This demonstration guide answers one field question about seats that swell and will not seal the needle. Measurements come before parts, and the notes are not a generic equipment FAQ.
Diagnose before replacing parts
Start with needle leak-down after a soak. That reading separates seats that swell and will not seal the needle from adjacent faults that share the same complaint.
If the measurement is out of spec, treat the needle and seat as the primary path. If it is in spec, do not replace the needle and seat just to complete a parts list.
Repair the failed system
The usual correction is the needle and seat, installed to the documented spec for this complaint.
A demonstration shop records the finding so the next technician is not repeating a guess about seats that swell and will not seal the needle.
Stop before you make it worse
Do not running 10% ethanol indefinitely because it "is all the station has". That shortcut hides the original measurement and creates a second failure.
If the machine still fails after the needle and seat is corrected, convert the visit to a broader diagnosis rather than stacking parts.
Written and published by Ridgeline Small Engine.