stihl 034 not idling, dies when running

[Old2stroke]

Compression may be low and would mean that a top end rebuild would be necessary to restore the saw to "like new" power, but since it starts when cold and actually runs a bit, compression is NOT the main issue. The carb obviously has to be carefully rebuilt no matter what you do to the saw, so why not rebuild it first and get the saw running, then you can evaluate it's performance and decide on the top end rebuild. You might be surprised on how well it runs with a few scratches on the P/C.

 

[BuckthornBonnie]

While I agree that scored pistons are often symptoms of underlying issues, I would also see that particular scoring as alarming enough to deal with now.

Yes to the fuel system inspection and overhaul, but I would not run that saw for long nor would I send it “down the road” like so many shadetree mechanics seem inclined to do. It has a problem... fix it.

 

[SteveSr]

Hey arborists, this is my first post so thank you for letting me into the forum. I have a stihl 034 saw that isn't performing after having sat for a few months.
It ran for a few minutes and then died. We started diagnosing L&H settings but these are ok. The spark seemed weak and plug was fouled so we replaced the plug and ignition coil. We cleaned the tank vent too. Unscrewing the gas cap doesn't help either so I don't think it's vapor lock. The current symptoms are as follows:
If the saw has been sitting for a while it will start up and idle ok for a few minutes but then when warmed up, it won't idle and it will die suddenly after a bit, even with the throttle open a bit to keep it running. When warm it won't start up at all unless it's on the fast idle setting.

I currently suspect the impulse line heats up, a crack in the line expands and the pumping action fails, starving the saw of fuel. However, when I pull the carburettor, plenty of vaporized gas exits the engine block.

The saw has seen a fair amount of use and we're okay with replacing it, but I really want to know what's wrong with it. I'm not a mechanic so I'm seeing this as a learning opportunity. I'd love to hear what you guys think is wrong with it.

That saw is old enough for an air leak in any number of places due to rotted rubber and/or crank seals. The saw is worth saving but NOT a quick fix. If you need something to run get a replacement and work on this one on the side.

Piston scoring could have been caused by an air leak, old fuel, or carb issue. The cylinder might currently be salvageable but if you keep trying to run it maybe not so much. BUT you need to find out WHY the piston is scored or you'll just do it again with a new one.