Low compression - 60 psi

[Bob_w]

The MS391used to run well. It is a used machine, that I have had issues with. I haven't run it for a few months, now it won't start. I have found that I have spark, The plug really does not get wet. So I checked the compression. I'm getting 60psi...not good. with the same pressure test unit, the 272xp reads 120 psi, so the tester is good. I just got a pressure vacuum test tool. What do I look for? The first thing I'm checking is the torque on the head screws. people have worked on this before!

My first problem with this machine was flywheel timing incorrect. I checked it again, and this is not the problem. nope. 60 psi. So what is next? Thanks

 

[rupedoggy]

Pull the muffler and see the piston all scored up

 

[Bob_w]

I will, It wasn't before, but I will check that. I'm suspecting that the previous owner was either smart or dumb. Smart, if he knew that the saw has an issue, but covered it. Or dumb if he took it apart and didn't put it back together correctly. Apparently, the flywheel molded key was stripped and the machine was way out of time when I got it. ... Probably not torqued to spec.

Thanks , I will look at that.

 

[hacskaroly]

Like rupedoggy stated, pulling the muffler will allow you to see the exhaust side of the piston to look for scoring, possibly even if the ring is stuck in place. Also when you lower the piston, you can see the cylinder wall on the input side for any scoring there. Pulling the spark plug will allow you to see the cylinder wall on the exhaust side. Unfortunately new cylinders for the 391 on Ebay/Amazon average around $140 USD, but a new Stihl kit will probably be pushing $350 USD. Depending on how bad the damage is, you might be able to get away with a little bit of honing and replacing just the piston head. I did this with a MS 291 that lost compression due to scoring on the exhaust side (ran with straight gas, no oil) and got it running again - cylinders for this unit are almost impossible to get without paying $300 USD, so I figured that I would give it a shot honing the scratches and dropping in a new $40 USD piston. Its running again!

 

[hotshot]

60# is less than half of a new MS 391, it’s time for a rebuild.

Don’t forget to perform your pressure & vacuum testing BEFORE you tear it down.

Doing that first will let you know what other new parts you’ll need, in addition to the new piston & cylinder.

 

[buzz sawyer]

120psi for a 272xp is not great either. I would expect 150+

 

[J D]

120psi for a 272xp is not great either. I would expect 150+

+1
If that 272 runs halfway decent then I suspect the guage is reading low or not one designed for small engines.
That aside the 391 is still comparably low so as stated the next step is to pull the muffler & inspect the piston.
Post some pic's & we'll go from there

 

[SteveSr]

I will, It wasn't before, but I will check that. I'm suspecting that the previous owner was either smart or dumb. Smart, if he knew that the saw has an issue, but covered it. Or dumb if he took it apart and didn't put it back together correctly. Apparently, the flywheel molded key was stripped and the machine was way out of time when I got it. ... Probably not torqued to spec.

Thanks , I will look at that.

While you are looking at things... On this series of saws the transfer port covers are GLUED on to the cylinder. If the saw overheats the glue softens creating an air leak which puts the final nails in the coffin.

 

[Cantdog]

If you find no bad scoring after removing the muffler I would suspect a stuck ring since it's a well used machine....especially if if it was running fine when you used it last Not a rare occurance with older saws when the ring loses some tension due to plain old wear and carbon builds up in the ring lands..

 

[Evan]

Worked when parked but no longer works.

Ill assume it still has spark.

If carb has gummed up whike machine was parked it may no longer be getting fuel. Compression will be higher with a carb working properly.

Ring on piston gummed up and stuck while parked.

Machine was taken for a test drive without owner being aware of and not put back in its oniwn parked condition

 

[Bob_w]

I do have some scratches. I poured some ATF into the spark plug hole and got 130 PSI. So at least, I have Hone and Ring. I will do pressure tests to see if I have more issues. I will look at the transfer port cover. To see if I have a problem there. Still didn't check if the Jug is bolted on securely. But it looks like rings and scores, are the issue. I will do pressure testing next.

 

[ballisticdoughnut]

That saw needs a piston and rings, possibly a cylinder if the plating is damaged.

 

[SteveSr]

I do have some scratches. I poured some ATF into the spark plug hole and got 130 PSI. So at least, I have Hone and Ring. I will do pressure tests to see if I have more issues. I will look at the transfer port cover. To see if I have a problem there. Still didn't check if the Jug is bolted on securely. But it looks like rings and scores, are the issue. I will do pressure testing next.

DO NOT try to hone that cylinder! The plating is only a couple of thousandths thick and you will ruin it.

Look for threads on aluminum transfer removal without acid if the transfer covers are still intact. If not it is time for a new cylinder or saw. The cylinders are pricey.

 

[lwn9186]

Two different cylinders on that saw so make sure and get the right one. I happen to have a new one here for the early style.

 

[Bob_w]

DO NOT try to hone that cylinder! The plating is only a couple of thousandths thick and you will ruin it.

Look for threads on aluminum transfer removal without acid if the transfer covers are still intact. If not it is time for a new cylinder or saw. The cylinders are pricey.

Thanks Steve! Wow, So the cylinders are just coated with iron or something like that with a very thin layer. Wow. How long can that last? I have an older 391 and a 2017 one, are they going to be better? Amazing planned obsolescence? My older saw has cut a lot of wood. MS391's are going for $700 at the local store.

 

[SteveSr]

Thanks Steve! Wow, So the cylinders are just coated with iron or something like that with a very thin layer. Wow. How long can that last?

The cylinders are coated/plated with Nikasil which is a combination of nickel and silicon carbide. It is an extremely tough coating but only a couple of thousandths thick. In normal use it will never wear out. This is why you can remove the aluminum transfer with abrasive without having to worry about damaging the plating.

I have an older 391 and a 2017 one, are they going to be better?

No. Every Stihl has been Nikasil plated for at least 3 decades.

Amazing planned obsolescence?

Not really. You can go through many pistons and rings before you will have to replace a cylinder

My older saw has cut a lot of wood. MS391's are going for $700 at the local store.

If you are cutting a lot of wood you should probably look at a professional saw like the MS261, MS362, or MS400. They will take a lot more abuse and don't have glued on transfer covers.

 

[Bob_w]

The cylinders are coated/plated with Nikasil which is a combination of nickel and silicon carbide. It is an extremely tough coating but only a couple of thousandths thick. In normal use it will never wear out. This is why you can remove the aluminum transfer with abrasive without having to worry about damaging the plating.


No. Every Stihl has been Nikasil plated for at least 3 decades.


Not really. You can go through many pistons and rings before you will have to replace a cylinder


If you are cutting a lot of wood you should probably look at a professional saw like the MS261, MS362, or MS400. They will take a lot more abuse and don't have glued on transfer covers.

The cylinders are coated/plated with Nikasil which is a combination of nickel and silicon carbide. It is an extremely tough coating but only a couple of thousandths thick. In normal use it will never wear out. This is why you can remove the aluminum transfer with abrasive without having to worry about damaging the plating.


No. Every Stihl has been Nikasil plated for at least 3 decades.


Not really. You can go through many pistons and rings before you will have to replace a cylinder


If you are cutting a lot of wood you should probably look at a professional saw like the MS261, MS362, or MS400. They will take a lot more abuse and don't have glued on transfer covers.

Thanks Steve. I saw a MS261 in action when some tree cutters cut a 24 in maple for me. They used a crane that lowered sections. I would love to have a saw that cut like that. I still have the rounds up at the log splitter.

 

[rupedoggy]

If you concentrate on your 272 and get it running as good as it can, you will have a saw that cuts like the ms261. The 272 is a professional saw but your other is not.

 

[Bob_w]

If you concentrate on your 272 and get it running as good as it can, you will have a saw that cuts like the ms261. The 272 is a professional saw but your other is not.

Good point, the 272 has always been strong when it was running. It is probably wearing out. I'm told that 120 psi is low for that saw, And I believe that it used to pull harder. I've had it maybe 20 years. but still, I can get it back running.