Stihl 028AV running problem

[zoltan]

Hi, I'm very new here. I don't work as an arborist but use chainsaws for firewood.

 

I'd appreciate some help diagnosing a problem with my Stihl 028AV

 

Its history: fairly well used Ebay purchase when I got it, been treated to a new piston and rings, a carb overhaul kit and a new bar, sprocket and chain. Was running nicely but has deteriorated in last couple of months

 

It starts fine and idles. Running 50:1 on Stihl oil and ordinary unleaded. Clean fuel and air filters

 

When it is warmed up and I try to cut, the power surges unless the saw is held down onto the wood with a small degree of pressure, held lightly then you kind of have to feather the throttle otherwise it will cut out. When running it has sufficient power to cut properly. It is impossible to up cut with the saw so my initial theory was a split in the rubber sleeve that connects the barrel to the carb. As if the downwards pressure was closing the split and it was gulping air and running lean without the saw being pressed down. I took this out at the weekend but is was perfect, no perishing, no damage.

 

Next area to check is if fuel hose is getting sucked flat but this doesn't explain why the saw is sensitive to being pushed down on its rubber mounts as the tank, hose and carb are all fixed together in the handle, separate from the engine

 

Anyone have any other ideas? I'm becoming somewhat stumped by this

 

Do dicky crank seals give these symptons?

 

Thanks

 

Colin

[tommer9]

Hello Colin. Welcome to arbtalk.

My first thought was crank case oil seals too. I have found that the idle was affected aswelll though.(funnily enough it was on a mates 028av!)

[geoff]

I would look to the carb if all the hoses are ok,sometimes a strip & clean sorts things out at no cost.Check the electrical conections& leads for problems too.

Hope you get it sorted without too much hassle.

[rxe]

The best indicator of an air leak is a very fast idle. Have you needed to bring the idle speed down? Where is the impulse hose in an 028? (I've never worked on one....) is it clear?

 

Is something odd (movement related) going on in the throttle linkage?

[rotax81]

always best to strip the plastics off and have a look. something is loose/cracked worn.

not too clued up on stihl but check the crankcase around the mounting points.

[zoltan]

Thanks for the replies and advice everyone.

 

I have new seals so I will fit those and take it from there. It looks like special tool time to get the magneto off the crank without damaging it

 

The impulse hose runs from the back of the crank case under the port through to the handle and carb. It is clear as I ran some air through it, I ought to check it for a split as it is the only other connection between the carb/linkage and the crankcase and would certainly be affected by movement between the handle and the rest of the saw motor

 

I suppose it is hard to say if I am compensating for the problem with the idle settings as I have tuned the saw around the problem when I thought it was a carb issue