Stihl KM 55 Engine Stalls (Causes & Fixes)
A KM 55 that stalls may have stale or incorrectly mixed fuel, a blocked filter or vent, a dirty air filter, a fouled spark plug, or an idle or carburetor fault. Start with safe checks and fresh 50:1 mix, then match the stall pattern to the cause. Stop if it races, overheats, or loses power.
A stall can turn a quick trimming job into a frustrating repair, especially when the tool ran fine last season. The useful clue is often when it stops: at idle, as you squeeze the throttle, or only after it warms up. Work through the checks in order, and do not guess at carburetor settings.
Equipment Setup and Safety Preparation
The KM 55 is a 27.2 cm³, air-cooled, two-stroke KombiEngine powerhead. It must be used with a KombiTool approved for the specific KM 55 version. Before inspection, stop the engine, let it cool, and disconnect the spark-plug boot.
Wear eye, hearing, hand, leg, and foot protection suited to the fitted attachment. Check the applicable tool manual, make sure the attachment is secure and unobstructed, and keep people clear before starting. The KM 55 is not a chainsaw; chainsaw cutting specifications and procedures do not apply.
Before troubleshooting, note the exact symptom and the attachment fitted. A head or tool that is jammed or damaged can place extra load on the engine and may look like an engine fault. With the engine off and cool, inspect the attachment and drive connection for debris or damage. Do not remove guards or bypass safety controls to test the machine.
Identify the stall pattern
A symptom is the condition that causes the engine to stop. Record whether the KM 55 stalls at idle, on throttle, or after warming, and whether it restarts right away. These details narrow the checks without encouraging guesswork or unnecessary carburetor adjustment.
| When it stalls | First checks | What the result may point to |
|---|---|---|
| At idle | Attachment movement, idle setting, air filter, fuel flow | Low idle, restriction, or carburetor fault |
| When throttle is applied | Fuel age and flow, plug, air filter, muffler screen | Restricted fuel or exhaust, ignition, or carburetor issue |
| Only when warm | Tank vent, restart behavior, unusual heat or power loss | Vent, heat-sensitive ignition, or fuel/carburetor fault |
| During attachment work | Approved fit, obstruction, tool condition | Excess load or attachment problem |
These patterns guide inspection; they do not prove a particular part has failed. If the attachment moves at idle, stop using the machine until the cause is corrected. Next step: identify the stall pattern, then check fuel and external restrictions before considering service work.
Field Operation and Stall Diagnosis
A controlled restart helps separate a basic fuel or filter problem from a fault that needs service. Follow the KM 55 manual’s cold- or warm-start procedure, and test only with the approved attachment installed and the work area clear. Do not run the attachment in a way that could injure someone.
Start with fresh, correctly mixed fuel and a clear work area. If the engine stops, note whether it sputters, fades, or cuts out suddenly, then allow it to cool before inspection. A sputter or gradual loss of power can occur with restricted fuel or exhaust flow, but those signs alone cannot confirm the cause.
If it stalls at idle: Check whether the attachment moves while the engine is idling. A moving attachment is unsafe; stop the engine and do not try to correct it by turning carburetor screws. If it stays still but the engine dies, inspect the air filter and fuel supply. A low idle setting or carburetor fault is also possible.
If it stalls on throttle: Check for old or incorrect fuel, a restricted pickup filter, damaged fuel lines, a dirty air filter, a fouled plug, or a blocked muffler screen. A fuel-metering problem may also stop the engine from responding to added throttle. Clean or replace service parts only as the applicable manual directs.
If it stalls after warming: A restricted tank vent can interrupt fuel flow as the engine runs. Heat-sensitive ignition or carburetor faults are also possible. If the engine restarts after cooling but repeatedly stalls again when hot, stop repeated testing and arrange diagnosis rather than assuming the plug is the cause.
A rising idle, surging, unusual heat, or sudden power loss may signal a lean-running condition. A lean condition means the engine is getting too little fuel for the air entering it. An air leak, restricted fuel supply, or incorrect carburetor setting can cause overheating and severe engine damage. Stop immediately; do not keep running it to see whether it clears.
Diagnostic scenario: Suppose a KM 55 starts cold, idles briefly, then dies when you open the throttle. I would first replace suspect fuel with fresh mix, inspect the air filter and pickup filter, and check the plug and muffler screen. If those checks do not resolve the symptom, a technician should assess fuel metering and ignition.
Next step: repeat a short, safe test only after basic checks. If the same stall returns, record when it happens and seek service instead of making unapproved adjustments.
Maintenance, Fueling, and Carburetor Service
The KM 55 uses a two-stroke engine, so gasoline must be mixed with two-stroke oil. The specified ratio is 50:1 with STIHL two-stroke oil, or the oil maker’s stated ratio. The tank holds about 0.33 L; use the machine’s manual for fueling and handling instructions.
Do not use straight gasoline or guess the oil ratio. Drain suspect or old fuel into an approved container, then prepare fresh mix using a clean, suitable container and the product instructions. Fuel mix that has been stored too long or mixed incorrectly can contribute to hard starting, poor running, or stalling.
Check the fuel system: With the engine cool and the plug boot disconnected, inspect fuel lines for cracks, leaks, or damage. Check the fuel pickup filter and cap vent as described in the manual. Do not start the machine if you find a fuel leak; have damaged parts repaired first.
Check the air and exhaust paths: Inspect the air filter and clean or replace it according to the manual. Check the muffler screen for blockage, and clean it only as directed. Restrictions can affect how the engine runs, but removing parts or running without required components is not a repair.
Check the spark plug: The commonly listed plug is an NGK CMR6H with a 0.5 mm electrode gap. Confirm the correct plug and gap for your machine’s manual and installed parts before changing it. Inspect for fouling or damage, and replace it if required by the manual. A plug check is useful, but a clean-looking plug does not rule out every ignition fault.
| Check | What to do | Stop and get service if |
|---|---|---|
| Fuel | Replace suspect fuel with fresh, correct mix | Fuel lines leak or are cracked |
| Tank vent and pickup filter | Inspect as the manual directs | Fuel flow remains poor |
| Air filter | Clean or replace as specified | Engine still stalls after basic checks |
| Spark plug | Verify type and 0.5 mm gap against the manual | Plug or ignition fault returns |
| Idle | Adjust only by the model-specific procedure | Attachment moves or engine races |
| Carburetor | Do not turn capped or unapproved screws | Stalling persists or engine runs lean |
The idle speed is approximately 2,800 rpm, but verify the correct setting for your exact machine in its manual or label before adjustment. Attachment-specific maximum speed is not an engine idle or tuning specification. Adjust idle only as instructed, and only when the attachment remains still. Do not tune by ear or turn limiter-capped screws.
Internal carburetor work may require special tools and checks. If a basic inspection does not solve a repeated stall, a qualified technician can test fuel metering, ignition, and air leaks. For suspected lean running, ask for a pressure test rather than trying to compensate with an arbitrary screw adjustment.
Next step: finish only the maintenance tasks the manual allows. If the engine races, overheats, leaks fuel, or stalls again, stop and arrange repair.
Shutdown, Storage, and Seasonal Care
A good shutdown and storage routine can reduce fuel and filter problems when the KM 55 is next needed. Stop the engine, let it cool, clean debris from the powerhead and attachment, and follow the manual for storage and fuel handling. Keep fuel in an approved container.
After a work session, check for loose or damaged parts and clear debris only with the engine stopped and cool. Do not put hands near a hot muffler. At the end of the season, follow the KM 55 manual’s instructions for fuel storage or draining; storage procedures can vary by machine and fuel product.
Before the next season, inspect the fuel lines, filter, plug, air filter, and attachment condition. Use fresh fuel mix, then follow the manual’s starting procedure. If it starts but stalls again at the same point, the symptom pattern is useful information for a service technician.
I use a simple rule: if the engine only runs after repeated restarts, or the same stall returns after fresh fuel and basic checks, stop treating it as a routine seasonal issue. Continuing to run a machine with a restricted fuel supply or air leak risks engine damage. Next step: store it safely, note the symptoms, and get a qualified diagnosis if the fault persists.
Frequently asked questions
Why does my KM 55 start and then stall?
Old or incorrect fuel, a blocked filter or vent, a dirty air filter, or a carburetor fault can cause this. Check the basics first.
Why does it stall when I squeeze the throttle?
Check fresh fuel, fuel flow, the air filter, spark plug, and muffler screen. If it still stalls, have the carburetor and ignition checked.
Why does it only stall when hot?
A restricted tank vent, heat-sensitive ignition fault, or fuel/carburetor problem may be involved. Stop repeated testing and seek service if it keeps happening.
What fuel mix does a KM 55 use?
Use a 50:1 gasoline-to-STIHL two-stroke oil mix, or the oil maker’s stated ratio. Confirm instructions in the manual and never use straight gasoline.
What is the KM 55 idle speed?
It is approximately 2,800 rpm. Verify the exact setting for your machine before adjusting, and ensure the attachment does not move at idle.
Which spark plug does it use?
NGK CMR6H is commonly listed, with a 0.5 mm gap. Confirm the plug type and gap in the applicable manual.
Can I adjust the carburetor myself?
Adjust only as the model-specific manual directs. Do not turn limiter-capped screws or tune by ear; have internal faults checked by a qualified technician.
What if the attachment moves at idle?
Stop the engine and do not use the machine. Correct the cause before restarting; a moving attachment at idle is unsafe.
Can I keep using it if it surges or runs hot?
No. Stop immediately. Surging, racing, unusual heat, or power loss may signal a lean condition that can damage the engine.
Is the KM 55 a chainsaw?
No. It is a KombiEngine powerhead for approved KombiTools. Chainsaw chain, bar, and cutting specifications do not apply.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
