This demonstration guide answers one field question about vacuum in the tank as fuel is used. Measurements come before parts, and the notes are not a generic equipment FAQ.
Diagnose before replacing parts
Start with vent airflow after a stall. That reading separates vacuum in the tank as fuel is used from adjacent faults that share the same complaint.
If the measurement is out of spec, treat the tank vent as the primary path. If it is in spec, do not replace the tank vent just to complete a parts list.
Repair the failed system
The usual correction is the tank vent, installed to the documented spec for this complaint.
A demonstration shop records the finding so the next technician is not repeating a guess about vacuum in the tank as fuel is used.
Stop before you make it worse
Do not loosening the cap as a permanent "fix". That shortcut hides the original measurement and creates a second failure.
If the machine still fails after the tank vent is corrected, convert the visit to a broader diagnosis rather than stacking parts.
Written and published by Ridgeline Small Engine.