Unigas G258A Manual Of Installation - Use - Maintenance page 39

Attention! if it is necessary to change the head position, repeat the air and fuel adjustments described above.
Only if necessary, change the combusiton head position: to let the burner operate at a lower output, loose the VB screw and move pro-
gressively back the combustion head towards the MIN position, by turning clockwise the VRT ring nut. Fasten VB screw when the
adjustment is accomplished.
CAUTION: perform these adjustments once the burner is turned off and cooled.
VB
VRT
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PART III: OPERATION
"MAX" head position
"MIN" head position