Stihl MS 170 Flooding After Carb Swap (Fixes)
If your MS 170 began flooding after a carburetor swap, stop using full choke and check the replacement parts before adjusting anything. First clear excess fuel safely, then inspect carburetor fit, fuel and impulse lines, gasket order, choke action, and the inlet needle. A leaking or incorrect carburetor can flood the saw again, so do not run it until the fault is fixed.
I remember the kind of repair that stalls a weekend: the saw ran before the carburetor swap, but now the plug is wet and every pull seems to make matters worse. Before buying another carburetor or paying for shop time, separate a one-time flooded start from a fault that keeps feeding fuel into the engine.
What flooding after a carburetor swap means
Flooding means too much fuel has entered the engine for it to start. A wet spark plug is a common clue, but the reason matters: the saw may simply have been started with too much choke, or the replacement carburetor may be leaking fuel into the engine.
The MS 170 is a 30.1 cm³ two-stroke saw. Its carburetor meters fuel and air, while the choke enriches the mixture for starting a cold engine. After a carb swap, a wrong part, misplaced gasket, trapped diaphragm, or misrouted line can upset that balance.
A useful first question is whether the engine flooded once during starting or continues to flood while parked. If it starts after clearing excess fuel and then runs normally, the cause may have been the starting sequence. If it repeatedly develops a wet plug, leaks fuel, or floods after sitting, stop and inspect the carburetor and fuel system.
| What you find | What it suggests | Next step |
|---|---|---|
| Wet plug after repeated choke pulls | Excess fuel from the starting attempt | Clear the cylinder, then use the manual’s starting procedure |
| Wet plug returns after clearing | Ongoing fuel delivery or a setup fault | Check carburetor fit, needle, diaphragm, gaskets, and lines |
| Fuel visibly leaks | A safety fault, not a tuning issue | Do not start or operate the saw |
| Engine races or chain moves at idle | Unsafe idle or control fault | Shut down and diagnose before use |
Takeaway: Clear a one-time flood, but treat repeated flooding as a repair fault.
Make the saw safe and clear excess fuel
Clearing a flooded engine means removing excess fuel from the cylinder so the spark can ignite the next starting charge. Work outdoors, away from flames, sparks, heaters, or smoking materials. Switch off the saw, engage the chain brake, let the engine cool, and wear eye protection and gloves.
Before inspection, remove the spark-plug cap. Keep fuel-soaked parts and fuel vapors away from ignition sources. Follow the operator’s manual for the saw’s controls and starting procedure; control markings and procedures can vary by version.
Clear the cylinder without adding more fuel
Set the control to the manual’s warm-start or run position and open the choke. With the ignition disabled, pull the starter until excess fuel clears. Do not keep pulling with the choke closed, and do not repeatedly prime or pump fuel while the engine is flooded.
If the plug is wet, remove it and dry it fully or fit the correct replacement plug. With the plug removed and the ignition disabled, pull the starter several times to clear fuel from the cylinder. Keep the plug hole pointed away from you and any ignition source. Refit the plug and cap before attempting to start.
Use the model-specific starting steps in the manual. Do not invent a carburetor screw setting to make the saw start. If it fires briefly but floods again, stop and move to the carburetor checks rather than repeating the same starting attempt.
Takeaway: A wet plug can be cleared; repeated wet plugs call for diagnosis.
Check the replacement carburetor and its installation
A carburetor application is the part matched to a particular saw version. MS 170 parts can vary, so verify the saw’s exact model and serial-number version and confirm the replacement carburetor’s part number and application before changing settings or ordering parts.
A carburetor that physically fits is not necessarily the correct one. A wrong application or an installation error can cause poor starting, excess fuel, or unsafe running. Check the saw’s manual or parts information for the correct carburetor and intake components.
Inspect fit, lines, gaskets, and controls
Work with the saw off, cool, and the spark-plug cap removed. Check the following against the correct service information for your saw:
- Carburetor part number and application.
- Fuel-line routing and condition, including cracks, loose ends, or pinching.
- Impulse passage and connection. The impulse signal helps operate the fuel pump in the carburetor.
- Intake boot and gaskets for damage, poor seating, or a blocked passage.
- Throttle and choke linkage for correct connection and free movement.
- Air filter for blockage and the plug for fuel contamination.
Gasket and diaphragm order matters. A diaphragm is a flexible membrane that responds to fuel pressure and engine pulses; if parts are reversed, damaged, or pinched, fuel metering may not work as intended. Compare the installed order with the correct diagram for the carburetor version. Do not guess from another MS 170, and do not bend the metering lever or defeat limiter parts.
Check for a leaking inlet needle
The inlet needle and seat control fuel entering the carburetor. If the needle does not seal, fuel can continue entering when it should stop. That can cause repeated flooding, including while the saw is parked, and may lead to fuel leakage or fire risk.
If fuel leaks, the plug becomes wet again after clearing, or flooding recurs without more choke pulls, do not operate the saw. The carburetor may need correct repair parts or replacement, but first confirm that it is the right carburetor and that its diaphragm, gasket, and lines are installed correctly. Do not try to hide a leak by leaning mixture screws.
Takeaway: Verify the part and installation before condemning the engine or adjusting the carburetor.
Compare the likely causes before buying parts
A symptom points to a test; it does not prove which part failed. I use the timing of the problem to choose the next check. If flooding began directly after the swap, inspect the work first. If the saw was also difficult to start before the swap, broaden the diagnosis rather than assuming the new carburetor is the only cause.
| Symptom after the swap | Check first | Avoid |
|---|---|---|
| Plug wet after full-choke pulls | Clear the engine and follow the manual’s start steps | More full-choke pulls |
| Flooding returns after a correct start attempt | Carburetor application, needle/seat, diaphragm and gasket order | Random screw adjustments |
| Fuel leak at carburetor or fuel line | Stop; inspect seals, hose fit, and correct parts | Starting to “see if it clears” |
| Choke plate does not open fully | Choke linkage and control connection | Forcing the control |
| Hard starting plus damaged or wet plug | Correct plug, condition, and air filter | Reusing a fuel-soaked plug without inspection |
A representative bench pattern is straightforward: the saw floods after a carb swap, the owner clears the plug, and the wet plug returns. The useful finding is not “the carburetor is bad” by itself. It is that fuel is still entering or the installation is still wrong, so the next checks are the needle and seat, diaphragm and gasket order, and fuel and impulse connections.
Takeaway: Use the symptom pattern to direct inspection; do not replace parts by guesswork.
Start and verify the saw before cutting
A successful start is not enough to return the saw to work. Verify stable idle, clean throttle response, a chain that stays still at idle, and bar-oil delivery using the manual’s check procedure. Stop if the saw leaks fuel, races, or moves the chain at idle.
Use fresh fuel mixed at 50:1 with quality air-cooled two-stroke oil: 20 mL of oil per 1 L of gasoline. Use fresh mixture and the correct air filter and spark plug. If you are unsure how old the fuel is or what is in the container, replace it rather than adding it to the saw.
For versions with the cited specifications, idle speed is about 2,800 rpm and maximum engine speed is up to 13,000 rpm. Confirm the correct figures in the saw’s manual or label. Do not tune by ear or exceed the stated maximum; use only the procedure and limits listed for the exact carburetor version.
Check the bar and chain combination against the saw and bar markings. Many MS 170 setups use 3/8-in Picco chain, but gauge can vary, commonly 1.1 mm (.043 in) or 1.3 mm (.050 in). Match chain pitch, gauge, and drive-link count to the bar and sprocket. Use chain-and-bar oil and confirm delivery by the manual’s procedure; there is no single oil viscosity or flow rate that applies to every climate and bar version.
Before cutting, use a sharp, correctly tensioned chain and suitable protective equipment. Follow safe chainsaw practices and any site rules. After work, shut down, engage the brake, let the saw cool, clean it, and inspect it for leaks. Follow the manual’s fuel-storage and end-of-season instructions.
Takeaway: Return to cutting only after the saw runs safely and oil delivery is confirmed.
Frequently asked questions
The short answers below help distinguish a starting mistake from a fault that needs repair. Use the manual for the exact saw version and carburetor; do not apply settings from a different model just because the parts look alike.
Why is my MS 170 flooding after a carburetor replacement?
Common checks include the carburetor’s application, fuel and impulse-line routing, gasket and diaphragm order, choke operation, and a leaking inlet needle.
How do I clear a flooded MS 170?
Open the choke, use the manual’s warm-start or run position, and pull with ignition disabled until excess fuel clears. If the plug is wet, remove and dry or replace it, then clear the cylinder with the plug out.
Should I keep pulling with the choke on?
No. Repeated full-choke pulls add fuel and can make flooding worse.
Can the wrong carburetor fit and still cause flooding?
Yes. Physical fit alone does not confirm the correct application. Match the part number to the exact saw version.
What if the plug gets wet again after I dry it?
Stop repeated starting attempts. Check the carburetor, needle and seat, diaphragm and gasket order, choke, and fuel-line routing.
Can I adjust the mixture screws to stop flooding?
Do not use screw adjustment to mask a leak, wrong carburetor, or installation fault. Follow only the specified procedure for that carburetor version.
Is a fuel leak safe to run?
No. Do not start or operate the saw with visible fuel leakage. Repair it with the correct parts or have a qualified shop inspect it.
What fuel mix does the MS 170 use?
Use fresh 50:1 fuel with quality air-cooled two-stroke oil: 20 mL oil per 1 L gasoline.
What chain fits an MS 170?
Check the bar and saw markings. Pitch, gauge, and drive-link count must match; do not assume every MS 170 uses the same chain.
When should I stop troubleshooting at home?
Stop if fuel keeps leaking or flooding returns, the engine races, or you cannot confirm the right carburetor and installation. A qualified service shop can diagnose it without risking operation of an unsafe saw.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
