Air leak at heat shield / muffler?

[gtmwoodworking]

Well, I ruined another piston and cylinder on my MS441 (yeah, I know everyone hates these saws). This time I noticed low compression and had trouble keeping it running at idle after using it on and off for at few hours. I pulled the muffler off and noticed some severe scoring on the piston. Here's where it gets interesting:

1. I did a vacuum test and pressure test before removing the cylinder: both within specifications holding 0.5 BAR for 20 sec.
2. The carb was tuned with a tachometer so I know I'm not over-revving the saw (<13K RPM running Stihl 50:1 gas)

I know that scoring on the exhaust side typically indicates that the saw is running hot and the mixture is too lean so here's my question: Is it possible that an air leak between the muffler and the head shield or between the heat shield and the cylinder could cause this lean condition? I ask this because I noticed a lot of "schmutz" (for lack of a better term) all over the heat shield and on the crankcase near the muffler. It looked like a combination of exhaust particulate, oil, and some sawdust. I know the muffler was torqued to the proper spec, but that doesn't mean it's airtight. The muffler is fairly new, but the heat shield is old. What do you think I should do after I get a new piston/cylinder on there? New heat shield and gasket? Re-torque it once its warmed up? Pressure test by somehow sealing up the exhaust hole on the muffler? Thanks.

 

[261cm]

Did it 4 stroke out of the cut at wot, and then clean up when in the cut? Personally I never use a tach to tune and never had an issue. Sounds like you had it to lean.

 

[huskihl]

The leak at the muffler didn’t hurt the engine. Did you clean the aluminum transfer off the cylinder the first time it scored the piston?
Need some pics

 

[L34]

Maybe start with the Stihl 50:1.
Just a thought.

 

[pioneerguy600]

Was it an OEM cylinder and or piston or aftermarket setup?

 

[gtmwoodworking]

Was it an OEM cylinder and or piston or aftermarket setup?

This go round it was an aftermarket set-up. Already have an OEM one in the mail to replace it. Do you think inferior metal in the aftermarket one contributed to the problem?

 

[pioneerguy600]

This go round it was an aftermarket set-up. Already have an OEM one in the mail to replace it. Do you think inferior metal in the aftermarket one contributed to the problem?

One never knows for certain but I do not trust any AM stuff to be ready out of the box for an install and would not use one of them for an actual work saw. They have been proven to work sometimes for light use setups as the quality of them are all over the spectrum. Not knowing how your rebuild was carried out, if the saw was pressure and vac tested after the build, how it was tuned or what types of fuel ran through it then only guess work for us.

 

[huskihl]

One never knows for certain but I do not trust any AM stuff to be ready out of the box for an install and would not use one of them for an actual work saw. They have been proven to work sometimes for light use setups as the quality of them are all over the spectrum. Not knowing how your rebuild was carried out, if the saw was pressure and vac tested after the build, how it was tuned or what types of fuel ran through it then only guess work for us.

On most saws that come in with AM cylinders, there’s a black slur on the cylinder walls and piston. Usually the owner didn’t know it was AM. That black oil slur says something didn’t fit quite right. Square peg-round hole type thing, or the opposite

 

[pioneerguy600]

On most saws that come in with AM cylinders, there’s a black slur on the cylinder walls and piston. Usually the owner didn’t know it wasn’t AM. That black oil slur says something didn’t fit quite right. Square peg-round hole type thing, or the opposite

You see more of it than I do,Kevin. But I have seen the same results fairly often. I have one early 066 with an AM setup on it, it has been faithful with no issues for over 10 years now running a 36" bar but I mix the fuel on the heavy side, 40:1 , did clean up all the rough casting, reshape the ports and get any loose or near loose plating out of the upper transfers before install. Since then the AM stuff has not been very consistent quality or fit so I avoid it. Saws come in with trashed P&C then its a bit of detective work to figure out what happened to cause it. Thrown C clips , pin keepers working loose and coming out and broken rings from catching in the exhaust port are most common, poor squish leads to poorer combustion and the results are a lot of spooge in the cylinders, out the exhaust and built up on the sparkplugs.

 

[huskihl]

You see more of it than I do,Kevin. But I have seen the same results fairly often. I have one early 066 with an AM setup on it, it has been faithful with no issues for over 10 years now running a 36" bar but I mix the fuel on the heavy side, 40:1 , did clean up all the rough casting, reshape the ports and get any loose or near loose plating out of the upper transfers before install. Since then the AM stuff has not been very consistent quality or fit so I avoid it. Saws come in with trashed P&C then its a bit of detective work to figure out what happened to cause it. Thrown C clips , pin keepers working loose and coming out and broken rings from catching in the exhaust port are most common, poor squish leads to poorer combustion and the results are a lot of spooge in the cylinders, out the exhaust and built up on the sparkplugs.

I’ve had 2 AM cylinders come in for port work that looked like they had a good 100 hours of use on them and they looked great. And probably 20 that were black inside or scored. The ratio isn’t good

 

[pioneerguy600]

I’ve had 2 AM cylinders come in for port work that looked like they had a good 100 hours of use on them and they looked great. And probably 20 that were black inside or scored. The ratio isn’t good

Too many foreign companies got into making the AM stuff , the prices dropped dramatically and so did the quality. Its too hit and miss to make it worth my while fiddling with the stuff, its OEM or nothing for me now.

 

[Maintenance supervisor]

I have 2 running and one I'm building MS441 saws. My M-tronic is all OEM and my carburetor one is a 52mm Aftermarket.
They both run fantastically but the AF cylinder was awful it looked like the plug and decomp holes were stabbed in with an ice pick and the transfers were almost closed off with chrome. I had to grind so much off the outside of the cylinder to get it to fit the chassis I thought I was going to break through the tunnels. All that being said it is a BEAST and I used it extensively for felling and buck firewood to see if it would grenade, running a 28" and 34" bar.
The biggest difference between it and the OEM is throttle response, the M-tronic is right now response.
You more than likely have an air leak or are leaning it out , an air leak can be sneaky and only present it self when the rotating assembly is hot or spinning, or the cylinder could be leaking from the base if you did a BGD .
Its hard to read the strato saws sometimes and if the coil is limited it makes it though. These saws don't sound like the old piston port saws I'm used to and dont burble hard unless I really have them too fat.
All the linkage has to be in sync also if your strato plate is hung up you'll go lean. The huge intake should also be supple and the impulse and fuel lines need replacing and I don't care how they look! They are bad unless new.
Long post and good luck ,pm me if you need help.