Stihl 028 AV Super - Powerless when warm.. Help?

[ShockValue]

Hi all -

Bought an old Stihl 028 AV Super to do some home firewood work. It runs great for about 10 minutes of cutting but then quickly loses power. It still revs good when not in the log, but as soon as it gets a bite it bogs and will stall completely if I put any pressure on it. Let it cool for 30 minutes and it runs fine again for a few minutes...

Brand new 92 oct fuel. 40:1 Oil ratio.
Brand new RMC3 chain.
Older air filter, but I cleaned it and made sure it was nice and dry before doing my testing.
Spring on choke flap seems strong, so I doubt it's closing while at full throttle.

I was inspecting and found that the vent for the fuel tank had a screw in the tube, with no apparent way for air to actually enter (it's was not a grub screw.) I was hoping that it was just a vacuum in the fuel tank, but even running without the screw it ran poorly when warm.

I was trying to look at the fuel filter but the hose is VERY short on mine for some reason. The line doesn't have enough slack in it to get the filter out of the tank.. If I pull the filter off, I'm going to have a heck of a job getting it back on.


Thoughts on what to check for?

If you need more info, let me know.

 

[sld961]

I would check compression. Compression drops when it gets warm.

 

[marti384]

What is the compression? Low compression can cause lack of power when warm. When it dies next time check for spark right away.

 

[marti384]

I would check compression. Compression drops when it gets warm.

Beat me to it.

 

[ShockValue]

I was afraid you guys might say that. I don't have a tester currently. I'll see if I can scrounge one up.

 

[redoakneck]

Ignition??? mega-fire II is twenty buck and easy to install.

A cheap off the cuff check for compression is will the saw hang from the pull cord or does it fall??? If it falls, prolly crappy compression.

The old bosch points on those go bad, mega fire will cure it!!!

 

[sld961]

Ignition??? mega-fire II is twenty buck and easy to install.

A cheap off the cuff check for compression is will the saw hang from the pull cord or does it fall??? If it falls, prolly crappy compression.

The old bosch points on those go bad, mega fire will cure it!!!

It doesn't die. It stays running and revs up as it should, it just loses power. I doubt it's the ignition.

 

[ShockValue]

Yeah, if you don't put it in the wood you wouldn't know anything was wrong. Revs fine, and will sit and idle OK. Just totally gutless with any resistance.

 

[Rockjock]

Beat me to it.

I agree test the compression. And pull the muffler and have a look at the piston.

 

[zogger]

Pull the muffler for a visual of piston and cylinder.

 

[SEAM]

Does the tank breather work OK? Could be clogged...

 

[ShockValue]

I can blow into the tank vent and move air when the fuel cap is off.

So I hung the saw by the rope this afternoon. I can lift the saw, but if I jerk it while in the air the rope comes out and the saw drops....

 

[bag-o-donuts]

The AutoZone on Tremont loans tools... maybe you could borrow a compression gauge there.

 

[nikb47]

Is it smoking a lot when it bogs and looses power?

 

[ShockValue]

No smoke.

 

[Rockjock]

did you pull the muffler and look at the piston? I had a 036 that did exactly what you describe and it was a air leak that only after the saw was warmed up it would lose power, pulled the muffler and the piston had some scoring. Intake boot was the issue.

 

[bag-o-donuts]

My 026 did exactly as rockjock describes, was the carb boot in that case as well, except that it was related to a bad a/v bushing. Ran great no load, lean on it and carb boot flexes opening tear, voila, saw has no power.

 

[nikb47]

I had a similar problem with my 020t. It turned out that I had put the metering diaphragm and gasket in the carb wrong. So I'm thinking a carb kit wouldn't hurt to try if you can't find anything else.

 

[Deleted member 83629]

heavy coked exhaust ports and muffler could also cause the problems seen it to many times

 

[erikvr]

Check also the impuls hose for rupture