STIHL 4-Mix seems to be starving for fuel

[Merkava_4]

STIHL FS 91 R string trimmer.

It surges at high RPM and I can't seem to richen it enough with the high speed mixture needle to get the surge to go away. It's never done this before. Normally Too Rich will cause an RPM drop off like it's supposed to. I may try fishing the fuel filter out of the tank for examination unless you guys got a better idea. All I want is the trimmer to run at a steady RPM at full throttle.

[bgsengine]

Sounds like an air leak to me. That or you got a pinhole in a fuel line, perhaps. Try the basic testing? Fuel system pressure/vacuum? crankcase leakdown? stiff diaphragms?

[Skywatcher]

Hi Merkava

A fairly common issue with those Stihl hybrid engines is valves going out of adjustment affecting top end performance. I forget what the specs are for those units, hopefully someone else here has them for you. All the very best,

Sky

[KE4AVB]

The valve clearance for 4180 engine is .004 (0.10mm) both valves.

Just note you will need a small width feeler gauge so you might just get the kit from Stihl has the gauge and a rocker cover gasket set. Stihl PN 4180 007 1005.

[KE4AVB]

From service manual.

Engine does not deliver full power
or runs erratically

Secondary air seepage through faulty gaskets on spacer flange > Install new gaskets

Piston rings worn or broken. > Install new piston rings

Muffler / spark arresting screen carbonized. > Clean muffler (inlet and exhaust openings), replace spark arresting screen (if fitted)

Air filter element dirty. > Fit new air filter element

Fuel/impulse hose severely kinked or damaged. > Fit new hoses or position them free from kinks

Pump diaphragm torn. > Install new pump diaphragm

Decompression lever sticking. > Install new cam gear