Docker,
I would check in this sequence.
1. Check gov hose is connected to engine block.
2. Remove carby and check the 'O' ring on the spout on cylinder.
3. Check spout for any movement.
4. Check starter housing bolts are secure.
Leave the starter 'O' ring till last because that is the most expensive one and the harder one to replace.
With 2 if damaged replace the 'O' ring and with 3 if you have any movement replace the 'O' ring behind it and tighten down properly.
After each step see if it corrects your problem if you have to replace any 'O' rings.
