Sounds like the timing chain is off either 180 out or a tooth off.
Might want to pull the cover and make sure the cam is timed right.
Also double check the wiring it is the most common start up issues plug wires crossed. Even seasoned mechanics do it from time to time. With the hold downs and all the wire close to each other its not hard to get one mixed up. I always make it a point to check it agaist the chart on my wall for firing order and which cylinder is which and cap position for the cyl. One wire off it runs fine one wire crossed it could stop the engine from starting if its always firing at the wrong time.