ORIGINAL: MJD
I have a NIB .74 I was given in about '93 that I've never run and will find it's way into an RC market place soon as I find the barrel/idle stop screw I removed for some reason, I think I borrowed it for a long-since-gone Fox .15. Of all the stuff I rooted through recently, that was the 1st and easiest choice for my ''auction site'' bin. I wouldn't waste 10 minutes on it myself. Years ago I bought a Fox .15BB RC and an OS .15 FP at roughly the same time. I gave away the Fox shortly after, and I still have the FP and it runs like a top and doesn't act up when I wear the wrong colour shirt to the field. Finicky carbs, overboard compression, metallurgy that threatens to lock up if you don't run equal parts of castor oil, maiden's tears and methanol.. [comments deleted in good taste]. Sorry if I offend Fox fans, I know they made a few engines that worked.
MJD
The first FP engines were also iron piston that required all castor fuel. The modern .15 BB does not have a finicky carb, it works just like an OS but with crude needles that work. I think you may have had an older BB .15 which may have an inferior carb. The Fox .15BB is also an extremely strong engine and does not reqire low nitro fuel.