John,
What motor is on it a power torque or a full crank engine?
If it is a Power Torque engine I would be looking at the top 'O' ring as well as the 'O' ring on the carby mount.
If it is a full crank engine I would be looking at the top seal and checking that first but until you tell us what motor is on it including the carby we are unable to give you the correct advice.
