Stihl 026 revving problem - newbie question

[LumberDutch]

Hi @spudulike, So you think the cylinder may be ok, right? I would be willing to replace it if that would solve the problem. When it runs, I really like the saw. Let me try to answer your questions:

1. compression when cold is good (I'm sure) but will need to double check if it is still ok when hot (bad memory for these things)
2. deep cut when cold: yes, it has excellent power, I was impressed
3. I checked the impulse line and it seemed to be ok, no obvious cracks or holes (I had it out to look at it)
4. I will check the manifold the way you suggest and report back
5. H&L screws are just over 1 turn out
6. I did not check the gauze strainer in detail; will do that; I also have a carb kit coming so will replace the pumping diaphragm and service the carb properly
7. fuel line and filter look ok, but I may replace them anyway; breather is breathing
8. Spark plug is brown/black - this may be an issue; there are also visible exhaust fumes
9. Kill and earth wires are good and not rubbing/bare

Thank you again for the excellent feedback and ideas. This turns out to be a bit of a project. I picked it up for next to nothing from a local old boy so don't want to bother him with returning it.

[spudulike]

It doesn't sound too bad then. if it cuts well when cold then it isn't completely knackered, just got to sort out the hot running issue. Does this happen on the 2nd, 3rd cut or after using the saw 15-20 mins?

Changing the plug is always worthwhile....it isn't often the issue but one or two have surprised me.

Check the stuff I have said - the following will help you see how the carb should work. I thought the guy made a lot of sense - 

Worth checking the coil to flywheel gap as well. Seen a few saws play up when this is out by an incorrect gap after others have fiddled.

 

[bmp01]

12 hours ago, LumberDutch said:

I picked it up for next to nothing from a local old boy so don't want to bother him with returning it.

Wasn't a facebook advert in Rugby by any chance.... just curious....

 

[LumberDutch]

Thanks for the further feedback and the video - very informative!

Update: put the saw back together to do the "pushing bar up" test; had to adjust the L screw to get it started and noticed it ran very fast (would go over 13k revs); adjusted H screw down to ~11k rpm (have tach meter) and got it cutting to warm up.

In short - the saw now seemed to work ok, no bogging down when hot. One time it stalled, but the fuel level was low (but not completely empty), topped it up and it ran again!

• Compression seems ok hot, but a slow pull works better than a very fast one for warm start
• pushing up bar test did not make an difference
• impulse line looks good, but (maybe) slides into the carb a bit easy... possible airleak?

Not sure what is going on here, maybe some dirt still in the carburettor which got flushed out (is that possible?)

Still have a carb kit coming and will replace the various filters and diaphragms and see if this makes it run (even) smoother.

Again, thank you all for taking the time to share your thoughts and provide suggestions. I may post a (hopefully) final update in a week or so after some more serious use this weekend.

And @bmp01no, it was not from FB in Rugby..... 

[spudulike]

Sounds like we are a bit closer. The 026 will rev up to 14Krpm but I would typically set it to 13 - 13.5Krpm just for safety. 

Fit the carb kit and see how it goes. The impulse lines can be a bit of a loose fit to the carb, seem that before. See how it goes once back together and has been run for a bit. If you fit a new impulse, only fit an original Stihl one as I don't trust the cheap aftermarket ones. 

Let us know how it goes but it is beginning to look promising.

[LumberDutch]

Hi all - update from me: 1 step forward, 2steps back! 

Serviced the carb, put new fuel in and ran like a dream. Cut wood for about an hour, started well between cuts. Stalled once but noticed that the fuel tank was just under half-full and with the short fuel line this seems to be a problem. Added fuel and all good again.

But then it cut out after another 10 mins, the saw was very hot and did not want to restart! Compression felt bad when pulling, so checked the cylinder and now it is completely scored! I actually now have a compression tester and where previously I had ~130psi (cold) now it is <90. 

So it looks like I need a new piston & rings (at least), maybe cylinder. But... is there any reason that this would happen. Leaking seals? Checked impulse line and manifold inlet and they are fine. Is there more I should check? Or is this likely due to the initial problem (possible worn/old rings).

As always, any suggestions are welcome. I have resigned to this being a "leaning opportunity"

[Stubby]

2 hours ago, LumberDutch said:

Hi all - update from me: 1 step forward, 2steps back! 

Serviced the carb, put new fuel in and ran like a dream. Cut wood for about an hour, started well between cuts. Stalled once but noticed that the fuel tank was just under half-full and with the short fuel line this seems to be a problem. Added fuel and all good again.

But then it cut out after another 10 mins, the saw was very hot and did not want to restart! Compression felt bad when pulling, so checked the cylinder and now it is completely scored! I actually now have a compression tester and where previously I had ~130psi (cold) now it is <90. 

So it looks like I need a new piston & rings (at least), maybe cylinder. But... is there any reason that this would happen. Leaking seals? Checked impulse line and manifold inlet and they are fine. Is there more I should check? Or is this likely due to the initial problem (possible worn/old rings).

As always, any suggestions are welcome. I have resigned to this being a "leaning opportunity"

Very likely an air leak from somewhere . Fix that first .

[openspaceman]

1 hour ago, Stubby said:

Very likely an air leak from somewhere . Fix that first .

Or fuel starvation except the saw up and down change in speed points to a leak.

 

Also does the OP recognise four stroking? The saw needs sufficient turns out on the HI screw to cause four stroking to limit the revs. If the revs are set to about 12k rpm from cold you should be able to sense if the revs creep up when hot.

[spudulike]

What was the saws revs set to? The 026 can go to 14k but as I said in a previous post, 13-13.5 krpm is about right. 

If the saw isn't revving over these figures AND there is enough oil in the fuel and the mix is fresh, it shouldn't seize.

Typically saws seize from: -

1) Bad carb adjustment

2) Old fuel mix or straight fuel with no oil being used.

3) Air leak raising the running RPM of the engine.

A Mix of the above can cause a seize and starvation of fuel through carb issues, blocked breather etc can exacerbate the issue if you can actually get the saws speed up rather than bogging down.

A Saw seize is caused by excessive heat in the engine melting the piston causing the rings to stick and hold in the grooves and transfer or smear the exhaust side of the cylinder with aluminium.....what we refer to as transfer.

the heat comes from bad lubrication and/or the machine running too fast.

if the saw was set to the parameters I said and lubricated with a decent oil and fuel mix, this shouldn't happen 

 

 

[bmp01]

"it was cutting really well..."

"reving great..."

"never run better..."

Happens so often, unfortunately it looks like you had 3 warning signs.

Before you strip the saw down better pressure and vacuum test to see if there is anything to learn. Do some searching online for what is involved and decide if you're up for it. But also note you dont always find leaks through cold pressure testing,... the way you've described it could easily be crankseals leaking badly once hot.

Not much to loose now, its a good time to learn.