Stihl MS 170 Carburetor Upgrade (Swap Guide)
A carburetor swap on an MS 170 should restore reliable operation, not promise more power. Before ordering, match the saw’s full model, serial range, market version, and original carburetor part number. Check fuel and air leaks first; then install the confirmed replacement, reconnect each line and linkage, and verify safe idle and operation.
Thirty pulls with no start can make a carburetor seem like the obvious fault. But a blocked fuel filter, damaged intake boot, or ignition problem can produce similar symptoms. Replacing parts before checking the cause risks wasting money, and a poor-fit or incorrectly adjusted carburetor can damage the engine.
I treat an “upgrade” as a reliability repair: replace a failed or worn carburetor with the correct part for that saw. The MS 170 is about 30.1 cm³, and its small engine needs the right fuel mix and a sound intake system. Work through the checks below before buying anything.
Confirm the correct carburetor
The correct replacement is the one specified for your exact saw, not simply one advertised for an MS 170. Model details and carburetor configurations can vary by version, serial range, and market. Verify the original part number using the saw’s identification details and reliable STIHL parts information before ordering.
Identify your saw and original part
Record the complete model and type information from the saw, along with its serial number. Use those details to check the applicable parts list or ask a STIHL dealer to confirm the carburetor part number. If the saw has been repaired before, the installed carburetor may not be the original, so compare its markings with the parts information rather than relying on appearance alone.
Do not assume that an aftermarket carburetor is compatible because a seller lists “MS 170.” Carburetors can differ in limiter arrangements, emissions setup, and adjustment features. An adjustable carburetor is not automatically an upgrade, and fitting an unverified part may create a tuning or compliance problem.
Compare the fittings before installation
Before removing the old carburetor, compare it with the replacement. Check the mounting pattern, intake and air-filter connections, fuel-line fittings, impulse passage, and choke and throttle linkage points. Make sure the specified gasket is included or available. A mismatch in even one connection can prevent proper operation.
| Check | What to confirm | If it does not match |
|---|---|---|
| Part number | Fits the saw’s model, serial range, and market version | Stop and confirm fitment |
| Mounting and gasket | Same mounting pattern and correct gasket | Do not force it into place |
| Fuel and impulse connections | Ports match the saw’s lines and passage | Do not guess which line fits |
| Choke and throttle linkage | Linkages connect and move freely | Do not run with a binding control |
| Filter and intake connection | Carburetor meets the correct air-filter setup | Confirm the specified parts |
Next step: Order only after fitment is confirmed. Keep the old carburetor until the saw runs correctly with the replacement.
Rule out problems that mimic carburetor failure
A carburetor meters fuel and air, but it cannot overcome a blocked fuel supply or a leak in the intake system. Check the air filter, fuel filter, fuel lines, tank vent, impulse passage, and intake boot before swapping parts. These checks help avoid replacing a good carburetor while the real fault remains.
Check the fuel and air path
Work with the engine off and cool. Remove the spark-plug boot before working near the carburetor, and clean debris from the area so it cannot fall into the intake. Inspect the air filter, fuel lines, and connections for dirt, cracks, loose ends, pinching, or damage. Check the in-tank fuel filter and tank vent according to the saw’s manual.
Look for a blocked or damaged impulse passage or line. The impulse signal helps operate the fuel pump in the carburetor; if its path is damaged or blocked, fuel delivery may fail even with a new carburetor. Also inspect the intake boot for cracks or poor seating. A leak can let extra air into the engine and create a lean condition.
Check for other signs before buying parts
If the saw has no start, verify the basic ignition and engine condition rather than treating the carburetor as proven faulty. Follow the manual’s procedure for checking the spark plug and ignition. If there is still no clear cause, a compression test or a pressure-and-vacuum test of the crankcase and intake system may be needed.
Stop if the saw races at idle, hangs at high rpm, or behaves unpredictably. An air leak or fuel starvation can make an engine run lean, overheat, and seize. A carburetor swap is not a safe way to test an unexplained high idle.
Next step: Repair damaged lines, filters, boots, or passages first. If the fault remains unclear, test the engine before installing a carburetor.
Replace the MS 170 carburetor step by step
A careful swap preserves the original hose and linkage routing and avoids introducing new faults. Work on a cool saw in a clean, well-lit area, and use the service instructions for the exact version. Do not rely on guessed fastener torque values or on memory when reconnecting small parts.
Remove and compare
Drain or handle fuel as directed by the owner’s manual, away from flames and other ignition sources. Remove the air-filter cover and filter as instructed, then photograph the carburetor, hose routing, and linkage positions. That record makes reassembly easier.
Disconnect the throttle and choke linkages without bending them. Note which fuel connection goes where before removing the lines. Remove the carburetor and gasket according to the applicable service procedure. Compare the old and new parts side by side, including ports, mounting, linkage points, and markings. If anything differs, stop and confirm the replacement rather than adapting it.
Install and inspect
Fit the specified gasket and position the confirmed carburetor without forcing it. Reconnect fuel lines to their correct ports and ensure they are not twisted, pinched, or stretched. Reconnect the impulse path and both linkages. Move the throttle and choke controls by hand to confirm free movement, then refit the air filter and covers as specified.
Use the exact STIHL service documentation for fastener torque and carburetor limits; these are not safe to guess. If you find a suspected intake or crankcase leak, have the system pressure- and vacuum-tested before running the saw. A new carburetor will not correct a leak.
| Stage | Pass condition | Stop if |
|---|---|---|
| Part comparison | Ports, gasket, mounting, and linkages match | Any fitting or feature differs |
| Hose routing | Lines are secure, unkinked, and correctly placed | Routing is uncertain or a line is damaged |
| Control check | Choke and throttle move freely | A linkage binds or fails to return |
| Leak check | No suspected intake or crankcase leak remains | Idle was racing or leak symptoms persist |
Next step: Check your work against the service procedure before adding fuel and attempting a start.
Start and verify the replacement
A successful start is not the only pass condition. The saw must idle safely, accelerate cleanly, and stop correctly. Use the cold- or warm-start procedure in the manual, with the saw stable and the chain clear. Do not tune by ear to raise engine speed or guess at mixture settings.
Use the correct fuel and test safely
For the fuel mix, follow the manual for your saw and the oil manufacturer’s directions. STIHL specifies a 50:1 gasoline-to-two-stroke-oil mix when using its specified two-stroke oil. Use fresh, correctly mixed fuel. Add suitable bar-and-chain oil as directed; oil delivery checks should follow the manual for the exact saw and bar setup.
Start the saw on stable ground, with the guide bar and chain clear of people and objects. Confirm that it idles reliably and that the chain stays still at idle. Check for clean acceleration and proper stop-switch and chain-brake operation. Verify bar-oil delivery using the manual’s check procedure.
If adjustment is needed, use the exact model’s service specification. Have a qualified STIHL technician verify carburetion with a tachometer. Do not guess at mixture settings or exceed the specified maximum engine speed. Stop if the chain moves at idle, the engine races, or it will not respond predictably.
Keep the cutting setup compatible
The MS 170 is approximately 30.1 cm³. Use only a guide-bar length approved for your exact saw version; 12–16 inches is a common range, but confirm the manual and installed parts. Match the bar, chain pitch, gauge, and drive-link count. Many setups use 3/8-inch Picco chain, while gauge can vary, so read the bar stamp rather than guessing.
Wear eye and face, hearing, hand, leg, and foot protection when operating the saw. Check the chain brake, throttle lockout, chain catcher, and stop switch. For tree felling, follow applicable rules and training; OSHA’s logging rule requires keeping people at least two tree lengths away in situations it covers.
Next step: If any safety check fails, stop and correct it before cutting. A carburetor swap is complete only when the saw passes its operating checks.
Common swap scenarios and what they mean
These examples are diagnostic situations, not claims about a particular customer or saw. They show why a carburetor should be replaced only after checking the parts that can cause the same symptoms. Match the symptom to a test before spending on a replacement.
Example: New carburetor, same no-start
If a confirmed-fit carburetor does not change a no-start condition, return to the basics. Check fuel freshness and mix, fuel-filter condition, line routing, impulse path, and ignition using the manual’s procedure. If these checks do not explain the fault, compression or leak testing can help narrow it down.
Example: Saw starts but races at idle
A racing or hanging idle is a warning, not a cue to turn adjustment screws at random. Check for an intake boot, impulse, or crankcase leak, and confirm the replacement part is correct. Stop using the saw until the cause is found; running lean can overheat and seize the engine.
Example: Replacement does not match the old part
Do not trim linkages, force hoses onto ports, or adapt a carburetor based on a listing photo. Recheck the full saw identification and part number. Carburetor and emissions configurations can differ, so confirm fitment with an applicable parts list or STIHL dealer.
Next step: Let the symptom guide the next test. Do not buy a second carburetor until the first one’s fit and the rest of the fuel and intake system are verified.
FAQ: MS 170 carburetor replacement
Can I put any MS 170 carburetor on my saw?
No. Confirm the part number against the saw’s model, serial range, and market version. Carburetor fittings and configurations can differ.
Is an adjustable aftermarket carburetor an upgrade?
Not by itself. Do not fit an unverified carburetor or use it to raise engine speed. Confirm compatibility and compliance first.
Should I replace the carburetor if the saw will not start?
Not without checks. Inspect fuel, filter, lines, impulse path, intake boot, and ignition. Consider compression or leak testing if the cause remains unclear.
Can a blocked fuel filter mimic a bad carburetor?
Yes. A blocked filter can restrict fuel delivery. Check it and the fuel lines before buying a carburetor.
Do I need a new gasket with the carburetor?
Use the gasket specified for the exact saw and carburetor. Do not reuse a damaged gasket or improvise a substitute.
Can I tune the carburetor by ear?
Do not tune by ear to raise speed. Follow the exact service specification; a qualified technician can verify settings with a tachometer.
What fuel mix should I use?
Follow the saw manual and oil instructions. STIHL specifies 50:1 when using its specified two-stroke oil.
What if the saw races after the swap?
Stop running it. Check fitment and inspect the intake and crankcase for leaks; have the system tested if needed.
How do I know the swap worked?
The saw starts, idles reliably with the chain still, accelerates cleanly, stops correctly, and delivers bar oil as the manual directs.
Does a carburetor swap increase MS 170 power?
A correct replacement restores the intended setup; it is not a safe power upgrade. Avoid parts or tuning that exceed the saw’s specifications.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
