With your MS251 press the primmer bulb 3 -5 times even if there is fuel in the bulb. Squeeze the throttle trigger completely WFO and move the "stop, run, start, choke" (SRSC) lever to the bottom "Full Choke" position. Pull the start cord until you hear or feel a "pop" or combustion. Nine out of ten times the saw will NOT start at this full choke position or point. When you feel that first pop move the SRSC lever up to the START or partial choke/high idle position WITHOUT squeezing or touching the throttle trigger. Pull the starter cord and it should start or come to life within one or two pulls. DO NOT attempt to feather the throttle trigger but let the saw run in the high idle position until you can feel warm exhaust at the muffler outlet with your hand. Once you have warm exhaust you can squeeze the trigger and kick off the lever from the start position and it will go right to the run. You should have an idle speed of around 2800 rpm to at the most 2950 rpm but nothing more. If this doesn't work then I'm tearing up my Stihl Silver Level Tech Certificate, throwing away all of my Stihl saws, blower, trimmers and buying corded and battery Milwaukee stuff .

MikeR

1: Prime 3-5 times.
2: Squeeze throttle trigger, move control lever to full choke, lock lever into full choke position.
3: Pull start cord until you feel or hear one "pop".
4: Move control lever up one position to partial choke/start position.
5: Start/pull start cord.
6: Let run at high idle until you feel the exhaust get warm before squeezing throttle trigger.
7: In the run and warmed up position the idle should be 2800-2950 RPM.