Bob, I have three fourstrokes that I broke in on Fox 15% nitro fuel with 20% castor and then ran them for two seasons on it before I became "educated" on the ills of using castor in a fourstroke. They are an Enya .46 MKII, a high compression Saito 1.50 and a really high compression Saito .80. If you wrap your thumb and index finger around the prop driver and twist them they do not leak compression at all. They have seriously uglied up mufflers but run strong. Their exhaust valves did not load up with crud they became gold colored and stayed that way but no problem. The 1.50 is black with chrome rocker covers, maybe the gold exhaust valve makes it a Golden Knight engine.