THis may be a stupid question but I have realized to take nothing for granted, particularly from new comers to our hobby. The fact that you requested a starting point for the low end needle valve indicates that you may not have much experience engine tuning.

DID YOU lean out the high end needle valve at all???? The high end needle should be turned clockwise in 1/8 increments with the engine running at full throttle until you reach max rpm as per your tach. Once at max lean rpm, richen (counter clockwise) the mixture so that the rpm drops about 400 rpm shy of max lean rpm.

Next time you run your engine at full throttle, pinch the fuel line to the carb briefly and take note whether the engine speeds up. If it speeds up, you are too rich and need to learn her out some more.

An OS F plug is not your problem. This plug does well in most 4 strokes and it is not the source of your problem (unless it is bad and should then be replaced).