RE: os fs 52s
I've got two of those engines and they are without a doubt the best engines I've ever owned. I'm guessing that your problem is just mixture settings. Try this setup and see how it works for you
First, you will need a tach to really tune the four strokes. Your ear just isn't going to be able to detect the top end like you can on a two stroke.
Get the engine running, the 12x4 prop is a good load, and let it warm up for a minute or so. Now go full throttle and very slowly lean out the top end for maximum RPM. It takes a bit for the four stroke to catch up with the mixture change, so count to 10 between each click of adjustment. Once you have found maximum, it should be around 11,000 rpm, depending on how well broken and prop, richen it up so it drops around 500 to 600 RPM from peak.
Now cut back to as slow as the engine will run with out stalling, what ever speed this is. lean out the idle for maximum speed. If it starts faltering as soon as you start to lean it out, richen it up to maximum speed then go 1/4 turn futher rich. Cut the throttle back again to full idle, and you should keep running at aproximatly 2000 rpm. Note that your carb will not be fully closed at this point.
Now re-adjust the high end again. Changing the idle when it is way off will affect the top end adjustment. Use the same 500 to 600 rpm drop from full
Back to idle, does it hold a reliable idle now?? If not, re adjust the idle. But this time, you are only going to turn the needle 1/16th turn at a time. Before you start, jam the throttle open. Does it die imedatly, or does it sputter and slowly gain RPM. If it dies right away, you are to lean on the idle, start by click (1/16th turn) at a time until you can jam the throttle and it barks to life. If it sputters, it is to rich. lean it by the same clicks. That engine will jump when you hit the throttle. When it is broken in, you can get it to bark like a gasser by quickly flicking up and down throttle.
Once you have it right, go back and adjust the high end once again, then give the low end it's final tweek to prefection. Once set, and the engine if broken in, you shouldn't need to touch the needles for a long while. I flew one of my 52's through two crashes, one breaking the mounting lug off the back plate, and after rebuilding the plane on the first and then building a new fuselage on the second, that engine fired up right away and required nothing more than throttle servo adjustments. Great engine.
Don
PS, If the engine has a gallon or more fuel through it, you may want to check the valve lash, but don't adjust them it unless they are way off.