Chainsaw problem, Help needed

21 years ago

IÂm having a strange problem with my chainsaw. IÂm not a mechanic, but IÂve been using chainsaws since the early 1970s, so IÂm generally familiar with them. My current saw (a Stihl Farm Boss, a shade over two years old) was working for me a few days ago, although it started to run rough after about an hourÂs use at that time. But since that point, itÂs been starting normally, that is when fully choked, but as soon as I move the choke lever for normal warm-up, the engine dies. Normal revving (trigger pulls) will not keep it going.

IÂm running new gasoline, fresh from the pumps a few days ago, mixed with Stihl-brand two-cycle oil at about 36:1 (recommendations on the container are for mixes in the range of 32:1 to 40:1).

So I gapped the spark plug. I cleaned the air filter. I took the carburetor off, and ran air through the fuel line from a compressor (at a fairly low PSI), just to unblock it if there was any dirt in it. I took the carburetor apart and the diaphragm was in good shape so far as I could see. I blew air through the various passages, remounted the carb, set the high and low speed jets according to the manual recommended settings. No real improvement.

It seems the engine wants an extremely rich mixture, because it wonÂt continue to run when the choke is off. So I richened up the jet settings, but even with both fully opened I could not overcome the problem. Even after many successful starts, which I believe warmed the engine somewhat, it still wants to die when the choke lever is moved away from full-choke.

Thanks for any advice.

Joel

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Comments (7)

markapp
21 years ago
I would try some new gas and recheck the breather.Could be you just got the dregs at the bottom of a barel that day. A mechanic told me once the first thing to do with 2strokes is always to get a fresh mix of fuel. step 2 new plug even if it looks clean and gapped. My cousin serviced small engines and warned me to always run my chainsaw dry if it was going to set over a few days. mix will dry in the carb and leave varnish. chainsaws tend to be idle during summer the hottest weather when drying is worst. you may need to soak the carb and clean the orifices if everthing else seems to be tripple checked.
joel_bc
Original Author
21 years ago
markapp, the gasoline mix is as fresh as can be. I just made up the mix from fresh gas and oil a couple days ago. But it's true... sigh... I left some gas in the saw over winter. I dumped that out and have refilled the saw with the fresh mix.

Are you referring to the saw's fuel-cap breather holes when you mention "the breather"?

Joel
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wjongew
21 years ago
my stihl has a fuel filter in the tank where it is drawing the gas that can get plugged up.
juan_mm
19 years ago
Joel,

Your problem may in fact be a too rich mixture. My Stihl Farm boss is designed to run at a 50:1 mix ratio. You might want to check that in your manual. I bet that is why it won't run.
wdchuck
19 years ago
50:1 has been the mix ratio on Stihls and many other 2-cycle equipment for 20yrs now, that would be one item to change.

The fuel filter on the pickup line in your fuel tank could be suspect, change that out, they are cheap.

The fuel tank vent should be clean, it prevents pressure build up.

I suspect a possible air leak, since the problem starts after the saw warms up, you may a crack somewhere or a gasket that isn't fitting as well as it should anymore. It'll take some patience to find an air leak, but it would explain why the saw is demanding more fuel.

The folks at arboristsite.com will be able to nail this down pretty fast if you still want more help, lots of pro's and saw mechanics on there.
pablo_nh
19 years ago
There's an air vent on the gas tank on that Stihl. There's like 6 little holes in a plastic cap. If they get clogged, then no air gets in to replace gas, there's a vaccuum, and it stalls. I had to do a lot of tooloing to get my carb. set up right (260 pro)- follow the book, and expect to do it again as it breaks in.

That's on top of the air filter and gas filter recommendations.
mountain_man_winterbourn_net
19 years ago
From what you said it's not a plug problem. It's firing. See if it will run with the fuel cap off. Sometimes that thing in the cap that lets air in gets screwed up. Make sure mud wasps or some other thing didn't plug your exhaust.