This demonstration guide answers one field question about lean high-speed mixture that scores the piston. Measurements come before parts, and the notes are not a generic equipment FAQ.
Diagnose before replacing parts
Start with tachometer rpm against the model ceiling. That reading separates lean high-speed mixture that scores the piston from adjacent faults that share the same complaint.
If the measurement is out of spec, treat the high- and low-speed needles as the primary path. If it is in spec, do not replace the high- and low-speed needles just to complete a parts list.
Repair the failed system
The usual correction is the high- and low-speed needles, installed to the documented spec for this complaint.
A demonstration shop records the finding so the next technician is not repeating a guess about lean high-speed mixture that scores the piston.
Stop before you make it worse
Do not tuning by ear until the exhaust note "sounds sharp". That shortcut hides the original measurement and creates a second failure.
If the machine still fails after the high- and low-speed needles is corrected, convert the visit to a broader diagnosis rather than stacking parts.
Written and published by Ridgeline Small Engine.