Something is wrong. Unless you are using 3 blade prop, even the 15x6 is way too small. Use a 16x8 or a 17x6 on a 20cc. You can even run a 18x6. Gas engines shouldnt run at more than 8000-9000rpm or there isint enough time for the combustion to complete before the next firing.
Oh by the way, they are always difficult to first start. You can either flip till you are mad or use a starter. Ideally, use a metal spinner, they arent too much heavier than a plastic one (atleast the TruTurn ones). Start with the choke closed and apply starter till engine fires once. Then open the choke and the engine will fire every time
Ameyam