Where are the vent drain holes?
There are 0.040 inch holes about 6 inches from the end of the vent. Here's the procedure from the service manual.
12-18. CHECKING FUEL VENTS. Field experience has demonstrated that fuel vents can become plugged, causing possible fuel starvation of the engine. Also, a plugged vent line may allow fuel in a full bay to pressurize the fuel bay cavity due to expansion. following procedure can be used to check the vent system.
(THRU SERIALS 177RG1051 AND F177RG0160)
a. Cover . 040 inch drilled holes approximately 6 inches from end of vent lines at trailing edges of wing tips.
b. Turn fuel selector valve to OFF and remove both fuel bay filler caps.
c. Using rubber tube attached to vent line at trailing edge of wing tip, blow into line. If air can be freely blown into fuel bay, vent line is open.
d. Repeat this procedure for fuel vent in opposite wing tip.
Caution: Be sure to uncover . 040 inch holes in vent lines at wing tips after completion of check.
(BEGINNING WITH SERIALS 177RG1052 AND F177RG0161)
a. Cover . 040 inch drilled holes approximately 6 inches from end of vent lines at trailing edges of wing tips.
b. Turn selector valve to OFF position and remove both fuel bay filler caps.
c. Plug vent line at opposite wing tip.
d. Using rubber tube attached to vent line at trailing edge of wing tip, blow into line. If air can be freely blown into fuel bay, vent line is open.
e. Remove plug at opposite wing tip and install both UNVENTED fuel caps.
f. Again blow into rubber tube. If the vent line interconnect is open, air will emerge from opposite wing tip vent line.
g. Repeat this procedure for vent in opposite wing tip.
Caution: Be sure to uncover drilled holes and ends of vent lines at wing tips after completion of check.