I have an OS40LA engine that has about 20 flights on it. Yesterday I was flying my plane and it just died in the air. I landed it and now I cannot get the engine to run at high RPM. It will idle fine. I have
1. replaced all the fuel lines.
2. changed out the muffler.
3. tightened all the screws on the engine.
4. Put the engine on a different, working plane.
5. Put a working engine on the original plane and it works fine.
6. Changed out the fuel with new fuel.

When I hold my finger over the exhaust it will run up to high RPM but as soon as I take my finger off the exhaust it dies.

Has anyone experienced these symptoms and have any ideas what to try next. I have only been flying for 2 months now and can not think of anything else to check.