Stihl 028 av super, starts runs but dies when choke is switched off

[mtown]

Bought a used 028 av super last year for my son. It started and ran fine for me with light use a year ago. He had a few problems with it running consistently right last year. I took it to a local guy that fixes saws for a friend that has a very large tree business. He said the linkage was in need of adjustment. $50 and saw ran fine the little it got used last year.

My son has not been able to get it running right again this year. I have inspected linkage and it seems correct to me. High and low jet screws are all the way in and then backed out 1 1/4 turn each. The saw starts fine when choke is on and runs at very high rpm with choke on. As soon as choke is flipped off it stalls.

It almost seems as though the off switch is killing it. I even disconnected the little metal end and taped it to see if it was shorting out. That did not help. Carb has been ordered and I am comfortable with that project, but I am doubting it is the problem.

Thanks in advance for any ideas.

 

[Rio95]

If its running at high rpm with the choke on, you've got an air leak - most likely in the intake between the carb and the cylinder. Basically, the saw is drawing air in somewhere other than through the carb.
Look to see if the rubber has any cracks, or if its not fitting together properly. When its running, you can squirt some WD40 around the rubber intake, when you squirt on the area that is leaking, it will bog down.

 

[pioneerguy600]

If the saw runs at a very high RPM with the choke on it just might have a bad air leak around the crank seals. It might just be the carb not passing enough fuel through it when the choke is off and barely enough to run the engine very lean with the choke on but either way a proper Vac test would tell a lot about what is going on.

 

[HarleyT]

Sounds like a badly clogged up carb.

 

[HarleyT]

If you are putting on a new carb, replace the fuel filter as well, and of course inspect the fuel line and intake boot, impulse etc..
The old fuel filters had replaceable foam inserts, and that foam starts to break apart after 20 or so years. The debris will collect inside the carb and cause trouble.
Here is the pic of one out of a junk 028...



The lead weight didn't want to come out for the pic.....

 

[mtown]

I assumed there was such a filter in the fuel tank. Thanks for that info.
What baffles me is that it runs so strong when choke is on, and stalls completely when it is turned off. Guess I will find out when new carb arrives.

 

[HarleyT]

When the choke is on, you are restricting the air flow so the fuel/air ratio is closer to the correct running ratio, when the choke is open, then there is almost no fuel at all in the fuel-air mix.

 

[HarleyT]

The new fuel filters are one piece, and cannot be dis-assembled.

 

[mtown]

Got a new carb and it came with 2 gaskets. Both have holes that match the studs that hold the carb on. There was no gasket on either side of the carb I removed. Can not find an exploded view to tell if I need to install gasket or not.

 

[HarleyT]

No gasket, just these.

 

[mtown]

Harley,
Thanks for all the input. I ended up using both gaskets. One at the carb to the intake manifold, and then added one at the carb to air cleaner. The saw was doing about the same as before but improved when I added the second gasket. I was thinking the air leakage may have been from the seal not being tight to the air filter.

In playing with those things, I accidently knocked loose a 2-3" piece of tygon tubing. It was right near the kill switch and appears to be some kind of breather vent for the fuel tank. It was only connected at one end to a small plastic nipple. Now for the weird part. I was inspecting it closely and attempted to blow some carb cleaner through it. Totally blocked. I pushed the plastic tip of the cleaner through it and pushed out the other end two tiny set screws that were completely blocking any flow through this tube. The new carb came with 2- 10" pieces of tygon, so I cut 3" off one and installed it.

Some minor tweeking of the H and L screws and the saw is running, cutting, and idles fine.
Do you think I am hurting anything by leaving the gaskets in place?

I got the new fuel pick up filter. Do you just use needle nose to fish the line out of the tank?
Thanks again.