Stihl 028 AV wood boss rings/piston

[Pim]

I bought an very old Stihl 028 Woodboss on a garage sale. I starts easily, didn’t tested it further.
I started cleaning it and noticed a lot of carbon build up on the top of the piston. So I lifted the cylinder. I noticed there was also carbon underneath the piston rings. So I removed piston and rings to clean them. Underneath the piston there was also a bit of carbon.
The cylinder looks good no major scratches etc.. The piston and rings also looks fine visually.
The diameter of the piston is 44mm.
I measured the ring end gap: 0.017" and 0.013"

Do I need to replace the rings or rings + piston or just try it without replacing anything?

 

[pioneerguy600]

Just replace the rings,carefully clean out the ring grooves ,a broken off piece of ring segment will do to carefully scrape out the carbon. New rings should start out with .009 ring end gap.

 

[Wood Doctor]

I liked PioneerGuy's response and I also like the way this saw runs today:

I ran it last week and it performed beautifully with an 18" bar and chain. Vintage 1986 or so? Not sure. I added the outside bumper spike and gave the carb a good cleaning and tune up. New fuel line and fuel filter finished it off. That's about all it needed.

 

[Pim]

Thanks pioneerguy! I received the new parts yesterday. Hopefully I can find some time today to assemble.

I also ordered new cilinder gasket, fuel line and fuel filter.

@wood doctor.
Your saw looks almost new! Did you paint it white?
Mine is also from 1985 or 1986. The original bar has 86 engraved. And the carburator 85.
I will post some pics when it is assembled.

 

[Pim]

Ok, assembled the saw and gave it a test run.
After 4 pulls it started. It idles fine.
But almost doesn't response to the throttle, the rpm's go up only a bit.
Tried adjusting the carb, no change

I'm going to replace the impulse hose and clean the carb again and see if that does the job

 

[JonCraig]

If you’re up for it, you might consider replacing bearings, crank seals, A/V buffers, etc.

Bearings are 30+ years old (so therefore suspect) and easy enough to do in an 028. Crank seals are cheap insurance, A/V buffers—aftermarket ones are cheap and have always worked flawlessly for me.

No special tools required for splitting an 028. Just some big sockets to push out old bearings and a leather (or soft rubber) mallet for the cases. Great one to learn on if you’ve never done it before.

(My $0.02).

Also, that saw is purty enuf that she deserves a new bar. ;-)

 

[pioneerguy600]

Check to see if your throttle linkage is installed correctly and is actuating the throttle butterfly to open completely. One of the things I have encountered on clients saws after they had the carb off. If it is working correctly then further investigation will turn up what is going on.

 

[Mad Professor]

That's a nice looking 028!

Not sure how you adjusted carb, but if it stumbles off idle when opening throttle it's usually a little lean on low. Might try turning L out 1/8-1/4 turn and see if that helps. May have to reset idle.

 

[Wood Doctor]

It's been awhile, but I believe I may have spray painted the front of the case only. The rest was hardly worn so I left it alone. Then I touched up the STIHL name on the sprocket cover with a black Sharpie.

I've been criticized for doing any of this by some firewood collectors and sawyers. They say the scrapes and bruises add character to the saw. I guess it's to each his own. The important point is (and I agree with them) that the saw starts and runs well with good power.

 

[Pim]

Today I changed the impulse hose and cleaned the carb again.
The carburator is a Tillotson HU-40D, and I don’t have a rebuild kit for this one.
But I have a walbro repair kit, and the diaphragm is nearly the same, so I thought let’s give it a try and replace the diaphragm.
Reset the carb screws L & H to 1 ½ turn out.

Gave it a quick test run.
It started good again. And I now it also response properly to the throttle!

But…………. next issue: it started to behave erratic. On idle the rpm’s go up and down again on its own.
So I suspect an air leak.

I can rule out fairly curtain the:

• Impulse hose -> new
• Cylinder gasket -> new
• Intake boot -> checked when replacing piston rings

That leaves the oil seal rings and case gasket (or did I forgot something?)
My next move would be to remove the flywheel and clutch, to inspect the oil seal rings.

Or do you have other suggestions?

 

[JonCraig]

I’d say crank seals are your next bet. I *think* they’re the same as those for an MS260, but double check with an IPL before trusting my memory.

 

[Wood Doctor]

I reread the entire thread, No evidence of a compression test. Have you measured the compression? If so, what was the reading?

 

[Pim]

Compression test is 120 psi.

 

[Wood Doctor]

Compression test is 120 psi.

That's a bit low. Usually you need about 130 psi to hold a good idle. You are on the borderline. I'd say run it the was it is, but don't expect too much from this saw. If it gives you more trouble in the future, rebuild the too end.

 

[billyboy]

The info i've seen says 1 turn on the H & L carb screws.

 

[alfbolin]

If I understood OP right , he just put rings in it?. Any chance they haven't seated yet-resulting in a lower compression reading? If it is just taking a couple of pulls to get it fired up you might want to check compression with a different gauge too. Seems like in my experience saws that start in a very few pulls (especially after being apart) usually have more than 120psi compression? I have been all over the place with one of my 028 wb saws on won't idle, think it is an air leak deal. 165psi compression. Pressure and vac tested. Seemed to ck ok. Decided I didn't trust my testing so I went ahead and did the crank seals. Used new oem seals. New impulse hose. Checked intake boot very carefully. etc, etc. Still acting goofy. Finally bought an am walbro and slapped on it to straighten it out. I had been through the carb twice with Walbro kit. Who'da thunk it? Went through a similar ordeal with a FS55R trimmer a couple of years ago. Bought a new Stihl carb for it. Problem solved. Somebody correct me if I am wrong , but on a 028, if you forget to put the switch in the off position when putting the air filter back on it can cause weird stuff to happen with the throttle and choke linkage, can't it? Seems like I went down that road once a long time ago.

 

[Pim]

If I understood OP right , he just put rings in it?. Any chance they haven't seated yet-resulting in a lower compression reading? If it is just taking a couple of pulls to get it fired up you might want to check compression with a different gauge too. Seems like in my experience saws that start in a very few pulls (especially after being apart) usually have more than 120psi compression? I have been all over the place with one of my 028 wb saws on won't idle, think it is an air leak deal. 165psi compression. Pressure and vac tested. Seemed to ck ok. Decided I didn't trust my testing so I went ahead and did the crank seals. Used new oem seals. New impulse hose. Checked intake boot very carefully. etc, etc. Still acting goofy. Finally bought an am walbro and slapped on it to straighten it out. I had been through the carb twice with Walbro kit. Who'da thunk it? Went through a similar ordeal with a FS55R trimmer a couple of years ago. Bought a new Stihl carb for it. Problem solved. Somebody correct me if I am wrong , but on a 028, if you forget to put the switch in the off position when putting the air filter back on it can cause weird stuff to happen with the throttle and choke linkage, can't it? Seems like I went down that road once a long time ago.

I think the compression is ok and it's more likely my very cheap compression set is wrong. I can't remember that I ever seen it over 125 ~ 130psi. (But I'm an amateur)
And 4 pulls on cold start after carb was cleaned sounds good (warm only 1 pull).

I do not have a vac tester.
I ordered the crank seals, will try replacing them first.

Thanks for the tip regarding the carburetor, I will try replacing it after I tested the saw with new crank seals
Found a cheap aftermarket carburetor on ebay for less than $8 incl. shipping. Sounds a bit too good to be true, does anybody has experience with an aftermarket carburetor?

 

[alfbolin]

Yes mine was an eBay chicom carb. I paid like 10 or 11 bucks and coulda got free shipping but opted for 3 day priority which cost me another 7 bucks. I was having a rash of USPS regular ground shipping stuff getting lost for a week or two about that time. By all means, do the crank seals while your tinkering. The way I figgered it- its a 30 yr old saw. Seals are not gonna be a bad investment in it's future. Leaking or not. turitrade was the eBay vendor I got the carb off of. There were several cheaper ones out there, IIRC, I was just comparing descriptions and seller feedback ratings. Oh, BTW, I am definitely an amateur in the 2 stroke realm as well. I enjoy hanging out on here and learning, mostly.

 

[alfbolin]

BTW, I did have to set the screws to spec and then adjust as usual. Pretty sure I set them at 1.25 turns out to start with.

 

[tomdcoker]

Ok, assembled the saw and gave it a test run.
After 4 pulls it started. It idles fine.
But almost doesn't response to the throttle, the rpm's go up only a bit.
Tried adjusting the carb, no change

I'm going to replace the impulse hose and clean the carb again and see if that does the job

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Very nice 028. Tom,