STIHL 028 Compression spec - spoke to Stihl

[abl1111]

Trying to troubleshoot the low compression on my Stihl 028 w/ a 44mm cylinder.

I read 100 cold ! 90 hot !

Trying to ascertain whether I need a cylinder, a piston or both ( in a perfect world, I would just have slapped on a new P&C and been done with it ! )

I already replaced the rings. No comp change. Still 100 !

Called up Stihl as there seemed to be a lot of info out there ( here and local Stihl dealerships ).

For this specific saw:

100 - is low.
120-130 - is a good running saw.
130-150 - is/was expected from a new saw

I am still at a loss as to what would give me the biggest bang for the $ and I don't want to waste $

I do not have a caliper to size up my cylinder and bringing it to Stihl dealers has been a waste - they're not equipped to say it's good or bad. Same w/ the piston. They just recommend to R&R them both. Again - a lot of $$$.

Buying a used P&C seems sketchy to me, unless there is a confirmed compression test, as I can be getting no better than what I already have.

I am a novice 2 stroke mechanic ( although, holysh*t, I have gotten much better of the last few weeks ). I use a saw minimally to cut wood and to just have a good working saw around.

If you were me, what would you do ?

 

[spudulike]

Simple question - are you pulling the saw over one single time or 3-5 times when checking the compression - your 90-100 psi is about what you should get with one tug, full compression should come within 3-5 tugs.

125psi is in need of some work 150 psi is good partially worn saw and 150-175psi is new decent compression.

The other thing you must do is to try your gauge out on a decent condition saw and do a comparison, you either have a poorly saw, the wrong method of checking compression or a dodgy gauge.

Hope this helps and hope you are just giving the saw one tug - just pull it over some more and reach top reading!

Spud

 

[oscar4883]

Still have to check on that 028 cylinder for you. I will try to get to it tomorrow. I would make sure to get some measurements on your p/c so you have a better idea of what you are working with. That way you can find out how far out of spec. they are and if it is still usable. Maybe a local machine shop could do it for a couple bucks. The last 028 I built had piston slap. Learned my lesson on measuring everything. Or, if you plan on doing another saw, buy a cheap set of calipers.

 

[Mad Professor]

Maybe you need to run a while for the rings to seat?

 

[BloodOnTheIce]

How does the saw run?
Does it start and operate as it should?
Does it bog down in the cut or does it have enough power?
What do the piston and cylinder look like?

Why were you going to replace the Piston and Cylinder?
I've have good luck with used top ends and a new set of rings.

Most saws with less than 100-110 pounds of compression will barely run, and if they do, they will die when put under load.

 

[abl1111]

Spud - I think 175 psi must be for different saws. Stihl's spec, as mentioned for the 028 is what I 'have' to go with... Maybe it is a 'low compression saw' to begin with ??? I don't know. I have gotten so many different #'s ...

The comp testing I'm doing is with many pulls; 3-5 at least. I am just taking a comp tester ( w/ a schrader valve ) - threading into sparkplug hole - throttle open, 5-10 pulls and reading gauge...

Oscar - THANKS.. And, I am going to take the cylinder and piston w/ me tomorrow to see if I can find a machine shop to mic everything ( maybe a motorcyle shop ?).

Mad - i've read that too, but I've also had guys say, the minute they put the saw together, the comp was where it was supposed to be ?.

Blood - never even put it to wood. It runs great - never tried under a load yet. Cylinder looks good to me - no scores at all. Piston looks good too; ring to ring groove tight - intake side more worn than other side...


Definitely a challenge...

 

[Saw Dr.]

If it runs good, why did you not try it out? I'm not sure what you're trying to fix if it runs good. I think you have a problem with your gauge.

 

[timberwolf]

I have seen bone stock A1 condition stihls range from 150 to 180 psi same gauge even just different years of same model. If a saw is down 140 psi and should be 160 plus it has lost something, will it run and start fine? You bet, but it won't have the same zip. Less than 120 PSI IMHO and forget it it won't run right period.

The only real way to know if you need a piston, cylinder or rings is to pull it down and inspect. Check ring end gap, piston clearance, wear in the piston ring grooves and skirts. look for scratches, chips or places worn through the plating.

 

[Saw Dr.]

He had pics of the top end on here on another thread. I think we all can agree that it would not even run at 100 PSI, so I'm thinking his gauge is junk. He says it runs fine. Low on power may be a different story, but I have not seen any saws that were borderline on compression that would idle for more than a few seconds.

To the O.P., will it idle?

 

[abl1111]

Guys - thx for all the replies.

A friend gave it to me because he said it would start, idle rough ( had to be goosed ), run for awhile, but when shut off he could not re-start it.

He brought it in to a Stihl shop and they said " low compression ".

I inherited it. I found the same thing. I did a compression test it read 105. Researched and was told it was low. I pulled the cylinder off and here I am...

I have put the cylinder on and off half a doz times checking this and that as people on the site have been trying to help.

Replaced the rings only. Comp the same.

I exchanged my comp tester w/ a replacement of the same kind.

I swapped the spark plug, adjusted the carb to factory - and that seem
ed to help with the idle and the starting issues.

Vac and pressure are perfect. Oil pump seems fine. It's a worthy saw to fix. It's just the price of a P&C is cost prohibitive.

And, prior to 2.5 weeks ago, I never touched a (2) stroke - did not know how they even worked.

I've learned a lot - thx to this site.

What about the phone call to the tech dept @ Stihl - direct - and his comments about compression for this saw ? Is he wrong ?

Humbly,

The 028 Kid !

 

[billmartin]

If you have a new ring in there... Take it to some wood:greenchainsaw:

I recently tore down my perfectly good 257 to install a used jug that I ground on for a bit. Put it together and happened to come across my comp. tester. I only had 130 on the gauge and was highly bummed.

Took it out to the wood pile and tuned it to the best I could(btw I' m pretty green on 2-strokes also) and did some cutting.

After a few random uses I had gone thru a half tank and decided to recheck my saw. Bang 160psi cold and probably going to gain a bit more

I say run it for a tank or so and see if the ring seats up....

Good luck on it. Oh does that model have a de-comp valve that might be leaky?

Bill

 

[abl1111]

No de-comp valve.

Funny thing is, I have no wood to cut right now !!!

I'll try to have a tree-buddy of mine drop off some wood that I can cut up.

I have the P&C off again. I just wish it was clearer as to " what is broke, so I can fix it ! "

 

[billmartin]

That's got to be frustrating. In the mean while... It's apart... Gotta grinder and some solder???

Open up the exh,intake and muffler then set your squish to around 20thou.

Well thats what I would do:hmm3grin2orange: If stock is your deal thats ok to.

Have you checked the squish? maybe it's really big? Just throwing out nonsense here

 

[wigglesworth]

No de-comp valve.

Funny thing is, I have no wood to cut right now !!!

I'll try to have a tree-buddy of mine drop off some wood that I can cut up.

I have the P&C off again. I just wish it was clearer as to " what is broke, so I can fix it ! "

If there is nothing visibly wrong, as in the piston wore out, the ring end gap is huge, the cylinder scored, than the only thing that would cause the low compression is too big of a chamber/squish area. That is it. If everything else is in good shape and to spec, that is the only reason for low comp. Period. Check the squish band clearance and see what you have.


From this pic you posted in one of your 6 threads about this particular saws compression.....

....it appears to me that the squish clearance is huge, It looks like the rings were stopping 1/4" from the top of the cylinder wall. CHECK THE SQUISH!!

 

[Rookie1]

opcorn: Im curious.

 

[wigglesworth]

This is a pic of your piston correct?

The machine mark's are still clearly visible. Not anywhere near wore out.

 

[abl1111]

Wolf - I checked the piston clearance - I assume that's the space between the piston and the cylinder -

With the piston set down w/ skirts:

- even w/ bottom of cylinder, I could fit a .003mm along the P&C - 1/4" down
- piston 1/2 way down, I could barely fit .003 only one side of skirt 1/4" down
- piston all the way down - could not fit .003 at all anywhere.

Space in ring gap :
The spaces for the ring ends placed in the cylinder pushed up beyond the intakes w/ the piston:

- old rings pulled out originally - .018
- new replacement rings - .011


Bill - when I get the comp figured out, I will find out about grinding things...

Wiggle - I will check squish tomorrow. Let's say my squish is huge - removing my .020 gasket ain't gonna cut it...

Gotta remember, I'm in the learning phase. With my other 'seasoned' hobbies, I'm actually a guy like you - very knowledgeable... But, a chainsaw - I'm a babe-in-the-woods. Learning hardcore, (2) stroke engine diagnosis and repair ain't an easy endeavor...

 

[mtngun]

Wolf
With the piston set down w/ skirts:

- even w/ bottom of cylinder, I could fit a .003mm along the P&C - 1/4" down
- piston 1/2 way down, I could barely fit .003 only one side of skirt 1/4" down
- piston all the way down - could not fit .003 at all anywhere.

Space in ring gap :
The spaces for the ring ends placed in the cylinder pushed up beyond the intakes w/ the piston:

- old rings pulled out originally - .018
- new replacement rings - .011

Repped you for the good data. You're doing good.

 

[abl1111]

ready for feedback on the measurements i took

- is ring gap good within spec

- cylinder / piston gap

 

[Fish]

If that is a pic of the piston, I must suggest that someone put in a piston
that goes to another model, as that one looks too new to be original.
And if it is the wrong piston, then that may be the problem, as suggested
by Wigglesworth.

Here is a pic of an 028 46mm, I may have a 44mm here somewhere, note the piston top too.