Gday Chris...Needle could be a large part of the problem...its the base for a rich or a lean mixture additional as mentioned above twins are harder to tune as you have two. Can be done at home but you have to know what your doeing and a couple off additional tools.
Could also be acouple of other thing...
Oil could be to think in the dash Pots.
Timming at the Dizzy or even at the timming chain
Points Gap (Dizzy) could be out and or tappets.
Carbies could also need a Kit
You have mentioned its only been on the raod for two weeks. Has then Engine just been throu a rebuild. If its just had a rebuild you have to give the engine time to loosen up a bit also
Yep a Dyno Tune is best way to go. Also try and get some one who knows Mini Engines and SU carbs.
Please let us know how you get on and also..where abouts are Y
I know I have not helped but it could be one thing or a collective