Have you checked for a compression loss? Head gasket or????? Not to point out the obvious.[)] I myself have several Vintage .15 size engines that run really well on high nitro. Even up to 20% with 20% castor. Not all engines are created equal as the above answers also indicate. I have a loop scavenged Thunder Tiger .15 that makes 15,000 on an 8X4 prop with 15% nitro (with all castor lube, my preference). And I have 3 Foxs that do just as well on the same combo. On the other end of the scale I have a K&B .45 that just LOVES 15% and makes Mongo power on it. I have also tried FAI fuel and it works well in my Magnum .46, just depends on the compression. And I can't get a Cox Pee Wee .020 to even start on less than 20% nitro. [sm=lol.gif]