Stihl 028 AV Super, stumbles at WOT.

[tlmartin84]

Looking for where to start diagnosing....


Saw has had a lot of new parts in the past 10 years, carb, fuel filter and hose, vent hose, air filter, clutch etc.


Saw is stuborn to start after sitting for awhile, Ive found pumping the gas to pull it up prior to starting it helps.



Anyhow, used the saw about 6 months ago with no issues. Picked it up this evening to go work a tree, ran fine for about 10 minutes, blocked about 6 cuts in a 24" tree.



It started to bog almost as if the chain binding. Pulled it up, let it recover and it bogged again.



It idles fine, will pull to wide open throttle, will stay there for about 5 seconds and then bogs and stumbles.



Clean filter, sharp chain, good gas, the vent and pickup hoses are all good.

 

[Brushwacker]

Make sure your spark plug is gapped 20 thousandths and there is no evidence of running hot or grey on the electrode. At some point you will do good to remove the muffler and check the condition of the piston. Check compression.(Some easy and I expensive things I would start with) ,drain all the fuel into a clear container, check condition of fuel and gas line.Correct anything u find wrong . Put in fresh new fuel recently from the pump.Start and take it for a test drive.

 

[alexcagle]

Greetings fellow insomniacs,
Plus 1 on the above.....also test tank vent. - it's right by the stop switch wire.
That, or it sounds like your fuel hose is suss, kinked, upside down, tangled in the cap retainer...etc.... (I've seen it all)
Make sure OEM pickup hose was used, and not aftermarket. They are awful and kink if you look at them wrong. A Stihl hose will have "Stihl" moulded into it. Might be hard to see it 'installed' tho.
A bubble of water will lay at the bottom of the tank, moving with gravity. Right where the pickup lays.

 

[tlmartin84]

The vent and fuel pickup are both relatively new and were OEM parts, I replaced them all about 10 years ago when I rebuilt the saw. I looked both over and do not see any issues. I also do not think it is the gas, same mix I run in the weedeater and no issues there.

Plug looked good and was running good.

I find it odd that it just randomly kind of happened with a cut, and over a matter of seconds. It was fine, and then it wasn't.

It is like it is starving, like the choke is flipped on when revved. I'm going to check the filter and flapper. Could it be the ignition?

 

[Brushwacker]

Crap gets in the gas tank, draining it good is easy preventative maintenance even if it isn't the problem. Double checking things has paid off many times for me.
When you move on if the problem persist I would look at wires, sometimes a loose bare spot that falls against the cylinder and grounds when your saw is turned in a different position. Your coil also could be malfunctioning or a diaphram in the carb may have a hole.

 

[tlmartin84]

 

[tlmartin84]

It is getting fuel....it is dumping it in and Im getting some blowback.

 

[Wood Doctor]

if you feel it has good compression, then I suspect I carburetor problem. These tend to fail with age. The air/fuel mixture to the combustion chamber is not supporting WOT. Before replacing it, have you made any carb setscrew adjustments?

 

[tlmartin84]

if you feel it has good compression, then I suspect I carburetor problem. These tend to fail with age. The air/fuel mixture to the combustion chamber is not supporting WOT. Before replacing it, have you made any carb setscrew adjustments?

Yes, Ive tried it all over the place. It has always been within a quarter turn of the recommended 1-1/4 turns.

Dumping the fuel like it is in the video is not normal for a 2 stroke carb is it? It should be a fine mist, right?

 

[Wood Doctor]

Yes, Ive tried it all over the place. It has always been within a quarter turn of the recommended 1-1/4 turns.

Dumping the fuel like it is in the video is not normal for a 2 stroke carb is it? It should be a fine mist, right?

Yes and you are getting that fuel dump every time the engine stalls out because not enough air is included in the the air/fuel mixture that the carb is supplying. The carb is letting lots of fuel in but not enough air is being supplied along with it.

Or, the engine is starved for fuel at WOT, but you seem to believe it is not.