Stihl 250 Chainsaw Won’t Start? | Fast Fix Guide

If your MS 250 refuses to fire, you can usually bring it back with a handful of simple checks. Start with fuel, air, spark, and safety interlocks. Then move to carburetor settings and compression. The steps below are field-tested and arranged from fastest wins to deeper fixes.

Why A Stihl MS 250 Won’t Fire: Quick Diagnostics

Small two-strokes are simple. They need fresh premix, clear air, a good spark, and the controls set in the right spot. When one of those goes off, starting gets tough. Work through the list in order so you don’t chase your tail.

Symptom	Likely Cause	Fast Fix
Pulls but no pop	Stale fuel or wrong ratio	Drain and refill with fresh 50:1 premix
Fires, then dies	Flooded or choke left on	Open half-choke, hold WOT, pull 5–8 times
Hard pullback kick	Chain brake off, chain moves	Engage brake before starting
Wet plug	Over-choking	Remove plug, air-dry 10 minutes, retry on run
No spark	Bad plug or switch	Fit NGK BPMR7A and verify stop control
Idles then stalls	Low idle or air leak	Turn LA screw clockwise 1/8 turn
Only starts warm	Weak compression	Check cylinder, ring, and decomp valve

Step 1: Set Controls And Safe Start

Flip the master control to full choke for a cold engine, set the chain brake forward, and place the saw on firm ground. Pull until the engine coughs. Move the lever to half-choke, pull again until it runs, then nudge the throttle to settle to idle. For a warm engine, skip full choke.

This routine keeps flooding at bay and keeps the chain still while you pull. If the stop control was left off, the ignition is dead. Confirm the lever sits in the run position.

Step 2: Fix The Fuel Side

Old premix makes hard starts. Ethanol draws moisture and goes off in weeks. Mix a new batch at 50:1 with fresh unleaded and a quality two-stroke oil.

Inspect the fuel line for soft spots or cracks. A split line lets air in and starves the carb. The pickup filter should rattle free and look clean. If it’s dark or gummy, replace it. When a saw floods, open the cap, pull the plug, and let the cylinder air out before trying again.

Step 3: Restore Clean Air

A clogged filter enriches the mix and soaks the plug. Pop the top cover, lift the filter, and tap it clean. If you see heavy dust packed into the pleats or a torn seam, fit a new one. Make sure the choke plate moves freely and fully opens in the run position.

Step 4: Get Reliable Spark

Remove the spark plug and check color. Tan is good, jet-black and wet points to a rich or flooded start. Install an NGK BPMR7A or Bosch WSR6F with a 0.020-inch gap. Clip the boot snugly and confirm the lead isn’t frayed. If you can’t get spark after a new plug, inspect the stop switch wiring and the ignition module air gap.

Step 5: Baseline The Carburetor

If the engine fires but won’t stay lit, set a neutral baseline. Turn the H and L needles in gently until they seat, then back each out 1 turn as a starting point. Start the saw and warm it. Turn the LA idle screw clockwise until the chain just wants to move, then back off until it stops. Fine-tune the L for crisp throttle and the H for a clean four-stroke burble at full load.

Never set the H needle so lean that the engine screams with no burble under load. That shortens engine life. If you’re unsure, leave high-speed tuning to a dealer with a tach.

Step 6: Rule Out Flooding

Over-choking loads the cylinder with raw fuel. The tell is a wet plug and a strong fuel smell. To clear it, switch to run, hold full throttle, and pull until it fires. If that fails, remove the plug, pull the starter ten times to vent the chamber, refit a dry plug, and retry at half-choke.

Step 7: Check Safety Interlocks

The front hand guard must be pushed forward for the chain brake to engage during starting. The throttle lockout needs a full hand on the rear handle to release. If the brake band is jammed or the guard is half-set, the saw can stall the moment it lights. Reset the guard fully, start, then pull the top of the guard back to release the brake once it idles.

Step 8: Inspect For Air Leaks

Air leaks lean the mix and cause erratic starting. Look for a loose carb boot, cracked impulse hose, or a leaky crank seal. Spray soapy water around joins while it idles; bubbles reveal a leak. Any leak near the crankshaft needs pro service, since seals and pressure testing call for special tools.

Step 9: Compression And Top End

Healthy compression makes cold starts easy. If your arm says the pull feels weak, test with a gauge. Numbers around the mid-hundreds are normal on a fresh top end. Scuffed cylinder walls, stuck rings, or a blown decomp valve drop that figure. A stuck ring often comes from straight gas or a lean run. If compression is low, a top-end refresh may be the honest fix.

Correct Starting Sequence At A Glance

Here’s the routine many owners follow with success. Engage the brake, set full choke, pull to the first burp, move to half-choke, pull to run, tap the throttle to set idle, then release the brake. For a hot restart, go straight to run or half-choke. Any deviation can flood the chamber and stall the first fire-up.

When The Idle Won’t Hold

If the saw lights then fades, richen the L needle one eighth turn at a time until it picks up cleanly. Then raise the LA screw slightly to keep the chain still without dying. If the bar oil tank runs dry while the fuel still shows, refill oil; some machines prefer a touch more idle to push oil on a cold day.

Pro Tips That Save Time

• Keep two fresh plugs in the case. Swapping a fouled plug is faster than cleaning one in the field.
• Use fresh premix from a clean can. Label the date; discard old fuel after a month.
• Check the muffler screen. A plugged screen chokes airflow and ruins starts.
• Seat the air filter evenly. A skewed seal lets dust in and shortens engine life.
• Confirm bar oil flows. A bone-dry chain drags the clutch and fights the starter.

Reference Specs And Baselines

Item	Spec	Notes
Fuel mix	50:1	Fresh unleaded + two-stroke oil
Spark plug	NGK BPMR7A	Gap 0.020 in (0.5 mm)
Idle speed	By ear, stable	Chain still at idle
Carb baseline	H 1 turn, L 1 turn	Fine-tune warm
Air filter	Clean, intact	Replace if torn
Compression	Healthy mid-hundreds	Gauge reading

Cold Weather And Flood-Prone Starts

Winter mix and cold air thicken fuel and oil. Give the saw a prime, then use shorter pulls. The first cough may take longer than usual. Once it lights, hold a little throttle to keep it alive while it warms.

Some owners like a small shot of premix down the carb throat after a long lay-up. That can help wet the crankcase and seals. Don’t overdo it. If the saw still acts flooded, open the throttle fully and keep pulling in the run position until it clears.

Starter, Clutch, And Chain Drag Checks

A sticky starter rope or a dragging chain wastes energy and makes the pull feel heavy. Pop the side cover and spin the drum by hand; it should turn freely. If the chain tries to move at idle, back the LA screw out until it stops. Check the clutch springs for damage and the drum bearing for grit.

Pull the starter with the plug removed. The rope should retract smoothly and the pawls should engage every time. If the rope frays or the spring binds, fit a service kit.

Quick Troubleshooting Flow

Work down this simple order. One, fresh 50:1 and a clean filter. Two, verified spark with a new BPMR7A at the right gap. Three, correct choke sequence with the brake set. Four, baseline the carb and set a safe idle so the chain stays still. Five, rule out leaks and low compression if starts remain erratic. That path saves time and avoids random tweaks that mask the real issue. Keep brief notes between steps.

When To See A Dealer

Some faults demand bench tools or parts you may not have. If you see fuel weeping at the carb, a split intake boot, or a drippy crank seal, book service. The same goes for a sheared flywheel key, damaged ignition module, or stubborn no-spark after a new plug and a wiring check. Good shops can pressure-test the crankcase and set the carb with a tach so the engine lives a long life.

Safe Start And Fuel Ratio Resources

If you want the factory playbook, skim the official starting steps and the premix ratio guide. Both lay out the control positions, the brake setting, and the exact 50:1 numbers. They match the procedures used above and are worth bookmarking.