Re: butto from australia
Welcome. What you have going on there sounds like it's four stroking. It is normal for this design to four stroke at cruise, and is especially noticeable until the engine gets a few hundred miles on it.
Have a look at your plug, and if it is really black and oily, then you'll need to make a couple of small changes.
Use a break in mixture of 24:1 at the very lowest! Do not use a 16:1 mixture.