I do not understand what you mean by a check valve on a 250. To check the oil flow on a 250 (or a 210 or 230, which make up the 1123 line of Stihl saws), pull off the bar and chain and run the saw. If there is no flow out the oil hole, and you cleaned the oil tank filter, then it can be several other things. If there is oil flow at the oil hole of the saw, then clean out the oil holes in the bar and you are done. Otherwise...
One possibility is a clogged oil tank vent valve. The vent is just in front of the bar studs. Use a compressor and blow on the vent from the outside to clear it. The next possibility is a clogged oil line. I have found that after a lot of use in dusty conditions, fines can build up below the oil pump in the 1123 saws. They are a pain to get to. You have to pull the top handle off the saw to get to the small oiler elbow at the bottom of the saw on its belly. Pull the elbow and use a blast of compressed air to clear the elbow. Then pull the oil line out of the hole that one end of the elbow fits into. Check for clogs or breaks in the line. Replace it if needed. Then check the area below the oil pump in the belly of the saw body. That is where I have seen the fines most commonly build up, and clog the oil flow, leading to foamy oiler output. The oil has to run through a small groove that runs along the side of the oil pump, and that is where clogs can and do occur.
If you cannot clear the fines below or in the oil pump with a compressor, then you have to pull the oil pump. You can buy the fancy spendy Stihl oil pump puller, or get a screw or bolt with the same fine threads as the oil pump base. I have one in my 250 parts bag, but I do not know the pitch or size of the threads. It is about an inch long though. The oil pump has threads at the base, and you screw the screw or bolt into the threads and use a hammer or crowbar to lever up on the screwhead and pull out the pump. The pump is tiny and made of chromed brass, and it is just pressed into place in the plastic case. Once levered out, you can clean it up, blow out the upper oil lines, and then press the oil pump back into place. Align the groove in the oil pump up with the groove in the case, and tap on the screw that you pulled the oil pump out with to seat the pump. Seat the pump in place so that the gears on the oil pump head are at the center of the oil pump drive gear (which is where the oil pump base is about 1/8 of an inch below the level of the case).
If that does not fix the problem, it may be a warn drive gear, or a broken oil pump drive. The oil pump drive is a thick wire that sits into a notch of the clutch drum. To check the oil drive and gear you have to pull the clutch drum and clutch (remember that the clutch is reverse threaded!). Once inside the clutch area you will see the oil drive and gear which are obvious, and you will see the gear head of the oil pump as well. Spin the drive and check that the oil pump gead head is spinning. In your case, as the oiler is foaming, the oil pump drive and gear are more than likely working, and your problem is a clog in the line someplace.
Good luck Mr Phelps, and should you or any of your team be caught or captured, the Secretary will disavow any knowledge that I told you anything about fixing the oil pump on a Stihl 1123 saw. :greenchainsaw:
BTW: there is no need to pull the muffler to deal with an oiler on a 1123 saw that I am aware of. ??? *Shrug*
