Stihl Chainsaw cuts out

15 Jan 2012
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United Kingdom
Hi all, I have a Stihl MS171 chainsaw from new for about 2 years, it's never really been worked hard, just used for cutting felled trees and branches into logs for my fire.

I've always used Stihl chain oil and 2-stroke oil correctly, and kept it serviced with genuine parts.

I came to use it the other day and it always starts fine, but after a few minutes of use it starts to lack power then stall. The choke isn't on and the fuel tank is full. I checked the air filter, but it was fine (it was serviced just before winter). I wouldn't have thought the crank seals would have gone this soon - or would they???

Any ideas welcome as I want it in use to cut up logs ready for next winter.
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No problems with the crank seals - this would give poor idling and unstable pick up, due to the ingestion of air.
Always keep a spare plug handy to try - they are worked hard and get very hot.
So, if the plug doesn't help, its time for a carb strip - this are effectively in two parts - the fuel pump and delivery side and the mixing chamber.
The carb is rather like a metal / plastic sandwich, with gaskets and diaphragms in between Its vital to keep these the correct way round on dismantling.
If you want to give it a go, start with the pump side which has the fuel; supply pipe going into it. You'll find a needle jet trapped by a pin held by a screw - if you take this out, watch for a tiny spring below the arm that holds the needle jet.
I wouldn't really attempt this work without some carburettor cleaning spray and compressed air but if you want to have a go, then cloths etc are strictly taboo!
Be lucky
John :)
Stale petrol can cause problems if its stood idle for a few months, if it runs and cuts out it could be a bit lean on two stroke oil, so you get a partial seize.
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Hi all, thanks for the replies. I thought about the petrol and swapped it for fresh, but made no difference. I'll strip the carb and clean it out. Is this a regular service task once a year?
Its just the luck of the draw really - the filtering in Stihl machines is very good, but the fuel passages in the carb are miniscule so its just as well.
The problems I get are usually fuel from dirty cans or worse still with a little water in there....this causes oxidisation and rusting inside the carb.
Other problems stem from blocked fuel tank air inlets due to a build up of wood dust around the carburettor.
As before, toluene carb cleaner is a must, and I'm fortunate to have a large air compressor to help me clean things out.
So, there's nothing really to worry about when dismantling carbs, but do look for split gaskets and thin diaphragms with tears or wrinkles in them, and keep the bits in order. Watch for that tiny spring I mentioned!
John :)

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