Pat,
If everything is tight and sealed up then it is likely you have a poor piston to cylinder fit. A new set costs $25 from Tower. We have found these engines do not like more than 18% oil. We have also found that they do not draw fuel as well as larger engines and you have to make sure it is getting fuel easily from the tank.

There is more about the OS LA.15 on www.georgiacombat.com in the glow combat section. It covers break-in, and fuels. Good luck.