems to be working ok, have also checked air filter. Have been running the bike on a 50:1 mix as recommended by previous owner. Any other suggestions?