new engine wont start

My shot in the dark is you're either not getting a spark or not getting enough compression.

clutch slipping?

My be lack of compression,...
in my case, I had taken the head off and dropped a few drops of 2 stroke oil in the area of the ports to give it some lubrication for it's first few times the piston was run up and down that cylinder, and when I put the head back on I must have not gotten seated correctly or I tightened 1 side down too much before snugging the other side, I loosened the head bolts, re-seated the head, and was real careful this time snugging the nuts and and rotating as I tightened them just a 1/4 turn everytime,... and then it started right up,...
but if you haven't messed with any of that, I'd start with,...
got GOOD fuel to the carb ?
got Spark at the plug ? (you'll have research how to test that)
got the wiring done right ?
good luck, let us know,...
scuba

air leak at the manifold to carb junction