Stihl MS 250 Oil-to-Gas Ratio (Fuel Mix Ratios)

The STIHL MS 250 uses a 50:1 mix of fresh unleaded gasoline and quality air-cooled two-stroke oil: 20 mL of oil per 1 L of gasoline, or 2.6 US fluid ounces per US gallon. Use gasoline rated at least 89 AKI, mix it in an approved container, and fill the fuel and chain-oil tanks separately.

Getting the mix right protects more than the engine. It helps prevent costly damage, makes fuel preparation repeatable, and removes one source of doubt before a cut. The key is to measure oil against the amount of gasoline you are mixing, not to estimate by eye or confuse the fuel mix with bar-and-chain oil.

I also check the saw’s own manual and nameplate before service. Equipment can vary by market or production version, and the manual is the final guide for the exact saw in front of you.

Equipment Setup and Safety Preparation

This section covers the correct fuel, the separate chain-oil supply, and the checks to make before starting an MS 250. Prepare the mix with an accurate measure, then confirm that the saw and cutting attachment are safe to run. A correct ratio is essential, but it cannot make a damaged or poorly maintained saw safe.

Mix the fuel at 50:1

A 50:1 ratio means 50 equal parts gasoline to one part two-stroke oil by volume. For each litre of gasoline, add 20 mL of oil. Use quality oil intended for air-cooled two-stroke engines and follow STIHL’s recommendation for the fuel you are preparing.

For a small batch, use a marked syringe or measuring cup that is reserved for engine oil. For a larger batch, use a container with clear volume marks. Do not rely on a bottle cap, an unmarked cup, or a guess.

Choose gasoline and mix safely

STIHL specifies fresh, unleaded gasoline with a minimum rating of 89 AKI, shown as (R+M)/2 on US pumps. The pump’s octane number may be displayed differently outside the United States, so check local labeling and the saw’s manual. Use a clean, approved fuel container and label it with the mix ratio and date.

Pour in part of the gasoline, measure and add the oil, then add the rest of the gasoline. Close the container and mix it thoroughly. Keep fuel away from flames, sparks, and hot surfaces. Never put straight gasoline in the fuel tank: a two-stroke engine needs oil in its fuel for lubrication, and too little oil can damage the piston and cylinder.

Keep chain oil separate

The chain-oil tank is not the fuel-mix tank. Fill it with STIHL bar-and-chain lubricant, then close both caps securely. Do not add two-stroke oil to the chain-oil tank or bar oil to the fuel can.

There is no single oil-flow rate or viscosity that fits every temperature and cutting condition. Follow the manual’s guidance for the saw and lubricant, and check that oil reaches the chain during operation. A full chain-oil tank does not confirm that the oil system is working.

Check the saw before starting

Wear a helmet, eye and face protection, hearing protection, chainsaw-protective legwear, gloves, and sturdy boots. Before starting, inspect the chain brake, chain catcher, throttle lockout, handles, guide bar, chain tension, and lubrication system. Clear the work area, plan an escape path when needed, and keep others outside the work zone.

The MS 250’s common cutting setups use a .325-inch chain pitch, but pitch alone does not identify the correct replacement chain. Read the guide-bar stamp and match the chain’s pitch, gauge, and drive-link count to the bar and sprocket. Confirm that the complete combination is approved for your saw.

Field Cutting and Landscaping Operation

This section explains how to start and use the saw after preparing the correct fuel mix. Safe operation still depends on condition, footing, and the work being done. Start with the chain brake engaged, check for proper idle and chain lubrication, and keep the guide-bar nose clear of objects.

Start and verify the saw

Set the saw on firm ground and follow the starting procedure in the manual. Keep the chain brake engaged during starting. Once the saw is running, check that the chain does not move at idle and that the chain receives oil. If the chain moves at idle or lubrication is not reaching it, stop and address the problem before cutting.

Do not use engine speed as a field adjustment target. STIHL service information lists approximately 2,800 rpm idle and 13,000 rpm maximum no-load speed for the MS 250. These are service specifications, not a reason to adjust the carburetor by ear. Carburetor work should follow the manual and applicable emissions rules.

Cut with control

Use two hands, maintain a stable stance, and keep the guide-bar nose away from the wood and nearby objects. Contact with the upper quadrant at the bar nose can cause rapid rotational kickback toward the operator. Keep the saw below shoulder height and use a properly functioning chain brake.

If the chain is dull, stop and sharpen it according to its maker’s instructions. A dull chain often cuts slowly and can make the operator push harder. Do not force it through wood. Set depth gauges to the chain manufacturer’s specification; for many STIHL chains, the reference is 0.65 mm (0.025 in), but the chain type governs.

For felling, assess lean, wind, overhead hazards, and tension or compression in the wood. Use an escape path and a sound hinge when felling is appropriate. Follow ANSI Z133 and applicable OSHA requirements; if the tree or site is beyond your training or equipment, use a qualified professional.

Maintenance, Sharpening, and Carburetor Service

This section helps you separate fuel-mix problems from chain, oiling, or tuning problems. A correct 50:1 mixture is only one part of reliable operation. Inspect the chain and bar, keep the air and fuel systems in serviceable condition, and use the manual for adjustments that affect engine speed or emissions.

Diagnose before changing the mix

If the saw runs poorly, do not assume that adding extra oil will fix it. Confirm the ratio, fuel freshness, and gasoline rating first. Then check for common maintenance issues, such as a dirty air filter, damaged fuel line, poor chain tension, or a chain that needs sharpening. Use the manual for inspection and service steps.

Adding more oil than specified does not correct stale fuel or a blocked filter. Nor should you reduce the oil amount to try to improve running. Keep the mix at 50:1 unless STIHL’s instructions for your exact saw and approved oil state otherwise.

Sharpen and tension to the chain’s markings

Use the file size and sharpening angles specified for the exact chain, not a guess based on the saw model. Check the chain’s markings or the chain maker’s chart. A wrong file size or angle can change how the cutters bite and can make the chain cut poorly.

Before adjusting tension, stop the engine and follow the manual’s procedure. The chain should sit correctly on the bar and remain movable by hand with suitable protection, while the drive links stay engaged. Recheck tension as directed, since a chain can loosen during use. Never run a loose or damaged chain.

Leave carburetor tuning to the correct procedure

The idle and maximum no-load rpm figures are service references. They do not replace the correct tachometer-based procedure, model-specific settings, or emissions requirements. If the saw will not idle correctly after basic checks, consult the manual or a qualified service technician rather than turning adjustment screws at random.

Shop scenario: A saw arrives with a 50:1 mix but still cuts poorly. I first check chain sharpness and tension, then inspect the air filter and oiling. This order helps avoid blaming the fuel for a problem that is actually in the cutting attachment or routine service.

Storage and Seasonal Fuel Care

This section covers what to do with mixed fuel when the saw will sit unused. Fuel ages, and storage instructions can depend on the fuel and the length of the break. Use the manual and fuel supplier’s directions rather than assuming a particular batch will remain suitable indefinitely.

Prepare only as much mix as you expect to use in the near term. Mark the container with the ratio and date, close it tightly, and store it in an approved location away from heat and ignition sources. Follow local rules for fuel storage and disposal.

If the saw has been stored, inspect it before use and follow STIHL’s instructions for preparing it to run. Do not pour old or uncertain fuel into the saw to “use it up.” If the fuel’s age or condition is unclear, consult the manual and local disposal guidance.

At the end of a cutting session, engage the chain brake, shut off the engine, and let the saw cool before refueling or cleaning. Refuel away from ignition sources. Clear chips, check the chain and bar for damage, and verify fuel and chain-oil levels before the next job.

Quick Answers: MS 250 Fuel Mix

These answers give the core numbers and checks at a glance. They do not replace the manual for the exact saw, chain, or service procedure. If a label or manual for your machine differs, follow the machine-specific instruction and confirm the model before mixing or servicing.

What is the MS 250 fuel ratio?
Use 50:1 gasoline to two-stroke oil.

How much oil goes into 1 L of gasoline?
Add 20 mL of suitable air-cooled two-stroke oil.

How much oil goes into 1 US gallon?
Add about 2.6 US fluid ounces.

What gasoline octane does STIHL specify?
At least 89 AKI, also written as (R+M)/2 in the United States.

Can I use straight gasoline?
No. The two-stroke engine needs oil mixed into its fuel for lubrication.

Is chain oil the same as two-stroke oil?
No. Use two-stroke oil in the measured fuel mix and bar-and-chain lubricant in the chain-oil tank.

Can I add extra oil for protection?
Do not change the specified ratio. Measure the mix at 50:1 and use the recommended oil.

Does every MS 250 use the same chain?
No. Match chain pitch, gauge, and drive-link count to the bar and sprocket, then confirm the combination in the manual.

What are the MS 250 idle and maximum no-load speeds?
Service specifications list about 2,800 rpm at idle and 13,000 rpm maximum no-load speed. They are not field tuning targets.

What should I do if the chain moves at idle?
Stop using the saw and follow the manual’s troubleshooting or service guidance before cutting.

(This article was written by one of our staff writers, David Hensley. Visit our Meet the Team page.)