Stihl MS 170 Throttle Linkage (Diagram & Setup)
The MS 170 throttle linkage connects the trigger to the carburetor’s throttle lever. With the engine off and cool, check that the rod is seated, routed as shown in the correct parts diagram, and clear of the cover and controls. The throttle must open smoothly and snap back to idle when released. If it binds, do not start the saw.
Does the trigger move freely, but the carburetor lever fail to follow or return? That points to a linkage problem, not a carburetor adjustment. A careful inspection can prevent a runaway chain and keep you from replacing parts that are still sound. The exact rod shape and route can vary by production version, so use the diagram and manual that match your saw’s serial number.
Throttle Linkage Identification, Setup & Safety Preparation
The throttle linkage is the set of parts that transfers movement from the trigger to the carburetor. On an MS 170, check the trigger, trigger lockout, connecting rod or link, carburetor throttle lever, and return spring. The goal is smooth opening and full return, not a particular rod bend or guessed position.
Before opening the saw, stop the engine, let it cool, engage the chain brake, and follow the cover-removal steps in the manual. Wear gloves and keep the saw on a stable surface. If you cannot identify a part or routing with confidence, stop and find the serial-specific STIHL parts diagram rather than forcing a link into place.
Functional diagram: what connects to what
This simple diagram shows the movement path, not the physical routing. Link shape and position can vary, so it is not a substitute for the correct parts diagram.
Trigger lockout (must be pressed)
↓
Trigger → throttle rod/link → carburetor throttle lever
↓
return spring → idle stop
The trigger lockout prevents the trigger from operating unless you deliberately press the lockout. When the trigger is released, the carburetor lever should return to its idle stop. The choke and starting controls are separate; a throttle rod must not foul them or get trapped under the cover.
Step-by-step linkage setup
- Check the parts before moving them. Look for a loose rod, missing spring, cracked or bent link, worn contact point, or signs that the cover has pinched the linkage. Take a clear photo before removing a displaced part.
- Compare the route with the diagram. Confirm the rod is seated at both ends and follows the original path. It should not rub the cover, snag on the choke or start controls, or press against nearby parts.
- Test movement with the cover off. Deliberately press the trigger lockout, then operate the trigger by hand. Watch the carburetor lever. It should move smoothly toward open throttle, without sticking or jumping out of place.
- Release the trigger. The lever must snap fully back to its idle stop. If it returns slowly, hangs up, or stays open, do not start the saw. Check for misrouting or interference. A damaged spring, lever, or link needs the correct replacement part.
- Refit the cover and test again. Follow the manual’s cover procedure. Operate the lockout and trigger through their full travel. A linkage that works with the cover off but binds after refitting may be pinched or touching the cover.
Do not bend the rod to make it reach, stretch or substitute a spring, disable the lockout, or turn carburetor screws to mask a linkage fault. If correct routing and parts do not restore free movement, have a STIHL service dealer inspect it.
What the movement tells you
| Check | Normal result | If it fails |
|---|---|---|
| Lockout pressed, trigger pulled | Carburetor lever moves toward open throttle | Check rod seating, routing, and damage |
| Trigger released | Lever snaps fully back to idle stop | Check spring, binding, or interference |
| Lockout not pressed | Trigger cannot actuate the throttle normally | Stop; inspect lockout and related parts |
| Cover reinstalled | Trigger and lever still move freely | Check for a pinched rod or cover contact |
A trigger that does not return is a no-start, no-use defect. If it sticks while the engine is running, stop the saw immediately using its stop control, and do not resume cutting until the cause is corrected.
Illustrative diagnostic cases
These are examples of how to isolate a fault, not reports of a particular repair. In the first case, the trigger moves but the carburetor lever does not. I would inspect both rod ends and compare the route with the parts diagram before considering any replacement.
In the second case, movement is free with the cover removed but binds after the cover is installed. That points to contact or pinching during reassembly. In the third, the lever moves but does not return. I would check for a displaced or damaged return spring and friction at the lever, not adjust the carburetor.
Field Cutting Operation and Work-Area Controls
Once the linkage passes its checks, confirm the saw’s safety parts and cutting setup before starting. A working throttle does not prove the saw is safe to use. The chain brake, hand guard, chain catcher, trigger lockout, bar, and chain all need to be present and in sound condition.
Start and functional check
Use the cold- or warm-start procedure in the manual for your saw. Start on stable ground, with the saw held as the manual directs and the guide bar clear of people and objects. Do not test a questionable throttle by revving the saw.
At idle, the chain must stop. Pull the trigger and release it: the engine should return to idle promptly, and the chain should stop at idle. If the chain keeps moving, the trigger sticks, or engine speed does not settle, stop the saw and correct the fault before cutting. Do not use the saw with a binding linkage or disabled lockout.
The MS 170 is a compact saw with a 30.1 cm³ engine and rated power of 1.3 kW. Select the bar and chain combination listed for the saw’s serial number and intended task. The chain is commonly 3/8-inch P pitch, but pitch alone does not establish a match.
| Item | What to verify |
|---|---|
| Chain pitch | Must match the bar and sprocket specifications |
| Chain gauge | Must match the bar groove; do not infer it from the model name |
| Drive-link count | Must match the fitted bar; count or verify from its markings |
| Chain tension | Follow the manual; chain must be correctly tensioned before use |
| Chain brake and hand guard | Must be fitted and function as specified |
| Throttle return | Trigger and carburetor lever return freely; chain stops at idle |
Cutting safely after setup
Wear chainsaw-rated leg protection, a helmet with eye and face protection, hearing protection, gloves, and suitable boots. Clear the work area, plan escape routes where relevant, and keep secure footing. Hold the saw with both hands, use a sharp, correctly tensioned chain, and avoid cutting with the guide-bar nose.
Match the saw and bar to the wood and task. The MS 170 is for appropriately sized wood, not heavy felling. Tree felling requires suitable training and safe, standards-compliant methods; do not treat correct throttle linkage as proof that a felling job is within your ability.
Maintenance, Chain Sharpening & Engine Tuning
Routine checks help catch linkage trouble before it strands a weekend project. Keep the linkage clean and inspect it for wear or damage when servicing the saw. Make adjustments only where the manual calls for them; do not change carburetor settings to compensate for a throttle rod that sticks or fails to return.
The MS 170’s idle speed is approximately 2,800 rpm, with maximum speed around 10,000 rpm. These are reference specifications, not targets to reach by ear. Use the current manual and correct service procedure; if speed is wrong or the engine will not idle, seek proper diagnosis rather than exceeding specifications.
Keep the linkage, chain, and oil system in view
After stopping the engine and allowing it to cool, clean the air intake and cover area. Remove debris around the bar groove and sprocket area as directed in the manual. Check that the throttle still moves freely and the return action remains immediate.
The saw has automatic bar oiling. Use STIHL bar-and-chain oil and follow the manual’s checks for oil delivery. Do not treat oil flow as an operator-adjustable setting. Keep the chain sharp and correctly tensioned; poor cutting is not a reason to alter throttle linkage or carburetor settings.
Fuel must be fresh and mixed at 50:1 with approved two-stroke oil, following the current STIHL manual’s fuel specification. Incorrect or stale fuel can cause running problems, but it does not explain away a linkage that physically binds. Diagnose the mechanical movement first.
Storage, Fuel Handling & Seasonal Care
Before storage, stop the engine, let it cool, and handle the saw with the chain brake engaged. Clean the cover and linkage area so debris does not hide wear or obstruct movement. Follow the manual for fuel storage and longer-term storage; do not leave fuel-handling decisions to guesswork.
Before the next use, inspect the linkage again, especially if the saw was transported, cleaned, or had its cover removed. Confirm the lockout works, the trigger returns, and the chain remains still at idle after starting. If any check fails, stop and repair the fault before cutting.
Final setup checklist
- Correct serial-specific parts diagram or manual consulted
- Link seated at both ends and routed without pinching or rubbing
- Return spring present and undamaged
- Trigger lockout works as intended
- Trigger and carburetor lever move freely and snap back
- Cover refitted without changing linkage movement
- Chain brake and other safety parts intact
- Bar and chain match in pitch, gauge, and drive-link count
- Chain stops at idle during the running check
Frequently Asked Questions
Which way does the MS 170 throttle rod face?
There is no safe universal direction to give without the correct version diagram. Compare the rod’s ends and routing with the serial-specific STIHL parts diagram, and make sure it is seated at both ends without rubbing or pinching.
Should the throttle lever snap back when I release the trigger?
Yes. The carburetor throttle lever should return fully to its idle stop when the trigger is released. If it sticks or returns slowly, do not start or use the saw until the cause is fixed.
Can I run the saw if the trigger lockout is broken?
No. The lockout is a safety feature. Do not bypass it or use the saw with it missing or disabled. Replace the correct part or have a STIHL service dealer inspect it.
Can I bend the throttle rod to make it fit?
No. Bending can change how the linkage moves and may cause binding or prevent full return. Verify the part and routing against the correct diagram; replace a damaged or incorrect part.
Can a throttle-linkage fault stop the engine from starting?
Yes. If the linkage prevents the throttle and starting controls from moving as designed, starting can be affected. Inspect for misrouting or interference, but do not force the controls or change carburetor settings to compensate.
Why does the linkage move with the cover off but bind after reassembly?
The rod may be pinched, misrouted, or touching the cover or another control. Remove the cover as directed, compare the route with the diagram, and refit it without trapping the linkage.
What chain size fits an MS 170?
Do not choose by model name alone. Match chain pitch, gauge, and drive-link count to the fitted bar and the saw’s serial-specific manual. A common pitch is 3/8-inch P, but the other measurements can vary.
What should I do if the chain moves at idle?
Stop the engine and do not cut. The chain must stop at idle. Have the idle condition diagnosed; do not operate the saw while the chain continues to move.
Should I adjust the carburetor if the throttle does not return?
No. A carburetor adjustment cannot correct a binding rod, damaged spring, or faulty trigger. Fix the linkage fault first, and use the proper service procedure for any separate engine-speed issue.
What fuel mix should I use?
Use a 50:1 gasoline-to-approved two-stroke oil mix with fresh fuel, following the current STIHL manual’s fuel specification. Do not use a fuel-mix change as a substitute for fixing a mechanical throttle fault.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
