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.