028 bogging when warm

[ja1724]

I have an old Stihl 028WB. When first fired up runs and cuts great for about the first 5 minutes. Once fully warmed up boggs down something terrible whether in the wood or free wheeling. Almost sounds like the chain brake is on, no power at all.

What sound I be looking at first? Carb adjustment?

Thanks.



Stihl 028WB
Shindaiwa 500
Shindaiwa 488
Shindaiwa 680

 

[Mastermind]

I have a habit of pulling the muffler off to check out the condition of the piston, easy to do and it rules out a scored piston before you sink a bunch of time into it.

 

[pioneerguy600]

Yep, check piston condition and compression, it falls off as the saw heats up.
If that is all good then it may be a bad tank vent.
Pioneerguy600

 

[Fish]

Look at the ignition as well.

 

[trouttrooper]

sounds similar to what my 048 did last year. Turns out it was the fuel line. It was so old and soft that it would collapse on itself when it got hot enough. I would tear the saw apart after it cooled down to find the problem, but by then the rubber would be firm again. Took me awhile to figure that one out.

 

[Fish]

sounds similar to what my 048 did last year. Turns out it was the fuel line. It was so old and soft that it would collapse on itself when it got hot enough. I would tear the saw apart after it cooled down to find the problem, but by then the rubber would be firm again. Took me awhile to figure that one out.

Yes, that is a good possibility as well, give the fuel line a good pinch just above the filter, if it stays together, your line is bad.
Also, after 20 years or so the foam inserts in the fuel filter breaks apart and collects like a wad of snuff in the screen in the carb, but as said before,
look at the piston condition by removing the muffler, first.

 

[ja1724]

Ok, here's where its at. Compression 130psi. I tore the whole thing down last night and here's what I saw:

Piston, rings, and cylinder all looked great for their age.

An air line (I'm guessing) which runs from the carb to a barbed fitting at the cylinder was extremely loose at the cylinder fitting. I replaced this line.

A semi-clear line which appears to be coming from the gas tank and ending near the air filter has a tiny screw in the end of it. I don't know if it is supposed to have that screw there or not.

I doubt its the ignition. It's just a simple magnet/magneto setup and these usually either work or not at all. Seems like a fuel supply issue, like a vapor lock or collapsing line. Hopefully we'll see later today.

 

[Mastermind]

I could be wrong, but 130 psi is a bit low for that saw, the last 028 I checked had 155, and my 028 super is at 180 psi without a base gasket. That's not so low that it won't run, but if I had it apart I would put rings in it. Just go ahead and replace all the lines and the filter, now's the time while it's down. And yes the screw piece is suppose to be in the vent line, but still could be too tight. If I'm having a problem with one of that style I'll back it out until I'm sure that it's not the issue. Good luck and welcome.

 

[Fish]

Your saw likely has points and condenser, look for wires going under the flywheel.
Did you replace the impulse line with the correct part, or just generic fuel line?

 

[adkranger]

130# seems low to me too. All four of my 028 supers hold better than 155. But it's possible it's your guage too, compare it against another saw??

Check out that vent tube real well (semi clear fuel line w/grub screw). If it is original and really stiff, I'd replace it. They tend to get hard over time and either pizz fuel all over you or lock up tighter than a drum and starve the carb for fuel. Common malady for this family of saws, but a really simple fix.

I'd also replace both the fuel and impulse lines while I had it stripped down. New fuel filter too. Then I'd take a look inside the carb and particularly where Fish mentions, the fine screen at the carb inlet. If your metering diaphram is stiff or otherwise suspect, put a carb kit in it.

If fuel delivery system is ruled out, time to pressure/vac test the case. Though I'm not thinking that's your problem. Check out your muffler too, see if your spark arrestor is plugged (if it's stihl in there...). Does the saw run smokey/rich? Or in the past? This I find more a typical problem with the little saws (009/011/012, 020, etc), but could be influencing yours too? Good luck.

 

[ja1724]

I'm going to replace some parts on this puppy while I've got it torn down. The spec sheet I have says this 028WB has a 42mm bore. I'm want to replace the rings but all I can find are 44mm ring sets.

Can somebody confirm what bore size a 028WB actually is? If it is 42mm, anybody know where I can get a set of rings or a piston/ring kit? I found some sets on ebay but all are 44mm or 46mm no 42mm.

Thanks.

 

[Mastermind]

Is the piston domed? The 42mm usually is not flat topped like the 44mm, but the super (46mm) has a pop up in the center of the piston. If you have to, have it measured, or better yet get an inexpensive caliper, less than 20 bucks.

 

[Mad Professor]

Is the choke (inside air filter) opening up all the way and staying open?

 

[Fish]

I'm going to replace some parts on this puppy while I've got it torn down. The spec sheet I have says this 028WB has a 42mm bore. I'm want to replace the rings but all I can find are 44mm ring sets.

Can somebody confirm what bore size a 028WB actually is? If it is 42mm, anybody know where I can get a set of rings or a piston/ring kit? I found some sets on ebay but all are 44mm or 46mm no 42mm.

Thanks.

Wow, you have it down that far? What about the points and condenser?

You could slap on an electronic coil pretty cheap.

I always like to know the problem before replacing parts, but that is just me..........

 

[ja1724]

Is the piston domed? The 42mm usually is not flat topped like the 44mm, but the super (46mm) has a pop up in the center of the piston. If you have to, have it measured, or better yet get an inexpensive caliper, less than 20 bucks.

Yes it is slightly domed. Does this mean its most likely 42mm. I tried measuring with my caliper but its kind of worn and I can't tell what the reading is. It looks like 43mm ??

 

[ja1724]

Your saw likely has points and condenser, look for wires going under the flywheel.
Did you replace the impulse line with the correct part, or just generic fuel line?

I don't have a flywheel puller so I haven't changed them. I used generic fuel line as I haven't found the OEM impulse line yet.

 

[Mastermind]

Wow, you have it down that far? What about the points and condenser?

You could slap on an electronic coil pretty cheap.

I always like to know the problem before replacing parts, but that is just me..........

Help me out (again) Fish. I've heard different tales, when changing from points to electronic on the 028, do you have to change the flywheel too? Or just the coil?

 

[ja1724]

Wow, you have it down that far? What about the points and condenser?

You could slap on an electronic coil pretty cheap.

I always like to know the problem before replacing parts, but that is just me..........

Yeah, I can tear this puppy down and put back together in about an hour (not sure if this is good or not). I agree I don't just want to throw parts at it but I figure if I got it torn down I might as well change the original rings, they've got to be worn being nearly 30 yrs. old. The piston looks ok.

Any leads on 42mm rings? Are the 42mm rings 1.2 or 1.5mm thick? I think I saw some 42mm/1.2mm on woodsman, but they listed them as for an 025. If there the same size they gotta work on an 028 as well.

 

[Fish]

Help me out (again) Fish. I've heard different tales, when changing from points to electronic on the 028, do you have to change the flywheel too? Or just the coil?

Just cut the wire to the points, and hook the kill wire to the terminal on the coil and go.
A used 0000-400-1300 goes real cheap on e-bay.

 

[adkranger]

Yeah, I can tear this puppy down and put back together in about an hour (not sure if this is good or not). I agree I don't just want to throw parts at it but I figure if I got it torn down I might as well change the original rings, they've got to be worn being nearly 30 yrs. old. The piston looks ok.

Any leads on 42mm rings? Are the 42mm rings 1.2 or 1.5mm thick? I think I saw some 42mm/1.2mm on woodsman, but they listed them as for an 025. If there the same size they gotta work on an 028 as well.

If the cylinder/piston stihl look good, I'd throw new rings in and go. After a good clean up of course. 1.5mm thick I believe.