Stihl 024 and Husk 245(?) both with same issue.

[organic guy]

Update today.

Got new fuel, local station only does E5 so have probably been running on it for a while.

Made up new mix for both saws, double checked cleanliness of everything.

Husky started easily, but bogged and died as soon as I open throttle. Restarted easily, left it on idle,it sat for 30 seconds then revved up and died.

Stihl started fine but died on revving up.

Any more clues please

[spudulike]

So, the saws are idling OK so the idle circuit is providing enough fuel for idle but not mid to high speed running.

The first checks would be fuel filter and gauze strainer - on the latter, I don't mean look at it, I mean take it out and view it against a bright light with a magnifying glass!

On from that....

Holed fuel line, blocked tank breather, holed pump diaphragm flaps, hardened metering diaphragm, dirt in the metering section of the carb, metering arm set too low, dirt under the welch plug (very uncommon), faulty accelerator pump (If fitted), blocked high speed check valve, blocked air filter, faulty choke flap, faulty ignition timing ramp on the coil, arcing on the HT lead/plug......and so on.

Highly unlikely that two machines would suddenly develop the same issue with many of the above causes. 

Have you tried opening up the high speed screw on the carb? 

Edited November 5, 2021 by spudulike

[organic guy]

Spudulike et al.

Many thanks for your help and patience!

I am stuggling to work out why two saws on different fuel mixes made up at different times should show such similar symptoms, particularly the stihl which been very reliable for many years.

On the bright side my 350 ran fine on the new fuel mix today.

It seems odd to me that the stihl would sit at idle but then of its own accord rev up and die?

I did take out the gauze filter and spray cleaner through it but did not look at it against the light.

I have not touched carb screws, feel that is above my pay grade, unless you say it is O.k

[adw]

They are just running too lean for some reason, there is no problem richening them up on the H screw a little, it’s leaning them off which is the danger area, seems strange they have leaned off, however on the 254 turn the H screw ( the one furthest away from the cylinder) 10 minutes on a clock face and try again.

[spudulike]

The idling and revving up then dying - what happens is you start the machine on choke, this fills the metering section of the carb with fuel. As the saw idles, it empties this part of the carb of fuel and what should happen is the diaphragm pushes against the metering arm opening the needle valve and another slug of fuel enters the metering section and shuts off as the diaphragm comes away from the metering arm. 

 

What your Stihl is doing is using the first slug of fuel and then it basically runs out of fuel as no more is coming through and that is why it exhibits the same symptoms of running out of fuel.

 

The Stihl needs the metering arm height checked and perhaps a new diaphragm in the carb. I have come across the same thing in BG86 blowers, exactly the same symptoms. Whilst you are in there, check the positioning of the metering arm spring and that the needle is moving freely up and down.

 

Why the new fuel should cause this....no idea as I don't know the history but your issues with the Stihl are exactly as I have said! You could check the metering arm height and see if you can get it to work just by doing that if the diaphragm is relatively soft and hasn't hardened.

 

If I get a machine like yours in, I just work through the carb and if a going through doesn't sort it, I fit a new kit.

 

If you want to prove it, let the saw run and then stop then strip the carb, the metering section will be dry and there will be fuel in the pumping section!

[organic guy]

Spud

Thanks for that brilliant explanation. 

I begin to suspect I need to replace carb parts. May have been lead up the garden path by 2 misbehaving at the same time.

The metering arm on the Walbro 047 carb is set 1mm below the rim of the carb, matched by a 1mm lump on the metering diaphram. Is there an accurate way to measure this?

 

[bmp01]

 

Spuds description of how carb works is great explanation of why the saw runs out of fuel off choke.

 

I'd consider the pump side of the carb might be dubious too. So even if the metering side of the carb works and the needle valve opens, could be no fuel arrives. In that case it maybe the pump itself is knackered or the impulse line driving the pump is leaking.

 

The 2 saws at once is either a coincidence or the rubber parts in the carbs have both been affected by bad fuel at some stage and its coming to light now further down line (so it's not down to the current good fuel). Treat them as individal carb problems  and see if there is a common fault afterwards.

 

 

 

 

 

 

 

 

 

[adw]

It would be interesting to see the metering diaphragm, if this has hardened it will lose convolution so will not function properly against the metering lever, I still think it would be good just to richen the H screw just to see if they then pick up, then strip the carb to investigate why they have leaned off.

[spudulike]

9 hours ago, organic guy said:

Spud

Thanks for that brilliant explanation. 

I begin to suspect I need to replace carb parts. May have been lead up the garden path by 2 misbehaving at the same time.

The metering arm on the Walbro 047 carb is set 1mm below the rim of the carb, matched by a 1mm lump on the metering diaphram. Is there an accurate way to measure this?

 

I rarely use it but Walbro do this gauge: -

 

[spudulike]

58 minutes ago, adw said:

, if this has hardened it will lose convolution so will

Have you been doing those crossword thingys again Or was it a new key word in a powerpoint, bit like "synergy and holistic" were back in the day!!

I would agree with giving the carb a wake up call on the H&L screw, it may be an easy fix and if not....back to the hard way.