15,000 RPM on a 91FX? 'Fraid not. I have no idea what a Falcon 120 is, but you're not going to get that RPM out of that engine. If you want a .91 size 2-stroke that'll hit that sort of high RPM, be prepared to shell out for a Jett. Never take specs from mainstream manufacturers at face value. If you want to see some realistic, actual figures have a look at the Tachometer Readings forum.