I was recently given an O25 chainsaw by a friend. I have used a chainsaw very little in my life, and most of that (but not quite all) has been my battery Greenworks one.

This Stihl I now have starts up easily and runs strong, for a few minutes. After making a few small cuts I can tell saw starts to run a little “anemic” and starts behaving like it wants to die. From that point I can keep it running for maybe another minute with some throttle, then it’s just done and shuts down. Once it’s down it won’t be started again.

This first happened a couple days ago and after I couldn’t get it to start back up I set it aside and finished up with my battery saw. This morning I grabbed it again and like before it started right up and ran fine for maybe 5 minutes before repeating the same issues as before. It seems to not run once warm and it kind of seems like it isn’t getting fuel, but that’s just a feeling from a 2 stroke engine ignoramus.

Brave AI search provided a list of potential issues and I am looking forward to tinkering, but I want to do it one at a time so I can positively ID the cause.

Can anybody help point to a “most likely” problem? Based upon the search results I was going to ensure the fuel vent is clear of gunk, but I’m open to suggestions.

I would change the fuel filter, air filter and spark plug. Clean out the carb and see what happens.

quote:

Originally posted by wcb6092:
I would change the fuel filter, air filter and spark plug. Clean out the carb and see what happens.

Never a bad place to start, but fuel overheating (or just bad fuel) would be my guess.

Is the fuel old, been sitting in it for more than a month or two? Ethanol?
Try to use no- ethanol fuel.

Try the basics as mentioned above. If that doesn't fix it, it could be the crank seals leaking once the saw warms up.

Yes, check the spark plug, change the fuel filter. Another thing I would do, remove or at least clean the spark arrestor located in the muffler.

First, the spark arrester, a fine mesh screen in the exhaust. Check youtube for your model.
I'm having a similar problem now on a Stihl leaf blower. i had the same thing a few years ago on a Stihl trimmer and started by replacing the carburetor (they're cheap online). That didn't fix it.
Eventually I stumbled onto the real problem. The coil has a gap to the triggers on the flywheel and needs a small gap.
Over time it had vibrated a bit closer until it started rubbing slightly and when it warmed up the expansion closed just a few thousandths more and grounded the coil.

This message has been edited. Last edited by: 220-9er,

quote:

Originally posted by 220-9er:
Fist, the spark arrester, a fine mesh screen in the exhaust. Check youtube for your

I am betting this is the issue - I say this because this exact same issue happened to me with my Husky chain saw a few years back. Removed the screen and the problem went away.

Yup, spark arrester. I need to clean mine about every other month.

Spark arrestee first, got it!

The age of the fuel is unknown. Probably a good idea to eliminate that variable.

Thanks!

Standard pump gas (with ethanol) will do that if its more then 6 months old.

Try using Recreational gas (no ethanol).

Is the choke disengaging properly?

Use non-ethanol fuel.
Corn is for cows, and good old mountain dew.

If you are in a place that does not sell "rec gas", big box stores, Wally World, Tractor Supply etc. sell non-ethanol fuel with oil in one quart cans. 40:1 or 50:1 2 stroke pre-mix.

If it has a fuel-priming bulb, make sure you prime it really well. My Stihl weed trimmer will do what you're describing unless it's primed well. I fully push the priming bulb 7 times and that is enough for my equipment.

Stopped up fuel tank vent?

quote:

Originally posted by Rightwire:
Standard pump gas (with ethanol) will do that if its more then 6 months old.

Try using Recreational gas (no ethanol).

Is the choke disengaging properly?

The choke switch seems to be working correctly… whether or not that means it is doing what it is supposed to I guess I don’t know for sure.

quote:

Originally posted by cparktd:
Stopped up fuel tank vent?

I looked at this real quick… the vent appears to be a little black tube coming out of the reservoir and there is a small flathead screw in the end of the hose. It looks free of debris but maybe there is an adjustment to be made to the screw? I’ll have to watch some videos, I think.

Check out YouTube “ married with small engines”.
Many Stihl troubleshooting videos.

I always use pre-mixed gas in a can. expensive but got tired of replacing carbs when the fuel messes them up

quote:

Originally posted by thumperfbc:
It seems to not run once warm and it kind of seems like it isn’t getting fuel, ...

This was exactly what my brand new Echo backpack leaf blower was doing. Turned out to be a defective ignition coil. They replaced that and the problem went away.

quote:

Originally posted by valkyrie1:
I always use pre-mixed gas in a can. expensive but got tired of replacing carbs when the fuel messes them up

Cannot second this enough. I have used the fake fuel in the small engines and have not had a problem since. You can get a can at Home Depot for $5.

quote:

Originally posted by valkyrie1:
I always use pre-mixed gas in a can. expensive but got tired of replacing carbs when the fuel messes them up

Bingo. The pre-mixed alkylate-based fuels are noticeably better fuels than what comes from a gas pump. Alkylate fuels are expensive, but IMO the cost isn't really significant in the long run -- unless you're cutting a lot. FWIW, I currently have 7 gallons of Stihl pre-mix fuel in my garage -- I cut a lot. I'm stocking up on holiday sales for my upcoming winter/spring cutting season.

I have reviewed a number of web-based tests on 2-cycle premix. Stihl often rises to the top. VP is also right up there, and it costs a little less than Stihl. Husky's fuel is respectable, but I don't feel it's quite as good as Stihl and VP.

Be careful of Trufuel -- the pre-mix alkylate sold at big box stores. Do a search on the web. Evidently their QC for the the fuel-to-oil mix levels can be spotty. To much oil and the engine doesn't run to its full potential. To little oil and the piston/cylinder is ruined. I'd go back to gas pump fuel before I would use Trufuel.