Well Rinspeed, I can see some confusion here in these posts so let's clear it up. Most STIHL chain saws are normally aspirated with a carb. Current MS 170, MS 250, MS 391 for example. Several pro models have the STIHL M-Tronic carb system. Saws with C-M designation: MS 362 C-M, MS 661 C-M etc. A C-M saw has no mixture screws on the carb, fuel is metered electronically by a microprocessor in the ignition module that also controls ignition timing. The carb has a square wave operated solenoid that can open and close 33 times a second to meter fuel at all engine speeds. Main reason for this is that the saw will tune itself as you change altitude (need leaner adjustment as you go up the mountain, richer as you come down), use different brands of fuel, air filter getting dirty (dirty filter causes rich running), better starting in cold weather, even adjusts to the load on a long cut with a long bar in large diameter wood. To re-calibrate the M-Tronic saw you start it with the Master Control Lever in the triangle position (it is not called choke, but it does close a shutter with a big air bleed and a switch that tells the microprocessor the shutter is closed) and leave it idling in that position for 30 to 45 seconds then kill the engine. This puts all the fuel trim settings back to factory default. This should only be done if you install a new clean air filter, or change a worn out spark plug or do any serious service to the fuel system. There is no reason to re-calibrate on some periodic schedule.
Davefr mentioned break in and he is right, with a two ring piston and a chrome plated cylinder it does take a few tanks of fuel to bed in the rings, but STIHL does not require the operator to really do anything different when new compared to later, but you can if you want. The reason is that the manufacturing tolerances and quality control used to make these parts is at the same level as automotive, and the fit and finish of the parts is really phenomenal. And this is true for the MS 170 all the way up to all pro models. There is no field calibration for the MS 500i. I can diagnose and reset it with the MDG1 which is a scan tool I connect to the computer with Bluetooth to interface with the fuel injection system.
So I will go ahead and go deep here on the fuel injection. This is low pressure fuel injection. Fuel pressure is created for starting by pumping the primer 10X, then start it up. Once running an impulse pump driven by the fluctuation in crankcase pressure maintains a regulated 100mbar so when the injector opens fuel flows, but the injector opens into the crankcase when the piston is moving up so negative pressure is present, so high pressure is not needed. You still mix oil with the gas and it needs to wash the crankcase on it's way to the combustion chamber just like any other hand held two-stroke.
There is temp sensor in the crankcase and one in the ECM so the saw knows if it is a warm start or cold start by comparing them. There is a pressure sensor in the crankcase feeding info to the ECM regarding if there is pressure or vacuum, so it can be interpreted as piston position. The RPM is measured by recording how many times a second the flywheel spins around the generator, which powers the ECM and also powers the ignition coil. This is not a magneto. This pic shows what all is happening when you pull the rope.
The other thing about starting is to pump the primer 10X even for a warm start to purge any vapor bubbles from the fuel system and insure there is pressure for starting injection. It is an amazing piece of technology designed to work in abusive operating conditions and my personal experience is that it works very well. You will enjoy cutting with it. Get someone to take a pic when you are cutting so we can see the smile on your face. Mine always makes me smile when I use it!
