I only ask. Do you have a friend(s) with more gas engine experience? The 15 is usually reliable and easy to tune. It's a cooking engine and not fussy apart from getting adequate cooling. Set and forget. You may have a rogue ignition or some other problem, but if you know others who run gassers, they may easily identify and solve the issue for you.
1 1/4 turns out on each needle should run it albeit a bit lumpy.
A bad HT connection in the cap will be very unpredictable. Is it on tight, is the spring OK.
Check your supply battery. Test it under load. It may read 5+ volts but drop suddenly in use. Cycle charge it at at least 1A. Auto charging small 4.8V NiMh packs can be iffy!
Prop is 14*7 or similar? It isn't happy on larger props.
The reed valve is very simple and unlikely to cause trouble. It's a simple single flap.
Any Oil between 30 and 40:1 should be good.