Small Engines (Lawn Mowers, etc.): Walbro Carburetor, carburetor body, jacobsen snowblower

Question
I have an old Jacobsen snowblower with a 2 cycle engine that has a Walbro carburetor I want to make sure is adjusted properly.  It has only one adjustment screw encircled by a spring that is labeled "LO".  There is what looks like a screw in the "HI" position that I thought was adjustable, but it is either a preset and unadjustable screw or just a plug or something.  It has no spring around it and the head of the screw is tight to the carburetor body. So my conclusion is only the "LO" screw is adjustable but when turned doesn't seem to make much difference in engine response. The carb does not have a throttle and only runs at one high speed when started.  Exactly what does the "LO" screw adjust and how do I know when it is adjusted correctly?  What is that in the "HI" position hole. My concern is that I not run it too lean or too rich.  Any information I've found about Walbro has two adjusting screws.  Thank you for any details you can offer.

Answer
Check out these Walbro links:

http://walbro.com/techtipsmetering

http://walbro.com/servicemanuals.aspx

I have a feeling the high speed is fixed on your carb.  You would not notice much of a difference adjusting the L "low" speed needle as this needle mainly adjusts the low speed circuit when the enigne is idling.

You could manually hold the throttle in the idle position to adjust the L needle.

Eric