Sounds like rev limiter at half throttle ttr90

HAVE YOU SET THE AIR MIXTURE ON THE CARB?

sounds like a fuel /air problem. It only shows at low revs because the requirement is met as there is no great pressure on the fuel delivery system but as revs rise and the fuel delivery becomes critical misadjustment of the system causes it to miss. You need a ferrari man to check this out.

There is not an actual rev limiter. The top end is controlled by the carb. You actually want it to sound a bit rich at full throttle so that it has enough fuel when cutting under a load. What causes me some alarm is the revving up part. If it revs too fast that is a lean condition and can quickly ruin the piston and cylinder much like running straight gas. A torn intake boot can cause this. The high speed spec for the TS400 is 9,600 rpm + OR - 500 RPM. Your saw may not have a needle to adjust the high side as some have only the low speed adjuster. Also when you disassemble and clean the carb always use a new kit with the gaskets and diaphragm. Any Stihl dealer can furnish the kits and they aren't terribly expensive. Often between $10-20.

there is a special tool that fits the splines on the carb screws, part no. 530035560, before adjusting the Hjet make sure the air filter is clean, and if it has a spark arrestor in the muffler outlet check it has not carboned up, both of these things will cause the engine to run rich.

well i would have the catalic converter checked if it's clogged it would cause this.

Sounds like your throttle cable is binding,possibly bent or not routed threw the bike frame properly. Have you fallin with the bike? check for damage to the twist grip or looseness to the mounting area Hopefully this helps.

If it's hitting the rev limiter before the red-line you need to have the ECU recalibrated or install a power commander.

You need to set the valves.

assuming is a bad thing. sounds to me like a clogged carb. also inspect the hoses that drain to ground

This is a classic symptom of a bad mass airflow sensor. Nissan designed this as a protective feature on the car. If the mass air sensor goes bad, the computer limits the car to a 2400RPM rev limit to prevent damage.

The sensor is under the center nose panel between the headlights. Take that off and on the back of the airbox (which fills this space) you'll see a pipe coming out that meets at a "T" with another hose going across and down under the headlights. That pipe off the back of the airbox is the mass air sensor (it has a harness plugged in).