hey, Im using an OS 91 fs ll 4-S. Its new but, has been run in with several tanks of fuel. My question is... how hot should the motor get after break in? The rpms should be between 200-12,000... using my Hanger 9 prop "tach. meter" it says Im only getting about 7500 max rpm.. If I start leaning up the mix jet it picks up rpm but, it runs hotter of course... the motor has a black cap on the cylinder should I be able to touch this for any length of time or should it be to hot to touch? Im using the rpm meter outside in sunlight... thanks for any help or tips.

How did you figure 12,000 RPM? That's theoretical and depends on the prop too.

the real top end for that engine is more like 10,200 swinging a master airscrew 14/6prop and that would be the before richening it for flight,I run mine around 9,700 or so with that prop using PowerMaster 10% fuel.

my 91's that I have 3 blades on, are spinning about 7,800 on the ground, but will unload when flying the book is for reference you dont want to over rev a 4st the valves will float and your engine won't last long,idle RPM's is about 1,500-2,000.

actual temp I have never hooked a thermomiter to it but they get hot you don't want to touch em right after a flight for a couple of min.

I hope you are talking about the needle valve when you are refering to the jet mix on a new OS91 spII you need to run several tanks at the factory settings on the carb, only using the needle valve to adjust RPM's these engines need to be rich for break in,and leaning the low speed adjustments too soon will burn a engine fast and it will never run well for you.
if you have moved these screws then refer to the manual and reset them to the factory settings for now untill you have sorted out the top end RPM issue.

the RPM meter will work in direct light but if the sun is shining into the sensor they can give you a bad reading if you are getting top RPM's by sound,(running the engine at full throttle and finding the peak by closing the needle from 3 turns out untill the RPM's begin to drop then turning the needle out untill the sound is at its peak then out another 1/4 turn)but its not showing RPM's at least in the 9,000 range on the meter then you should do 3 things.
#1change position
#2make sure you have a good battery in the tach
#3 check the calibration with a flouresent light as per the specs to make sure your getting good readings from your meter
that would be a start there's alot more to 4 strokes but the nice thing is once you get the dialed they run great and sip fuel with loads of power.

The rocker cover (black cap you are talking about) is one of the hottest parts of my 4 strokes along with the muffler. I have never been able to hold on for even 1 second. So do not touch these parts. You are most likely running way to rich. IC engines get hot, don't worry about the heat . Just make sure they are slightly richer than peak.

If you are really concerned about heat some of us heli guys have learned to richen until you can hold the engines BACKPLATE (one of the coolest parts of the engine) for 3-4 seconds.

But on my planes i have never bothered to check. I lean my plane engines to peak and back off about 200-400 rpms.

also the aprox. needle setting for the 91 is about 2 turns out each engine will vary of course,but like cambo just said there's some hot parts you won't want to touch.
all of my 91's are in cowls so its a little harder to come in contact with the engine, but I did get a nasty burn off the muffler a while back,if your engine is overheating it will usually die or loose power pretty dramaticly,and is quite noticable.

the 200-12000 rpm is what the manual stated.. prop is 13x8

hey, Im using a 13x8 2 blade prop... I havent touched the low end screw just the needle valve. I think iVe run enough tanks of gas through it for break in and now Im trying to get it set for flight... thanks for all of the suggestions. Ill try it again tomorrow... with a new battery in the rpm meter.. when should - and why should I adjust the low end screw???

in your first post you said you had adjusted the mixture jet, that would be the screw that controls mixture page 15 of your manual goes into the terminology as well as the idle mixture adjustment procedure.

you posted you have run several tanks of fuel,you should have run 3 for the initial break in,pg.13/14 then at least 10 tanks or flights before adjusting the low speed mixture.
when properly adjusted the power and fuel economy will really increase,I made the asumption that the engine was already installed in your plane,break in can be done this way and a engine stand is not needed.

if you have only been running your engine at 7,500 RPM and your not mounted in your plane, then you might want to do the install and do a short run in/break in raising the RPM's to the recomended max RPM's as diagrammed in page 13/14 of the manual after doing the break in check all bolts etc.. temp is not something you need to worry about unless the engine is laboring in flight, its the proper RPM's for the metal to wear in properly that are the most important in the beginning of the engines life.