https://www.partstree.com/models/tec-640025c-tecumseh-carburetor/carburetor-0/ 
 
 
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The screw on the bottom of the float bowl is ALWAYS the high speed adjustment.
 
Low speed screws (if even there), are closer to the head, quite a bit smaller and 
very close to the throttle bore itself.  It's also placed (again if equipped) between the 
throttle and intake.   
 
Most new engines do not have ANY adjustments (EPA stuff)..  
Adjustments would be made by changing jet sizes or manually opening up a jet
(Very, very small jet drill bit's).  This would richen the mix being bigger.  Once drilled
you can't go back (short of buying another jet)..
 
For machines that have adjustment screws, to set them:
  1.   I push the throttle WIDE OPEN momentarily and adjust for smooth operation..
         
  2.   Adjust the low speed screw for a smooth, solid idle with no bogging when accelerating..
 
 
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