STIHL FS56, FS91 and FS111 fuel and service differences
The FS56, FS91 and FS111 all need 50:1 mixed fuel, but they are not the same engine. FS56 is a two-stroke; FS91/FS111 are four-strokes with mixture lubrication and valve-service requirements.
The latter still need premix rather than straight gasoline and crankcase oil. Match the exact model before selecting a plug, filter or service procedure.
Shared fuel ratio, different service hardware
| Model | Engine /tank | Approved plug /gap | Valve information |
|---|---|---|---|
| FS56 family | 27.2 cc two-stroke /0.34 L | CMR6H or USR4AC /0.5 mm | Two-stroke design |
| FS91 family | 28.4 cc mixture-lubricated four-stroke /0.71 L | USR7AC /0.5 mm | Inlet/exhaust 0.10 mm |
| FS111 family | 31.4 cc mixture-lubricated four-stroke /0.71 L | USR7AC /0.5 mm | Inlet/exhaust 0.10 mm |
The manual’s valve figure is not a complete adjustment procedure. Hard cranking or lost power can call for a qualified valve check on the mixture-lubricated models; copying that task onto a FS56 does not describe its engine. A shared 0.5 mm gap also does not make USR4AC and USR7AC interchangeable.
The FS56 felt filter is not a washable foam element
The FS56 manual permits knocking out or blowing out its felt element as a temporary measure and explicitly says not to wash it. Replace a damaged or unserviceable filter. Keep loosened dirt out of the intake while removing it.
Air-cleaner instructions from another trimmer can require a different treatment. Compare the fitted housing and element before cleaning or buying a kit. STIHL’s 41800071042 kit lists FS91/FS111 and related machines; that listing does not make it the FS56 kit.
Log the operation rather than guessing a year of use
The cited manuals call for plug replacement after about 100 operating hours, sooner if badly eroded, and for closer service in demanding conditions.
For an eight-hour working week, twelve weeks add 96 running hours; four more hours reach the 100-hour plug milestone. Log the replacement separately from fuel mixing or filter work.
If the cutting attachment moves at idle or a control fails to stop the machine as specified, stop using it and obtain the required adjustment or repair. Do not remove the guard or select an unapproved cutting attachment to compensate for poor cutting.
Prepare fuel in an approved container using the matching market manual’s fuel and oil requirements. For two liters of gasoline, 50:1 means 40 ml oil. The instructions for the driven attachment, deflector and harness also remain part of safe use.
STIHL FS 91 and FS 111 trimmers: fuel mix, spark plug, valve clearance and maintenance
The STIHL FS 91 and FS 111 trimmers are four-stroke engines that run on a 1:50 mix of gasoline and STIHL two-stroke oil, with a 0.71 liter fuel tank. The spark plug is a suppressed Bosch USR 7 AC gapped 0.5 millimeter and replaced after about 100 hours, and valve clearance is 0.10 millimeter on both the inlet and exhaust. STIHL USA’s trimmer service kit, 4180 007 1042, holds the air filter, fuel filter and plug.
Stop the engine, let it cool and disconnect the spark plug boot before service, and always fit the deflector that goes with the cutting attachment. They print metric units, so the US measures below are converted.
Two four-stroke trimmers that still take mixed fuel
The STIHL FS 91 is a 28.4 cc trimmer rated 0.95 kW (1.3 hp), and the FS 111 is a 31.4 cc trimmer rated 1.05 kW (1.4 hp). The R models have a loop handle. These are STIHL’s four-stroke engines, so the manual gives a valve clearance, but they use mixture lubrication, which means fuel is still gasoline mixed with STIHL two-stroke oil and there is no sump to fill.
| Item | FS 91 and FS 91 R | FS 111 and FS 111 R |
|---|---|---|
| Displacement and output | 28.4 cc; 0.95 kW (1.3 hp) | 31.4 cc; 1.05 kW (1.4 hp) at 8,000 RPM |
| Fuel to oil ratio | 1:50 with STIHL two-stroke oil | 1:50 with STIHL two-stroke oil |
| Fuel tank | 710 cm³ (0.71 L), about 24 US fl oz | 710 cm³ (0.71 L) |
| Spark plug | Bosch USR 7 AC (suppressed) | Bosch USR 7 AC (suppressed) |
| Electrode gap | 0.5 mm; replace after about 100 hours | 0.5 mm; replace after every 100 hours |
| Valve clearance | Inlet and exhaust 0.10 mm | Inlet and exhaust 0.10 mm |
| Idle speed | 2,800 RPM | 2,800 RPM |
| Dry weight, without cutting attachment | 5.8 kg (FS 91); 5.5 kg (FS 91 R) | 5.8 kg (FS 111); 5.5 kg (FS 111 R) |
| Gasoline | STIHL two-stroke oil, 1:50 |
|---|---|
| 1 liter | 0.02 liter (20 ml) |
| 5 liters | 0.10 liter (100 ml) |
| 10 liters | 0.20 liter (200 ml) |
| 20 liters | 0.40 liter (400 ml) |
STIHL says to pour the oil into an approved safety fuel canister first, then add the gasoline. If the machine is low on power, hard to start or runs poorly at idle, it says to check the spark plug first. Fit a new plug after about 100 operating hours, or sooner if the electrodes are badly eroded, and use only the suppressed type STIHL approves.
The service kit and the schedule
STIHL USA’s trimmer service kit, part 4180 007 1042, is listed for the FS 91 and FS 111 and holds an air filter, a fuel filter and a spark plug. It is on the STIHL service kit page.
The manuals’ maintenance chart lists checking the machine for condition and leaks, testing the throttle trigger, lockout, stop switch and choke, cleaning the air filter and replacing it when worn, cleaning the cooling air intakes and cylinder fins, setting the idle speed so the cutting attachment does not turn, setting the plug gap, checking the valve clearance when power is low or the engine is hard to crank, and lubricating the gearbox. STIHL says to shorten the intervals in dusty conditions.
1:50
Fuel to oil
20 ml of oil per liter of gasoline.
USR 7 AC
Bosch plug, both
Gap 0.5 mm; replace after 100 hours.
0.10 mm
Valve clearance
Inlet and exhaust on both trimmers.
STIHL FS 91 and FS 111 trimmers: questions
What fuel mix does a STIHL FS 91 use?
1:50, which is one part STIHL two-stroke engine oil to 50 parts gasoline, or 20 millilitres of oil per liter. The FS 91 is a four-stroke engine with mixture lubrication, so it has no sump to fill and runs on mixed fuel. Pour the oil into an approved safety canister first, then the gasoline.
What spark plug does a STIHL FS 91 or FS 111 use?
A suppressed Bosch USR 7 AC, with an electrode gap of 0.5 millimeter. STIHL says to fit a new plug after about 100 operating hours, or sooner if the electrodes are badly eroded, and to install only suppressed plugs of the approved type. Check it first if the trimmer is hard to start.
What are the valve clearances on a STIHL FS 91 and FS 111?
0.10 millimeter for the inlet valve and 0.10 millimeter for the exhaust valve, on both models. STIHL’s chart says to have the valve clearance checked if power is low or the engine is hard to crank. Adjusting it is work for a servicing dealer.
How big is the fuel tank on a STIHL FS 91?
710 cubic centimeters, which is 0.71 liter, on both the FS 91 and FS 111. The FS 91 engine is 28.4 cc rated 0.95 kilowatt, and the FS 111 is 31.4 cc rated 1.05 kilowatt at 8,000 RPM. Both idle at 2,800 RPM.
What is the STIHL trimmer service kit for the FS 91?
STIHL USA’s trimmer service kit is part number 4180 007 1042. It holds an air filter, a fuel filter and a spark plug, and it is listed for the FS 91 and FS 111 along with the FC 91, FC 96, FC 111, FS 111 RX, KM 91, KM 111 and HT models. Some kits also include a spark arresting screen.
What is the difference between the STIHL FS 91 and FS 91 R?
The R has a loop handle and the FS 91 has a bike handle. The FS 91 weighs 5.8 kilograms dry, without the cutting attachment and deflector, and the FS 91 R 5.5 kilograms. The engine, plug and fuel figures are the same, as they are for the FS 111 and FS 111 R.
How do I set the idle on a STIHL FS 91?
The maintenance chart says to adjust the idle speed so the cutting attachment does not turn, and to check it at regular intervals. Idle is 2,800 RPM. If the cutting attachment turns at idle, or the engine stalls, have a servicing dealer adjust the carburetor.
How often should I clean the air filter on a STIHL FS 91?
The maintenance chart lists cleaning the air filter and replacing it when worn, and cleaning the cooling air intakes and cylinder fins. STIHL says to shorten the intervals in dusty conditions or when you work longer than normal each day. The replacement air filter is STIHL 4180 141 0300, which is also in kit 4180 007 1042.
Does the STIHL FS 91 trimmer require gearbox grease?
The FS 111 manual includes a section on lubricating the gearbox, and the chart lists gearbox lubrication. The manual shows where the lubrication point is and which lubricant to use, so follow its section rather than guessing an amount.
What cutting attachments can I use on a STIHL FS 91?
The manuals list mowing heads, metal mowing tools and, on the FS 111, circular saw blades, each with its own deflector or barrier bar. STIHL says to use only cutting attachments and accessories explicitly approved for the power tool. Sound power level is 104 decibels with a mowing head.
How long is a STIHL FS 91?
The overall length without the cutting attachment is 1,800 millimeters. The manual’s dry weights are 5.8 kilograms for the bike-handle model and 5.5 kilograms for the loop-handle R model. Add the cutting head, deflector and fuel, and a harness helps for long jobs.
How long can I keep mixed fuel for a STIHL trimmer?
STIHL’s manuals say to store fuel mixture only in approved containers, in a dry, cool and secure place, and its blower manual says not to keep more than a 30-day supply. Mix only what you need, and use STIHL MotoMix if you use a trimmer only occasionally.
STIHL FS 56 string trimmers: fuel mix, spark plug and air filter
The STIHL FS 56 family runs on a 1:50 mix, which is 20 millilitres of two-stroke oil per liter of gasoline of at least 90 octane, and uses a suppressed NGK CMR 6 H or BOSCH USR 4AC spark plug with a 0.5 mm gap. The tank holds 0.34 liter. STIHL says to fit a new plug after about 100 operating hours.
Shut off the engine and disconnect the spark plug boot before servicing, except for carburetor and idle speed adjustment.
A 27.2 cc trimmer on a 50:1 mix
The STIHL FS 56, FS 56 R, FS 56 C and FS 56 RC are 27.2 cubic centimeter single-cylinder two-stroke trimmers rated at 0.8 kW (1.1 hp) at 8,500 RPM, with an idle speed of 2,800 RPM. They weigh between 4.7 and 5.2 kilograms dry, and the fuel tank holds 340 cubic centimeters (0.34 liter).
| Item | Specification |
|---|---|
| Engine | Single-cylinder two-stroke, 27.2 cc; bore 34 mm, stroke 30 mm |
| Fuel | High-quality gasoline, octane at least 90 ROC; alcohol content over 10% not for manually adjustable carburetors |
| Two-stroke oil | STIHL two-stroke oil, or oil classified JASO FB, FC or FD, or ISO-L-EGB, EGC or EGD |
| Mixing ratio | 1:50 (1 part oil to 50 parts gasoline) |
| Spark plug | Suppressed plug, NGK CMR 6 H or BOSCH USR 4AC |
| Electrode gap | 0.5 mm |
| Fuel tank | 340 cc (0.34 L) |
| Idle speed | 2,800 RPM |
| Maximum output shaft speed | 8,600 RPM |
How much oil for 1:50
| Gasoline | Two-stroke oil |
|---|---|
| 1 liter | 20 ml |
| 5 liters | 100 ml |
| 10 liters | 200 ml |
| 15 liters | 300 ml |
| 20 liters | 400 ml |
| 25 liters | 500 ml |
Pour the oil into an approved safety canister first, then the gasoline, and mix until the oil is dispersed. STIHL says fuel mixture deteriorates with age: mix only as much as you need for a few weeks and do not store it longer than 30 days. It says pre-mixed MotoMix keeps longer.
Air filter, spark plug and the schedule
| Item | What the manual says |
|---|---|
| Spark plug | Fit a new one after about 100 operating hours, or sooner if the electrodes are badly eroded. Readjust the gap when it needs it. |
| Air filter | A felt element. Do not wash it: knock it out on your palm or blow it out with compressed air, and fit a new one when it is damaged or cannot be cleaned. |
| Idle speed | If the cutting attachment keeps running at idle, have the machine checked and repaired by your dealer. |
| Fuel pickup body | Check it in the tank and replace it when the schedule says. |
| Fuel tank | Clean it, and drain and clean it before long storage. |
The maintenance chart
| When | What |
|---|---|
| Before starting work and after each refuelling stop | Inspect the machine for condition and leaks, check the control handle, the idle adjustment (the cutting attachment must not run) and the cutting attachment and its tightness; before starting work also check the manual fuel pump. |
| After finishing work or daily | Clean the machine and inspect the cooling inlets. |
| Weekly | Inspect the air filter visually. |
| Every 12 months | Replace the pickup body in the fuel tank. |
| If a problem occurs | Clean the air filter, clean the fuel tank, check the pickup body and readjust the spark plug electrode gap. |
| Every 100 hours | Replace the spark plug. |
| If damaged or as required | Replace damaged parts, the air filter and safety labels; sharpen a metal cutting attachment; retighten accessible screws and nuts; adjust the idle speed. |
Parts to buy for the STIHL FS 56 string trimmers
| Part | Part number |
|---|---|
| Service kit | 4144 007 1800 |
| Air filter | 4144 124 2801 |
| Fuel filter | 0000 350 3513 |
| Spark plug | 0000 400 7011 |
The kit numbers for every STIHL saw, trimmer and blower are on the STIHL service kit page.
1:50
Fuel mix
20 ml of oil per liter of gasoline.
CMR 6 H
NGK plug
Or BOSCH USR 4AC; gap 0.5 mm.
100 h
Plug replacement
Or sooner if badly eroded.
Who the FS 56 is meant for
Review N Picks describes the FS 56 RC-E as a gas trimmer with a 27.2 cc 2-MIX engine delivering 1.07 to 1.1 brake horsepower, suited to serious homeowners and light commercial users, while Fire and Saw calls it very good but notes that the pro-level STIHL trimmers are much better.
That positioning explains the service kit: it is a homeowner-grade trimmer used often, so the fuel filter and the spark plug are the parts worth replacing yearly.
Idle speed and the air filter on an FS 56
The STIHL instruction manual covers the carburetor in two short cases. If the engine stops while idling, warm it for about 3 minutes and turn the idle speed screw (LA) slowly clockwise until it runs smoothly with the cutting attachment still.
If the cutting attachment turns at idle, turn LA counterclockwise until it stops, then another 1/2 to 3/4 turn the same way. If it still turns, the manual says to have a servicing dealer check the machine.
The air filter is a felt element. Move the choke lever, undo the screw in the filter cover, ease the cover over the choke lever, lift it away and clean loose dirt from around the filter before you take the element out. STIHL says to fit a new element, and that as a temporary measure you can knock the old one out on your palm or blow it out with compressed air, but not to wash it.
It also says to fit a new spark plug after about 100 operating hours, or sooner if the electrodes are badly eroded.
Line feed and storage for an FS 56
With the TrimCut head, the manual says line feed operates only if both lines are still at least 2.5 cm (1 in) long, and to shut off the engine before adjusting line by hand. It says never to use wire or metal-reinforced line in place of nylon.
If the trimmer will be out of use for about 30 days or longer, it says to drain and clean the fuel tank in a ventilated area, press the manual fuel pump at least five times if one is fitted, then run the engine at idle until it stops. Remove, clean and inspect the cutting attachment, coat a metal one with corrosion inhibitor, and clean the machine, paying attention to the cylinder fins and air filter. The manual lists the FS 56 at 5.1 kg, the FS 56 R at 4.7 kg and the FS 56 C with ErgoStart at 5.2 kg, with a 27.2 cm³ engine and a 340 cm³ fuel tank.
- Service kit: Fits the FS 40, FS 50, FS 56 and FS 70, FC 56 and FC 70, HT 56 and KM 56.
- Air filter: Earlier number 4144 124 2800.
- Spark plug: NGK CMR 6 H; gap 0.5 mm.
STIHL FS 56 string trimmers: questions
What is the fuel mix for a STIHL FS 56?
1:50, which is 1 part STIHL two-stroke oil to 50 parts gasoline: 20 ml per liter, 100 ml for 5 liters, 200 ml for 10 liters.
What spark plug does a STIHL FS 56 use?
A suppressed plug, NGK CMR 6 H or BOSCH USR 4AC, with an electrode gap of 0.5 mm. STIHL says to install only plugs of the type it approves.
How often do I replace the STIHL FS 56 spark plug?
After about 100 operating hours, or sooner if the electrodes are badly eroded. Check the plug first if the engine is down on power, hard to start or runs poorly at idle.
What gasoline does the STIHL FS 56 need?
High-quality gasoline with an octane rating of at least 90 ROC. For manually adjustable carburetors, STIHL says not to use gasoline with more than 10% alcohol.
What oil can I mix in a STIHL FS 56?
STIHL two-stroke oil, or another high-performance oil classified JASO FB, FC or FD, or ISO-L-EGB, EGC or EGD. STIHL specifies HP Ultra or an equivalent to hold emission limits.
How big is the STIHL FS 56 fuel tank?
340 cubic centimeters (0.34 liter). The engine is 27.2 cc, rated 0.8 kW (1.1 hp) at 8,500 RPM.
How long can I keep mixed fuel for the FS 56?
Mix only as much as you need for a few weeks and do not store the mixture longer than 30 days. STIHL says pre-mixed MotoMix can be stored longer.
Can I wash the STIHL FS 56 air filter?
No. The manual says not to wash the felt filter: knock it out on your palm or blow it out with compressed air as a temporary measure, and fit a new element when it is damaged or dirty.
What is the idle speed of a STIHL FS 56?
2,800 RPM. If the cutting attachment continues to turn at idle, have the machine checked and repaired by a servicing dealer.
What is the maximum speed of the FS 56 output shaft?
8,600 RPM at the cutting tool holder, per the specification. That is separate from the engine’s 8,500 RPM rated power speed and its 2,800 RPM idle.
How much does a STIHL FS 56 weigh?
Dry, without cutting attachment and deflector, 5.1 kg for the FS 56, 4.7 kg for the FS 56 R, 5.2 kg for the FS 56 C with ErgoStart and 4.8 kg for the FS 56 RC with ErgoStart.
What are the STIHL FS 56 filter and service kit part numbers?
The service kit is STIHL 4144 007 1800. It holds the air filter 4144 124 2801, the fuel filter 0000 350 3513 and the spark plug 0000 400 7011, and fits the FS 40, FS 50, FS 56 and FS 70 as well as the FC 56, FC 70, HT 56 and KM 56.