Stihl 026 query - purchased 2nd hand

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I’ll run it at the weekend and see how it cuts.
I don’t have a pressure/vac tester but might try the starter fluid behind FW & clutch to see if there’s a vacuum issue.
Either way I’ll pull the cylinder to get a better look at the piston/cylinder/rings to assess them.
 
Decided not to run it yet until checked a few things

Got myself a vac/pressure tester

Holds good Vacuum at -10psi for over 5mins
Pressure 10psi dropping to 8psi after 5mins
I think that’s a decent result
I plan to run it briefly just to get a feel for it and look at then pulling the cylinder and see what I’ve got. Best case I guess would be a cylinder cleanup and likely new piston.
 

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Did some cutting this weekend or at least tried to. Saw idles beautifully but when cutting at WOT the revs drop and it wants to die unless I pulse the trigger and it might pickup but same then continues to happen.

Think it might be a fuel issue so have ordered a carb kit for the Walbro WT-426. Will sonic clean the carb body and service out the carb and hopefully that’ll sort it?

That and maybe the fuel pickup filter?
 
So some food for thought…if you haven’t tried adjusting the high speed jet then I would definitely do that. If you have…then I would suspect two main possibilities. First that engine compression is low, don’t remember if you tested compression or not? Second, I would check what I call the “last chance” filter inside the carb body under the diaphragm plate. It’s a micro mesh metal cup shape. I am not certain if this saw has that screen though as many of these saws have different carbs. I personally put a WT-194 in mine, as it has a larger bore diameter and the saw likes it. Back to the filter, it gets clogged over time if the tank filter ever fell off and previous owner ran it then micro saw dust makes it in. Many saws I’ve seen will idle beautiful but wont flow enough fuel when RPM goes up and very often this is the root cause.

I am sure others will bring up some other possibilities, because there are others, but this is what I check first because it is free, easy, and commonly a problem.
 
Did some cutting this weekend or at least tried to. Saw idles beautifully but when cutting at WOT the revs drop and it wants to die unless I pulse the trigger and it might pickup but same then continues to happen.

Think it might be a fuel issue so have ordered a carb kit for the Walbro WT-426. Will sonic clean the carb body and service out the carb and hopefully that’ll sort it?

That and maybe the fuel pickup filter?
Don't try to run it like this or you will likely destroy it if you haven't already. You might want to pull the muffler cover and check the condition of the piston.

All carbs have the internal screen which can get clogged. You will see it when you install an OEM carb kit. You also need to check the fuel line for a possible crack/leak and replace the fuel pickup with an OEM one if you haven't already.
 
I bought it new and OEM, paid $79.00 USD on eBay. I don’t think you need to replace the carb, and wasn’t really suggesting that, was just adding some info. If the problem is the carb, and you had to replace it then I would urge you to put the WT-194 on it, but for now i would try to solve the problem before shotgunning any parts.
 
I haven’t adjusted the High speed at all so will give that a shot,

I had stripped the carb and sonic cleaned everything and all visually seemed ok.
Otherwise the carb kit and pickup seem to be a good call for now, I’ll see where those gets me 👍

Compression was quite good 188psi
 
Where are you getting the wt-194 from?
The saw only cost me £200 these carbs seem to go for £100-£140
Your WT-426 is fine for that saw stock (unported). It is a "bypass" carb where only a percentage of the fuel goes through the needles. This makes it somewhat easier (less touchy) to adjust.
 
The 194 is a very good choice, I put one on every 026 that comes my way, everything Greg mentioned as well as check that the impulse hose is tightly fitting on both ends. Also what Steve posted as I was typing.
I have had a couple of saws die after being reved up due to a restrictive in tank fuel filter
 
I’d already pulled the muffler
There is slight wear on the piston which I was aware of. Don’t think that’s the issue though
 
I’d already pulled the muffler
There is slight wear on the piston which I was aware of. Don’t think that’s the issue though
Have you checked again since you tried to run it? Piston damage can happen FAST in a saw running too lean under load.
 
When you say too lean, are you suggesting it’s not the fuel to oil mix ratio that’s too lean but the fuel:air ratio?
 
Time for back to basics. Does the fuel flow out the fuel hose and stay running? Is there a pulse from the pulse hose?
In the shop there is a diagnostic process so that there is no skipping over an issue.
So tough to cure these issues over the internet. Diagnostics is still a hands on operation.
Have questions which may be answered by looking back through the posts.

Looked back. Still not happy with the pics of the piston. Some one mentioned the skirt affecting the pulse. Think of it as a burned valve, Not holding compression, but losing the pressure from the pulse on the underside of the piston.
You HAVE to verify internal condition before replacing outside parts. Time to get the cylinder off.
 

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