Stihl 026 revving problem - newbie question

[LumberDutch]

Hi all - I recently bought a 2nd hand Stihl 026 and initially it seemed to run fine. However, I noticed it started to bog down when hot/cutting at full throttle, and then had trouble starting it again. After cooling down, it starts ok.

Also, (and I'm not sure this is related) when I tip the machine up, it revs up, and if the bar points down, its revs reduce.... I read somewhere this could indicate a problem with the intake manifold boot, but since it is not easy to replace this, I hoped someone here might have some advice for me.

It has new fuel, sparks ok, I cleaned the airfilter and cleaned the small filter in the carburettor (which was initially dirty)....

Any advice would be welcome!

[Dan Maynard]

Next thing to check would be take the exhaust can off and have a look at the piston, does sound like air leak possible but that might have led to lean running. That could have caused the damage which leads to bogging when hot.

[ChrisNewport]

ebay purchase? I'll be 99 percent sure the piston is scored. As above take exhaust off and check the state of the piston before spending any money on anything else.

[Stubby]

Typical symptoms of low compression . All the above . 

[openspaceman]

46 minutes ago, Stubby said:

Typical symptoms of low compression . All the above . 

Even the tipping up and down bit?

 

Only time I had such a problem  the choke wouldn't set unless the saw was pointing bar up. Eventually I checked it out and it was the screw holding the butterfly had come loose so the butterfly was jamming the choke tube when below horizontal.

Edited June 27, 2022 by openspaceman
paragraph added

[LumberDutch]

Hi all - thank you for your quick replies. Cylinder issues was not what I was hoping for, but seems to be a real possibility. I had a look at the cylinder and it does not look badly scored, but I have no real reference.... Some vertical marks, but no real ridges/gaps.

Regarding compression, it has a much harder pull than my little Echo saw, so I was hoping that it would be ok.

I wondered indeed about the tipping up and down; but maybe I have a multitude of problems (just my luck!) 

[Stubby]

1 hour ago, openspaceman said:

Even the tipping up and down bit?

 

Only time I had such a problem  the choke wouldn't set unless the saw was pointing bar up. Eventually I checked it out and it was the screw holding the butterfly had come loose so the butterfly was jamming the choke tube when below horizontal.

The change in running when tipping up or down is most likely the opening and closing of the air leak that caused the piston to nip up in the first place ( this needs to be addressed before a new piston is fitted or it will happen all over again ) . 

[spudulike]

Best get some images of the piston taken through the exhaust port and let us make a judgement.

The tipping thing.....may  have been the grot in the carb gauze strainer moving about causing different running in different positions, possibly shot AV mounts.....difficult to tell when you can't see the machine.

I have had these saws in that work on their side but not upright.....transpired that the impulse line had come off and it was not drawing fuel when upright and worked just about when it was on its side. It is a common issue on this model saw.

[LumberDutch]

First of all - I want to thank all of you for taking the time to try to help me out here! I really appreciate it and I had no idea there were this many helpful people out there!

Second - it turns out not to be easy to get my iphone to properly focus on the piston. Here are a few pics, but I'm not sure they are of good enough quality to tell you much. Please let me know - I can try my other camera.

 

[spudulike]

Well, it hasn't been seized so at least that part is OK. There don't appear to be machining marks on the lower skirt so it has done some hours but that would be normal for a model as old as an 026. Is returning it to the eBay seller an option?

The bogging and not starting warm is a classic sign of lack of compression, without measuring or feeling the compression on the recoil handle, it is difficult to judge if it is a compression issue or not.

When the saw is cold and you do a deep cut immediately, does the saw have decent power? If it does, at what sort of run time does it start falling over?

One area to check is the impulse line where it connects to the union at the base of the cylinder between the air box (where the carb and air filter are housed) and the cylinder - this can cause unstable running but can be masked over by a carb tune....sort of! It is a common issue and can cause similar issues.

You can test the manifold - basically you need to pull the chainsaw upwards whilst the bar is held in place against a log/tree or something solid and if the saw picks up, you have a split manifold - this exercise basically stretches the manifold and causes air to enter through the split. An engineer would pressure test it but that is a bit specialist.

Many issues are carb related....are the H&L screw around 1- 1&1/4 turns out? When you cleaned the gauze strainer, did you pull it out and inspect it under a magnifying glass? They can look clean but still be blocked with scum. Was the pumping diaphragm OK? The holes under the small flaps can wear holes in them.

It is worth changing the fuel filter and also checking the fuel line for splits. The tipping could be covering and uncovering a split in the fuel line causing the issue. 

What is the colour of the spark plug? 

The fuel tank breather is worth checking but is generally reliable on these machines.

Lastly - the kill and earth wires can rub on the top AV mount - worth checking as I have seen it cause a misfire but doesn't sound too much like your issue.