To amplify what I've said above, there is a small block of metal which incorporates the non-return valve, and has the main jet pressed into it. This block is pressed into the carburetor body from below (i.e. from the metering chamber) until it reaches a stop, which ensures that it projects the right distance into the venturi. From the workshop manual it looks as if this may be one of the cases where to remove that block, you are supposed to drill a hole in it from the metering chamber, then screw in the self-tapper-like thread on the end of the little slide hammer puller that is included in Walbro's toolkit. You extract the original block, then you drive or press in a new block. However I don't see why you would want to do that until you had tried blowing air or carb cleaner into the yellow-circle drilling, to see whether it comes out through the main jet. If it does, all is good. If it doesn't, after some head-scratching it might be time to resort to the slide-hammer puller, provided you can get one, plus you can get another jet block cheaper than you can get another WT carburetor from a junked chainsaw. However I'm guessing that if you blast some carb cleaner into the yellow circle passage, it will come out through the main jet and all will be well.
