Fox Engines
I'm not an engine expert but have had some experience's with Fox R/C engines.
If you can find someone that knows how to set them up, they are top notch performers. The first Fox R/C engine I bought seemed to be a real piece of junk - until someone that knew what they were doing set it up for me. This someone happened to be a Fox dealer - but he was not a hobby shop owner. Seems like Fox used to have some "dealers" that were just ordinary modelers - you didn't have to own a business to be a dealer. Don't know if it's still that way. Anyway, this guy knew instantly what my engine needed. If I remember correctly (and I admit it's been a few years back) my particular engine used the 2-needle carb. You had a needle for both low-speed and high-speed. My problem was in the mid-range. It would be "blubbery" in the mid-range even though low and high speeds seemed perfect. When he heard this, he immediately took the carb apart and made a scratch (with a needle file) on the rotating barrel in the carb. The scratch went from the middle of the opening through the barrel and about 1/3 way around. So when the barrel was closed (or nearly so) at idle, it allowed just the tiniest bit of additional air to be added to the mixture. As the barrel rotated, more additional air was allowed to enter the mix because of the scratch. I know this doesn't seem like it would be of any significance at all, but believe me it made all the difference in the world. It made that engine run totally perfect in all ranges. The scratch allowed just enough air to the mixture to cure the blubbering at the mid-range. By the way, you wouldn't even think about trying to make any settings until the engine was pretty well broken in.
Fox's 2-needle system was a problem for a lot of folks (including myself). If I remember correctly, you were supposed to adjust the idle needle first. When you were satisfied with low speed performance, THEN you worked with the high-speed needle (for some reason, folks always seem to want to do it in reverse order). But often, with the 2-needle system, there would be problems with the midrange and the "scratch" on the rotating carb barrel seemed to cure it.