RE: 60LX mid range
Two things ...
First - Give it some time..... its new. Do not adjust anything on the low end mixture until you have at least 20 minutes on it.
If you just receive the engine, perform the run-in with a 9x7 on it ..... not the 10x6. Do no skip the run-in proceedure. Instructions provided with the engine. Put the 9x7 on it, dial up the needle so it is running 18K rpm (will still be a bit rich) and run it there.
Second - the engine will tend to load up a little bit in midrange. It is suppose to do this actually. It is a mixture curve is designed for the higher timing and the tuned exhaust. When the engine stages up and down off of the pipe, it gets in a situation where it needs either more fuel or less fuel than you would normally need. Biggest issue is when throttling down - the tuned muffler tries to keep the engine singing at its own rpm, but you have throttled back and cut off fuel. That creates a point where it gets lean rpm but no fuel to support it. That is actaully where most folks blow a plug. So the carb is set up to support that point. So conversely, that mixture endes up slightly rich when you throttle up - lots of fuel, engine struggles briefly to get up on the pipe. That is what you are seeing.
Get some time in the engine with the lighter prop. Then switch up to the 10x6.
If it is still a problem, you need a hotter plug. Rossi 2 or 3, K&B HP plugs are not bad, McCoy 56/59 are pretty good. The OS A3 is an ok plug, but some folks have struggled with it (not sure why).
Let me know how you make out.
also....... let me know when you received your engine (what date)
Bob