New rebuilt engines are usually tight and somewhat tough to start the first time. I know when I have done a few piston and ring jobs that has been the case. It has been told to me several times down through the decades, that upon that first start to run the engine at a higher R.P.M. for about 15 to 20 minutes to allow for the initial rings to seat/set themselves. In this manner the engine will then be able to start smoother upon subsequent re-starts. After a few gentle rides (break in period) all will be corrected in such a manner that it will start smoothly each time. I typically find resetting the Timing, and also using a vacumm Gauge to reset the carb is quite useful at this time as well.
Tom