get a billet
A little tack and seal gasket seal works well on the screws to keep gas from dripping
generally I've found more oil helps carburetion- start on the high scale and then try thinning from there- better than blowing the engine with too thin.
But also more air- a few more small holes drilled in the plastic air cleaner housing never hurts-
What kind of carb nd motor do you have- the stock carb shipped with most 66cc's was designed for the 50 cc motor and carb complaints like yours are common.
A newer 66 carb was introduced this winter- I don't know if it's shiping with any kits yet, (haven't been here much this summer), but the new NTS 66 carb is available separately for about $25.
It's most easily identified by the new shutoff valve where the fuel line meets the carb- if you have no shutoff valve there, you probably still have the older carb designed for the50cc. So this may be an option-
I installed - a billet intake last winter, and tout it here often- from Creative Engineering and Pirates Cycles about $15. I also got a 66 carb then and still haven't installed it- the older carb works well, and I've not been riding.
the billet may have clearance problems- it lowers the carb- see how close it puts the air cleaner to my cruiser frams? but is a shorter intake with no bend- works really great!
The poo poo pipe really helps the noise level, but I had to cut this one to run under the bottom bracket