Yes funny you say this. I just watched a YouTube video on this. I guess the black hose falls off this orange nipple but you can't see it. it's like under/between your chainsaw. Only way to check it is getting 2 screws off under the throttle handle and one screw off the side to separate the saw a little to check it. reattaching it is another job in itself. long needle nose pliers without serrations would help.
This could be my issue but then again, why does my saw start up and run perfect everytime and only begins bogging after 1 hour of a tree falling and multiple cuts. This air hose that assists in air pumping with gas mixing would be happening all the time. so my issue is likely something happening after this 1 hour runtime. then it goes away and starts all over again.
it's driving me nuts. I have to cut firewood out of 6 dead oak trees from 40 to 50 feet high and this saw pulling this after 1 hour is just plain annoying. I've already put in a new filter, ngk spark plug, fuel filter, oil filter, oil pump works great, clutch is new, carburetor works great (until 1 hour of runtime), spark arrestor is clean, chain is brand new, bar is brandnew, chainsaw is brand new too. it's had less than 10 hours runtime.
Going to try a Walbro carburetor but don't know which part number to get. the cross reference chart for walbro has many part numbers for Sthil for the Ms 250.
anyways I'll check that hose. next im going to check the piston under the muffler. after this I'm getting a carburetor. if it works after that I'm selling it and getting a farm boss.