Chainsaw Repair
Husqvarna - Stihl - Poulan - Jonsered - Dolmar chainsaws and more => Stihl => Topic started by: hessaacd on May 03, 2014, 08:25:53 am
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My stihl MS250 doesn't have very good compression - 90 dry, 120 PSI wet.
The images attached to this post show the piston and cylinder. I was curious if I should replace them or not.
Any help is greatly appreciated!
Alan
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Look on Ebay at the pistons and cylinders or complete power units offered?
It looks pretty ragged to me? I'd be trying to determine what happened?
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+1, I agree pretty rough. You already obviously have it torn apart, replace the cylinder and piston. Also given the scoring on the piston you may have a leaking seal. They are cheap so you should also replace them. Have you checked the bearings?
You can get everything needed here:
http://www.hlsproparts.com/category-s/82.htm (http://www.hlsproparts.com/category-s/82.htm) Use 'chainsaw' in the coupon code field and get 10% off your order.
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I agree with the others....you need to replace the piston and cylinder. While you have your saw torn down, check your impulse line and your intake manifold boot for any leaks or dry rot cracking. I also agree with the others...looks like you got a air leak!
On the re-build, unless you have or have access to the Stihl hookless circlip installation tool, you will most certainly have fun installing the "hookless" type of circlips! Since I retired and I do not own the Stihl tool, I like to use the "G" type circlips as seen in the picture on this link. They are very "controllable" during installation with good needle nose pliers. Also the price is right....
http://www.ebay.com/itm/Cylinder-and-Piston-Kit-Fits-Stihl-MS230-MS250-023-025-Free-Shipping-Low-Price-/271143412712?pt=LH_DefaultDomain_0&hash=item3f216823e8
Replace your crank case seals too. On the clam shell re-builds, I use and Dirko HT sealing compound.
Tip, when installing the crank shaft seal on the clutch side, put a layer of electricians tape around the crank shaft stub over the retaining ring groove. Put some light grease on the seal and ease it into place, The electricians tape will keep you from damaging the seal when it goes over the groove.
Dave
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Neat trick with the tape Dave!
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He does some real kinky chit with Duct Tape!!!!!
Or in his case DUCK tape......
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As everyone else says replace the piston and rings.
They aren't the highest compression saws tho and off the stihl line only pulled about 120-125 psi when new, obviously that went up a bit as they where "run in"
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Many thanks for the ideas. Just ordered new piston and rings.
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Make sure you figure out what caused the scoring on the piston and correct it! Otherwise you will be doing this again.
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After checking for dry rot on the hoses and intake boot and doing a soap bubble test on the crankcase seals it seems they are OK. The rubber gasket between the carb and the crankcase is a little torn around the edges. Could air leaking in there cause the pistons to wear?
I don't understand how air leaking into the crankcase can cause the pistons to get damaged like that.
Alan
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The saw breathes through the case. Excess air coming in from the outside leans out the fuel/air mixture. Too lean fuel/air mixture creates excess heat causing premature wear/damage to the saw. Lean=more air than necessary.
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Yes, one could melt the piston on a brand new saw just by adjusting the carb incorrectly. You can sit it on a bench, and slowly turn the high screw in and get it really screaming, say 16,000 to 18,000 rpms for 40 or so seconds.
You need to "richen" the mixture to slow them back down to 13,000 or so, depending on the model.
Also, since the lube is in the fuel, the leaner the mix, the less lube there is for the engine, which makes it overheat
exponentially. So if one is using his saw, and it starts acting squirrelly and dying, he needs to shut it off and fix the problem.
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+1 Fish, good advice as usual!
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WOW! Fish said it all and then some! ;D
Those that want to set up their saws by "sound"....usually end up with scoring of the piston and cylinder. Buy a good reliable tachometer, then tune! I have seen many a good saw go to ruin by those that relied on "set to sound" method of tuning. In fact, the shop where I retired from, all new Stihl saws came with a nice little orange handled screw driver...we actually had several hundred of them all bunched up like a bouquet of flowers in a big vase....in the shop area! All for the simple reason we didn't want the "new owners" tuning their new saws! When we sold a new saw, we serviced it before the new owner got it into his hands. Hardly ever a time we had to actually "tune" a NEW Stihl product! I can't say that about the others we sold or serviced! ;)
We also had a saying in our shop with "some" warranty claims that came in....warranty doesn't cover stupid!
Dave
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The newer designed chainsaws do not have such a definite two cycle - four cycle break point as the chainsaws from 20 or more years ago.
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Well, I put in the new piston and rings, it started normally, I ran for about 15 seconds, felt like it had good compression, then I turned it off for the night, planning to put on a bar and chain today.
Now it's back down to 90 PSI.
Does this indicate I might need a whole new cylinder/piston assembly?
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Try a different gauge JMO
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Compression psi only tells you that an engine will run, not how well. It also takes a little longer than 15-20 minutes of run time for your new rings to seat onto the cylinder wall. Run it through a couple more heat cycles and then test it again. Keep the carb rich for the first few tanks of fuel and try a different gauge. If the cylinder has alot of time on it, it may also need to be replaced. To give you an idea how long it may take for those rings to seat, some engines take 6-10 tanks of fuel before everything seats.