80cc kit keeps stuttering and has barly any power

Break it in at 24:1. 18:1 is for the straight 30wt that is commonly used over in China, not a good quality pre-mix that I bet you are using. (you are using a quality pre-mix, right?)

It could be a bad spark plug. The factory ones are junk.
It could be iffy wiring connections. The factory plug together connectors are junk.

What color is the spark plug? I bet it is black as night, but you'd be better off checking it to be sure what direction the carburetor needs to be tuned to.
Also, barring a carburetor tuning issue, a clogged exhaust can cause the symptoms you are describing too. Brand new does not mean made properly, especially with these cheap engine kits.

Check out the simple things before digging into the hard things.

Here's some good reading too:
http://motorbicycling.com/f30/guide-bicycle-motor-preparation-care-354.html
http://motorbicycling.com/f30/spark-plug-color-chart-motorized-bicycles-24873.html

Shoot, just read all of them, you will be miles ahead in the knowledge needed to make these things play nice.
Classic Instructions for Building and Repairing Motorized Bicycle Engine Kits - Motorized Bicycle: Engine Kit Forum