Have you checked the valve timing?

For comparison, mine has it's low-speed screw recessed in .04" from the throttle arm and the high-speed needle is out 2.5 turns plus 4 clicks. This engine has had about 5 tanks through it on the bench.

Try starting somewhere near there or a bit richer and see if you get anywhere. You may need to send it back to Horizon for a check-up.