Wont rev past 6500rpm, feels like fuel /air, filters clean, revs ok at standstill not under load, tacho. steady (doesn't seem electrical)

The 4.0 and the 5.2 engines are dauntless. I expect something external to the engine such as I've listed

Have the OBD scan checked for engine error codes!!

1. Very dirty air Cleaner/Mouse nest in air box
2. Dirty fuel filter -starving for gas
3. Failing Fuel Pump
4. In tank fuel filter
5. Plugged catalytic converter

Have the fuel pressure checked

When you say it won't rev past 6k what happens? Does it bog down and eventually choke itself out?

sounds like the carb needs a good cleaning. Have you replaced the fuel and air filter? If you take the air filter off does it run better or worse?

As the saw runs out of fuel the revs increase due to lean fuel mix, so would guess the saw is not hitting its 12500rpm at full revs, this is either a blocked air filter or the carb adjustment, the revs should be checked with a suitable tacho before leaning off the H screw on the carb, increasing the revs beyond 12500 could result in a cylinder seizure.

sounds like it may be running out of gas under aload it might be carb trouble

REPLACE THE CAMSHAFT !!!,,, HAVE A LOOK AT CAMSHAFT , EXHAUST LOBE IS PROBABLY WORE RIGHT OFF....

The 300M is equiped with a rev-limiter. The technology is designed to protect the engine. It keeps you from blowing it up when it has no load on it...If it is driving and shifting ok, and you have no check engine light on...there is nothing wrong.
Why would want to rev the engine past 4000 rpm in neutral anyway? Max torque comes in around 3950 RPM...
If you keep reving it up in neutral you might throw the computer in to LIMP mode, then you wont go no where.
Hope this answers your question...

I have seen this before a few things to do too make sure. First take the air filter out and shine a light use your fingers to move the slides up and down I suspect the springs that go on the slide is not seated in center you can tell from the slide going halfway and hanging up. Next, the boots on the slides only go on 1 way there is a notch on the carbs for them. I would guess this would fix you up if not we can get deeper into it with air-fuel mix screw settings.good luck

check ur valves and the gaps, try checking ur spark pug gaps

possible air mass meter