Hello, and sorry you are having troubles with your motor. I would like to suggest that you work through your problem and learn about the motor in the process, you will need the knowledge you are gaining to maintain and repair your bike over time. I would also note that it pretty much doesn't matter what company you buy your motor from, it is the luck of the draw if you get one that runs great right out of the box. That said, working out these problems usually isn't all that difficult.
First of all, I would like to stress that you need to know for SURE the engine is running (when you say it idles briefly). Often I see beginners mistake the sound of the motor turning over without firing as the motor running, when really you are just pedal powering the motor and it is not running at all. If your motor is RUNNING you can pull in the clutch and it will remain running, if you pull in the clutch and the motor stops it is not running in all likelihood.
Your motor needs proper air/fuel mix, spark and compression to run, lets assume you have compression on a new motor for now. Next you need spark, if your engine truly does idle then you have spark (yet to be determined IMHO). The best thing for you to do is pull out the spark plug and take a look at it. My first kit flooded out before I ever got her started, had the same scenario you describe and I was frustrated. When I pulled the plug it was wet with fuel and looked new like it has NEVER fired. I dried the plug out and rolled the bike around a bit with the plug out (and clutch released to force the motor to turn over), this forced any built up gas out of the cylinder. (be careful not to let flame or spark ignite any fuel that is being ejected from the spark plug hole during this process) Once I put the plug back in it fired up pretty much right away. The spark plug boot on these motors is crap, they break easily and don't always make good connection. You also MUST remove the cap from the top of the spark plug and push the plug wire boot directly on to the THREADED TIP of the spark plug. If you do not remove the cap from the spark plug your wire will not connect and you will not get spark.
Fuel delivery can also be an issue on a new kit, the fuel tanks are usually full of all sorts of debris from the factory, if you don't clean it out before you put it on your bike you can assume your fuel filters will clog immediately. So make sure your fuel tank is cleaned out and your fuel filter(s) not clogged.
From what I read you seem to have the CNS carb, same as I have. They are much harder to tune than the NT or Speed carbs, but it can be done and they work nice when tuned properly. My carb tilts forward slightly, the float was set way high and it was flooding the **** out of my motor at first. I had to adjust the float to lower the fuel level in the bowl before my CNS carb worked right. I wrote a long thread about what I did to get my motor running and tune the CNS carb, here is a link to that thread.
http://motorbicycling.com/f4/cns-carb-working-great-32736.html
I would also suggest that you clean up the carb inside, there are 3 jets on this carb, the main jet, the pilot (or idle) jet, and the enrichment (choke) jet. Make sure none of your jets are clogged by running something threw them (like the wire from inside a twist tie), read the above thread for more details.
Your spark plug tells the story of what is happening inside your motor, has it fired at all? Is it dry? Soaking wet? When tuning a carb you need to read the spark plug to determine the condition of your motor, get used to it....
