I have a Stihl Pro 026 that was not starting; see my post:
http://chainsawrepair.createaforum.com/stihl/026-pro-down/20/I rebuilt the carb last spring. Started it up, made several cuts with the saw and it ran great. Then two months later I went to start the saw and it never fired. I dumped the fuel put in fresh and still nothing. The only way to get it to fire was to squirt fuel on the face of the air filter. So I figured; screw it and yesterday I installed a new WT 194-1 carb (NOT the WT403B like I should have). I swapped the piece out that fits the air filter from the old carb. After a dozen pulls, the saw fired up. A few initial adjustments of fuel mixture screws I went to try it out. Saw ran great but over the course of a couple of hours it slowly degraded. First it got harder to start, then the low speed performance fell off. I made an adjustment to the low speed air screw and reset the idle and for a short time the saw work well but then it started running worse than ever. When I finally shut it down it was a 6-8 pull starter and would only run decent at full throttle. I had to feather the trigger to keep it running between cuts.
So now I’m wondering if it’s the fuel I’m using. I don’t use chain saw pre-mix. I use the same pre-mix as I do in my dirt bike, which is Maxima Castor 927 @ 40:1. This is great oil in off road 2-stokes and I’ve run in a several Stihl products but I’m wondering if the oil doesn’t agree with this style of carburetor.