Welcome to the forum!
Here are some links about teinsioners. If your bike has the sort of clearance that has the chain rubbing where the bottom frame trailing arm goes to the point the wheel bolts up too. Saying that is where the rubbing is? The teinsioner serves a duel purpose here it will keep the chain frame eating away and ruining your frame. These links have numerous ideas. Tensioner Links Look Here 1st for your Motorized Bicycle. - Motorized Bicycle: Engine Kit Forum
Now for the kit carburetor is leaking fuel there could be a float issue inside. Thrash at the needle valve seat just a tiny crumb of dirt not permitting the needle valve to seal. You need to have a in line fuel filter before the carb as well. Maybe just a simple float tang adjustment. Here shows the carb in better detail. Your float maybe too low? http://motorbicycling.com/f4/motoriz...all-195-2.html