stihl MS170 chokes at high revs when hot- Question
Question - what I should do if my MS 170 chokes and high speed and has no power when it wams up ?
So far i have done:
- changed fuel line and filter , air filter.
- rebuild carburetor - cleand and replaced all membranes ( I tried brand-new carburetor but in was not original and did not work)
- I have checked compression (lifting on the starter rope) and it is Ok.
when engine is cold - it is ok but after heat-up -> fails.
When I adjusted carb : screw counterclockwise ( decreased speed) it helps a little bit - at least power go up and i have impression that i smell less gasoline from exhaust (comparing to normal adjusted screew ) so i suspect that there is to much fuel supplied to engine, bit also it can be to little air.
Is there any procedure to diagnose problem?
I have beed told that i should have change spark plug - but i do not have problem with engine starting.
1
u/Ticrotter_serrer 4d ago
The screw you've adjusted only set the idle speed. There is no carburator adjustment for this model as you probably know.
Have you experienced this outside in the heat ? It might be vapor locked.
In that case the solution is to cool the saw , work in the shade, use better high quality gas (as you did everything else) .
Vapor lock in a chainsaw is a condition where heat causes the liquid fuel in the fuel lines to turn into vapor, creating a vapor bubble or plug that prevents the fuel pump from pushing liquid fuel to the carburetor. This leads to a lack of fuel pressure, a loss of power, and causes the engine to stall, have difficulty starting, or run poorly. It typically occurs in hot weather or when the engine is running hot, and can be resolved by letting the chainsaw cool down for a period.
1
u/2dof 3d ago
"Have you experienced this outside in the heat ? It might be vapor locked." - every time when chainsaw heats up.
I know that screew is for iddle speed but i noticed that after screw adjusting ( as shtihl handbook says) I have ipression that is a lot of unburn fuel in exhausting has (by smell) .
1
u/iscashstillking 2d ago
How old is this saw and what kind of fuel are you using?
If your machine starts easily on the choke and runs for several seconds on choke then your hot performance issue is likely an unmetered air leak.
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u/2dof 1d ago
I think it is abaout 8 -10 years, it was used couple times in first n 2-3 years and then my father put it away. year ago I used for 1 our with no problem - but then i learn that my father "performed servise" by himself and since that - problems started.
Fuel: 50:1 ratio mix of petrol_95 to oil.
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u/Candid_Ad8624 3d ago
Changing the spark plug is a cheap thing to do to rule out it’s not the problem. Spark plug doesn’t just “start your engine”, the spark plug fires spark continuously the whole time the saw is running. This happens every revolution of the crankshaft (so thousands of sparks per minute at high RPM) Why it matters..If spark is weak or cutting out when hot, the saw will lose power or misfire even if it starts fine cold. Not saying the plug is the issue, but wouldn’t just assume it isn’t because it starts.