🚨 Stihl FS 91 R Trimmer Engine starts and idles but hesitates and stalls out when revved up
Top Culprit: Loose or out-of-spec 4-Mix valve clearance
Why It Happens: The special hybrid 4-Mix engine relies on absolute valve timing; loose rocker arms decrease intake valve lift, starving the cylinder for fuel.
🛠️ The 60-Second Diagnostic Test
- Remove the top plastic engine shield shroud and take off the tiny chrome valve cover plate on top of the head.
- Set the piston to Top Dead Center and check the lash; if a standard feeler gauge slips through too loosely, adjustment is required.
💡 The Quick Fix Steps
- Loosen the valve locknut and set both valves to a tight 0.10mm clearance using a precision engine feeler blade.
- Torque down the retention locknuts, reinstall the valve cover with a fresh rubber gasket, and test throttle snap.