When your Stihl MS 170 chainsaw drops its power under full load, it generally means there is an improper mixture of fuel and air in your carburettor. When you depress the trigger, a vent on the carburettor opens, allowing more gas to reach the mixing chamber. If the saw drops at full throttle, not enough gas or air is reaching that chamber.
Check the Airflow
Checking the flow of air into and out of the engine is the simplest and quickest place to look for your mixture problems. Engines operate much like the human's breathing system, clean, cool air must flow in unobstructed and dirty, poisonous gas must be vented away. If this breathing system gets compromised, you will start noticing sharp power losses. Clean the air intake areas around the air filter, carburettor and air filter; replace bad or worn out filters. Also, clean around the cooling system near the starter side and around the fins on the cylinder. You should also check the function of your muffler and spark arrester screen to make sure gas is being properly vented away.
Check the Fuel Flow
The flow of fuel must also remain consistent during operation. If you're dropping power under load and your air systems are clean, then somewhere gas is leaking from the system. You'll want to start at the tank, checking the vent hole on the tank to make sure it's not plugged. Continue up the fuel system by checking the fuel filter to make sure it's not plugged or clogged; check the fuel lines to make sure they aren't cracked or have come loose from their valves on the carburettor. If your gas lines are more than a couple seasons old, the problem can be fixed by replacing the main fuel line, the impulse line and the fuel filter.
Adjust the Carburetor
If, after cleaning the air filter and replacing the fuel lines, you notice the power has improved a little but still drops under full load, then you can readjust the carburettor. Adjusting the carburettor's mixture screws will allow you to regulate the air and gas mixing at different speeds in the carburettor. Even small adjustments will have big effects on performance, so you need to take special care when adjusting these screws. Improper adjustment, especially of the high-speed screw, can result in permanent engine damage.
If you're still having problems with power during operation, then it's likely time to service your carburettor. The diaphragms inside the carburettor, especially the fuel pump diaphragm, tend to warp and bend out of shape over time. When they warp badly, they no longer regulate the mixing of fuel at high speeds. You'll also need to check the main jet bores and ports where the fuel gets pumped to see if it's backing up in the bore. Only Stihl service dealers should disassemble and service the carburettor due to the degree of skill required.