Stihl FS 100 Fuel Mix Ratio (Specs & Mixing)
The STIHL FS 100 uses a 50:1 gasoline-to-2-cycle-oil mix, even though its 31.4 cc 4-MIX engine has four-stroke valve operation. Measure 20 mL of suitable air-cooled 2-cycle oil per 1 L of fresh gasoline. Do not use straight gasoline or add crankcase oil. Check your model’s manual for fuel and service details.
A four-stroke label can mislead FS 100 owners into reaching for straight gas or crankcase oil. Don’t: this 4-MIX engine gets its lubrication from the fuel-and-oil mixture. Getting that mix right is a simple step that can help prevent serious engine damage and avoid a frustrating start to yard work.
Fuel Mix, Equipment Setup & Safety Preparation
The FS 100’s fuel ratio is the first specification to get right: 50 parts gasoline to 1 part 2-cycle oil by volume. The engine uses four-stroke valve operation, but it is designed to run on premixed fuel. Check the operator’s manual for your exact model year and market before fueling.
The correct FS 100 fuel ratio
Use fresh, unleaded gasoline rated at least 89 AKI, with no more than 10% ethanol, and quality oil intended for air-cooled 2-cycle engines. AKI means anti-knock index, the rating shown on US and Canadian pump labels. Do not use four-cycle crankcase oil in the mix.
| Gasoline | 2-cycle oil for 50:1 | Approximate finished mix |
|---|---|---|
| 1 L | 20 mL | 1.02 L |
| 2 L | 40 mL | 2.04 L |
| 5 L | 100 mL | 5.10 L |
| 1 US gal | 2.6 US fl oz | About 1.02 US gal |
The ratio is based on the amount of gasoline, not the final volume after oil is added. Use a marked measuring container or a proper premix bottle. A small measuring error matters more when mixing only a small batch, so measure rather than estimate.
The tank holds about 0.53 L, or 18 US fl oz. That is the amount it can hold, not a reason to prepare a large batch. Mix only what you expect to use, and keep it in a clean, approved fuel container.
How to mix fuel
- Work outdoors, away from flames, sparks, cigarettes, and other ignition sources. Use a clean, approved fuel container.
- Measure the fresh gasoline and the correct amount of 2-cycle oil.
- Add the measured oil to the container, then add gasoline. Close the cap and gently mix. Follow any mixing instructions printed on the oil product.
- Label the container with the fuel type and mixing date. Do not use a container that held an incompatible chemical or has dirt or water in it.
Fuel should be mixed before it goes into the trimmer. Never pour oil directly into the FS 100 tank and guess at the ratio. If you are unsure what is already in the tank, do not top it off with a different fuel. Drain and handle the old fuel according to the manual and local rules, then refill with a known, correctly measured mix.
Before refueling or starting
Wear eye and face, hearing, hand, leg, and foot protection suited to the attachment and job. Before fueling, stop the engine and let it cool. Refuel outdoors, wipe up spills, and secure the fuel cap before starting. Never refuel a running or hot machine.
Before operation, check the guard, cutting attachment, harness, controls, fasteners, and fuel system. Use only an approved attachment and matching guard. Keep bystanders clear, and do not start the machine with the cutting attachment close to people or objects.
Key takeaway: Use 50:1 premix, not straight gasoline. When the fuel history is unknown, replace it with a correctly measured batch instead of guessing.
Field Trimming and Landscaping Operation
Correct fuel is only part of a reliable start. The FS 100 also needs the proper starting procedure, a clear work area, and a sound cutting setup. Follow the manual for cold- and warm-start steps, since control layouts and instructions can differ by version.
Set the machine on stable ground before starting, with the cutting attachment clear of people and objects. Use the manual’s cold-start procedure when the engine is cold and its warm-start procedure when it has recently run. Do not improvise by changing carburetor screws or using an unrelated model’s instructions.
As a field check, I separate fuel questions from starting questions. If the machine has been sitting, confirm the fuel is fresh and mixed at 50:1 before treating a hard start as a carburetor fault. This simple check can prevent unnecessary adjustment or part replacement.
In-use checks
Once running, keep a firm grip and maintain control of the machine. Watch for hidden obstacles, such as wire, stones, or debris, that could strike or damage the cutting attachment. Stop if the attachment guard is missing, damaged, or wrong for the attachment.
Stop the engine and wait for the attachment to stop before moving through a work area, clearing debris, or inspecting the cutting head. If you notice a fuel leak, unusual vibration, loss of power, or a damaged part, stop and investigate before resuming.
A common troubleshooting scenario is a trimmer that starts, then runs poorly after storage. Old or incorrect fuel is one possibility, but it is not the only one. Check the fuel type and age first, then inspect for leaks, blocked components, or damage as directed by the operator’s manual. Do not assume the carburetor needs adjustment.
Key takeaway: Start and operate by the model-specific instructions. Stop the engine before inspection, and do not keep working if the guard, attachment, or fuel system is damaged.
Maintenance, Inspection & Carburetor Service
Routine checks can help distinguish a fuel problem from a mechanical one. After work, switch off the engine, wait for the cutting attachment to stop, and let the machine cool before cleaning or inspection. Use the FS 100 manual for service intervals and procedures.
Remove grass and debris from the machine, taking care around hot parts. Check the cutting attachment, guard, harness, fasteners, controls, fuel lines, and tank area for wear or damage. If you find a leak, do not run the trimmer until the issue has been repaired.
| Symptom | First checks | Avoid |
|---|---|---|
| Engine will not start after storage | Confirm fresh 50:1 fuel; follow the manual’s starting steps | Repeatedly cranking with unknown or stale fuel |
| Engine starts but runs poorly | Check fuel age, mix, leaks, and visible blockage | Changing carburetor screws by guesswork |
| Engine loses power or runs unusually hot | Stop; confirm correct fuel and check for damage or blockage | Continuing to run it on straight or lean-mixed fuel |
| Fuel is leaking | Stop and keep ignition sources away; arrange repair | Starting the engine to “test” the leak |
The FS 100’s carburetor controls the fuel-and-air mixture delivered to the engine. Idle and maximum-speed settings are model-specific; do not use chainsaw tuning values or settings copied from another STIHL model. If an adjustment or repair is needed, use the exact manual or ask an authorized STIHL dealer.
Straight gasoline or an incorrect lean premix can cause rapid overheating and engine seizure. If you discover that the machine ran on the wrong fuel, stop using it. Correcting the mix does not reverse damage that may already have occurred, so have the trimmer checked if it now has poor compression, unusual noise, or continuing performance problems.
Key takeaway: Check fuel and visible condition before suspecting carburetor settings. Leave model-specific carburetor adjustment to the correct service instructions or a qualified technician.
Storage and Seasonal Fuel Care
Storage choices affect whether the FS 100 is ready when the season starts. Use the operator’s manual for instructions on fuel remaining in the tank, long-term storage, and fuel-system care. Do not assume a fuel mix stays usable indefinitely.
Prepare only the amount of premix you expect to use, and mark its mixing date. Keep fuel in a tightly closed, approved container, stored as directed by the fuel maker and local rules. Keep it away from heat, flames, and children. If fuel is old, contaminated, or its mix is uncertain, do not pour it into the trimmer.
Before putting the machine away, stop it, let it cool, remove debris, and inspect for damage or leaks. Store the trimmer as its manual directs, with the cutting attachment protected and out of reach. Before the next season, check the fuel, machine condition, and starting procedure rather than assuming last year’s setup is ready.
For a trimmer that sat through the off-season, a practical sequence is:
- Check the manual’s storage instructions and inspect the machine for leaks or damage.
- Dispose of questionable fuel safely, following local guidance.
- Prepare a fresh 50:1 batch using measured gasoline and 2-cycle oil.
- Refuel only when the engine is cool, then use the manual’s starting steps.
Key takeaway: Fresh, correctly measured fuel and a pre-use inspection are better than trying to rescue a questionable batch after the machine refuses to start.
Frequently Asked Questions
What is the fuel mix ratio for a STIHL FS 100?
The FS 100 uses a 50:1 gasoline-to-2-cycle-oil premix. That equals 20 mL of oil per 1 L of gasoline.
Is the FS 100 a four-stroke engine?
It has STIHL 4-MIX four-stroke valve operation, but it is lubricated by premixed gasoline and 2-cycle oil. It does not use separate crankcase oil in normal operation.
Can I use straight gasoline in the FS 100?
No. Straight gasoline can cause rapid overheating and engine seizure. Use the specified 50:1 premix.
How much 2-cycle oil goes in one US gallon of gas?
Use about 2.6 US fluid ounces of 2-cycle oil for 1 US gallon of gasoline to make a 50:1 mixture.
What gasoline does the FS 100 need?
Use fresh, unleaded gasoline rated at least 89 AKI and with no more than 10% ethanol, subject to the instructions for your model year and market.
Can I use four-cycle oil instead of 2-cycle oil?
No. Use quality oil made for air-cooled 2-cycle engines. Four-cycle crankcase oil is not a substitute for the specified premix oil.
How much fuel does the FS 100 tank hold?
The tank capacity is approximately 0.53 L, or 18 US fluid ounces. Check your specific manual for the stated capacity.
Should I add oil to the crankcase?
No separate crankcase oil is specified for normal FS 100 operation. The engine’s lubrication comes from the 50:1 fuel mix.
Can I use chainsaw carburetor settings on the FS 100?
No. Carburetor settings are model-specific. Use the FS 100 manual or consult an authorized STIHL dealer.
What should I do if the trimmer has old or unknown fuel?
Do not guess at the ratio or top it off with fresh fuel. Handle and dispose of the questionable fuel as directed by local rules, then use a fresh, correctly measured mix.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
