MS290 Rebuild

[stephenson]

So - second time rebuild - still doing it for a friend and it jacks me up that I am doing it again! Figured must have done something wrong - duh.

Same issue as first time - rings welded into the pistons lands and heavy scoring on the piston. First time there was some scoring on the cylinder wall aligned with the exhaust port - I could catch my fingernail on it so not sure if that is considered heavy or light or somewhere in between - in any case, I smoothed it out quite a bit with sandpaper and then completed the remainder of the rebuild with new piston, rings, etc.

I was quite proud of helping my buddy out, and he hadn't said anything so I asked about it a couple of months ago and he said it had stopped working again ... I took a look - looked same as the first time.

I asked all the usual questions about oil ration, etc - I checked gas - looked fine and he clearly knew what he was mixing and the ratios.

So ... here are a couple of photos - would appreciate any advice at this point - this is way simple engine with little to go wrong ... could this all have been caused by being lean? I didn't mess with the carb when I put it back together ... is the cylinder salvageable? I am assuming these cylinders are coated with something like nikasil - can I sand it down again? I know I am pushing it out of round, but short of replacing cylinder, are there any other options?

ebay has an engine for $100 - clearly easy to replace engine, but I hate to give up on something so simple ...

Thanks in advance!!!

 

[Stihl #1]

If all the damage is on the ex side, from the middle working towards the sides, that is a typical lean running failure. If there is damage on both sides down the middle that is a typical lack of lube failure. So it looks like lean from your pic, since you didn't say the intake side was scored. Lean faults can be from many different places. If air comes through the carb but doesn't get enough fuel, you are lean. If the carb is working great but air comes in somewhere else, you are lean. If every thing is right and the carb is set too lean, failure may occur. And lean is lean, once on top of the piston. Take away the cooling effect of the fuel on the top of the piston, and since a saw doesn't have rev-limiter, then RPM goes up, temp goes up, and failure is eminent. The only way to know what caused this is to check for faults before taking the saw apart. Evaluate the fuel system: filter OK, fuel line holds pressure, tank vent opens under vacuum, fuel pump screen clear, impulse OK, diaphragms pliable. Test the crankcase: holds pressure, about 7 PSI steady for at least 20 seconds, holds vacuum steady while rotating the crank back and forth slowly.
After reassembly, repeat the pressure / vac test to verify that there are no air leaks. Once running tune to about 13K with bar and chain attached. If there is a lean fault you will have a hard time tuning for even running at 13K. After a few tanks set it to 13500.
I have a 290 I am getting ready to do a full tear down and rebuild, and I will take pics and post it like I did on the 034 a while back, but it will be a week or so before I have it posted.

 

[Stihl051master]

Make sure you check the carb boot also to verify it isn't cracked or not sealing somewhere also. I've seen many snowmobiles and dirt bikes punch their last tickets due to a leaky boot.

 

[stihl025]

Once running tune to about 13K with bar and chain attached. If there is a lean fault you will have a hard time tuning for even running at 13K. After a few tanks set it to 13500.

13,500? Kind of excessive, don't you think? My service manual says 12,500 max.

 

[West Texas]

Congratulations for being willing to tear down a 029/290 not once but twice. Been there and done that. ha. I'd rather work on almost any saw than these. It is difficult to get the piston and rings in that cylinder for sure; but, two points to ponder. Was the arrow on the piston pointing towards the exhaust port? And were the rings 'right' side up so they would compress around the little pins on the piston? I know those are both stupid questions maybe; but, at my age, I sometimes do stupid without realizing it. Good luck with it. You deserve a medal.

 

[stephenson]

Thanks for comments, guys ....

Lines, hoses, etc all look good without any cracks ... I should have checked the carb tuning prior to giving it back to him .... someone may have messed with it earlier to get higher revs and got it too lean ....

Examined the piston, again .... clearly scored centered on exhaust port, but also lighter, but distinct scoring about 120 degrees back to the left side (looking down on piston) toward the intake port ....

I like the lean answer since it means I didn't do a bad job previously ... I didn't replace the crank seals then, but it all looked so clean and new it didn't seem reasonable, plus didn't want to removed crank, just replace top end.

So, should I forego even trying to hone it smoother and get a new cylinder? they don't appear to be too expensive ...?

 

[stihl025]

13,500? Kind of excessive, don't you think? My service manual says 12,500 max.

I apologize fellas, I read the wrong thing. It is 13,000 max according to my manual

 

[stephenson]

Just rechecked - the other area of scoring covers about a third of the intake port .... not sure if is significant .... everything appears to be oily, including some residue in the case below the crank ...

 

[THALL10326]

Congratulations for being willing to tear down a 029/290 not once but twice. Been there and done that. ha. I'd rather work on almost any saw than these. It is difficult to get the piston and rings in that cylinder for sure; but, two points to ponder. Was the arrow on the piston pointing towards the exhaust port? And were the rings 'right' side up so they would compress around the little pins on the piston? I know those are both stupid questions maybe; but, at my age, I sometimes do stupid without realizing it. Good luck with it. You deserve a medal.

Hey West I got a few 290's uphere I'd like to send you to rebuild, why, just becasue I likes ya. Messing with ya Tex, I know how you feel about tearing those 290's apart,LOL

 

[THALL10326]

Just rechecked - the other area of scoring covers about a third of the intake port .... not sure if is significant .... everything appears to be oily, including some residue in the case below the crank ...

Have you got a tach to set the Rpms with?, if not your rebuild may be all for not. Gotta keep that thing within the safe range and its hard to do by ear nowdays.

 

[Stihl051master]

One other thought since you have the saw apart (I know it's not your own saw). If you have to go the new cylinder route, why not look for a MS310 or better yet an MS390 piston and cylinder? It'll bolt right on, and give you some more free displacement and power all at the same weight. The only downside is they seem to be a bit hard to find unless you pony up the $$$ for new OEM. Just a thought since the saw is torn down anyway...

 

[stephenson]

Rog on the tach - don't have one for small engines, but will look around for one. Since it seems to be richness related, I know what to look for now ...

Have been looking for 310 or 390, but as you said, hard to find.

This seems like a fair price for a complete engine .... $100(http://cgi.ebay.com/COMPLETE-ENGINE...trksidZp1638Q2em118Q2el1247#ebayphotohosting), or should I go for aftermarket P&C at $80 or so?

 

[Stihl051master]

It's kind of a tough call on that engine. You could just bolt it in and be done, but then considering it is used it's kind of a gamble on what you're getting. If you put on a new p/c, at least you'll know exactly what you've got.

 

[Lakeside53]

He says it's "excelent and no scoring".... go for it...

 

[stephenson]

Bought the whole engine - no major issues, but it does seem Stihl designed this saw for initial production without major consideration for rebuilding it - things like fuel lines and electrical cabling are not particularly logical in layout. In spite of rebuilding this same saw twice, I still spent an inordinate amount of time putting it together then pulling it back apart .... I figure I would need to do about 3 more times prior to it making sense! (maybe it's just me, too

OK - so, assuming the issue on previous two burned pistons was leaness, I took a look at the carb - low jet was about 1/4 turn from the far clockwise position (there are limit caps on the screws) and the high jet was about 1/8 turn .... this would point to lean as previous cause?

Prior to startup I reset both to half way positions and it sputtered at the fifth pull - popped choke off and it started and ran OK on the next pull. I don't have a tach - is there a "close enough" way to adjust carb without one so I am on safe side?

Thanks again for all the advise from everyone!

 

[Lakeside53]

Given your saw history... I would pull the limiters and retest the carb settings -they should be close to 1 turn out for decent idle and slight burble at WOT.

And the the usual - fuel lines, filters, etc..