MS 390 muffler mod

[Shawn.54]

I was using my MS 390 in a warm day when I noticed how hot the saw was, it was uncomfortable on my bare hand on the handle bar so I decided to do a he muffler mod I’ve been reading and watching videos of, not for power increases but to help with heat.
All I can say is that the temperature of that saw went way down. I don’t see a major increase in power but my buddy who is letting me cut wood on his property says it definitely does cut better I know it definitely uses more fuel as my oil and fuel consumption were almost equal with just a little bar oil left when it ran out of fuel now it is 1/3 tank of oil left.
I’m not complaining about fuel consumption.
The fact that they sale these saws so chocked
that they will cook them selves if used in hot weather or long periods. I cut a lot of trees into firewood so my saw gets a break when I add fuel or sharpen the chain. I was a little hesitant to make this mod but after feeling how hot the saw was I decided to try it and now I wish I would’ve done it sooner.
Now tell me if I’m crazy thinking it lowered temp that much.
Shawn

 

[Deleted member 149229]

Did you retune the carb after doing the muffler mod?

 

[Shawn.54]

Yes removed the the tabs and adjusted them.
Like I said I didn’t do it for power increase as much as I did it for heat the saw was just running to hot before I did the mod I even thought maybe I was running it to lean to cause the heat I had a local logger check it and he said it was not to lean. Plug was a dark brown color. This saw even melted the plastic above the muffler at some point.
Shawn

 

[Deleted member 149229]

Not saying this is for sure but many times a muffler or muffler gasket leak can concentrate exhaust heat causing the melting problem. If you retuned so it’s not lean then you should be good.

 

[Shawn.54]

I may have been a little conservative with my opening up the slots and that’s ok I can always do more later tomorrow evening I’ll take some pics and post them. But the heat in my hand was almost non existent and I was cutting an ash tree that was 18’ to the first Y in it and 20-24” at the narrow point probably 36-38” at the butt and the only time I stopped was to add fuel/bar oil and cutting it to 16 inch lengths so was working it hard. I am pleased with it now.
Shawn

 

[s sidewall]

Those have converters in the muffler?

Steve, Samsung On5 using Tapatalk

 

[Shawn.54]

No converter just tiny exhaust outlets
Shawn

 

[rd35]

Scientifically speaking, the higher average pressure inside that muffler canister would certainly cause it to get much hotter due, purely, to the higher concentration of heat inside from the added exhaust pressure. By opening it and reducing the pressure the temperature would naturally be less. Your hand (and the plastic top) is feeling the radiant heat from the muffler being so hot. Knowing the history of this series of saw, I would expect that your saw was running lean which added to your heat problem. Melted plastic above the muffler is not uncommon. You did your saw engine a REALLY BIG FAVOR by richening up the mixture just a bit. Many an 029, 031, 039, MS290, MS310, and MS390 have been toasted (piston and cylinder damaged) by being set too lean from the factory and the adjustment limiters preventing proper adjustment. I don't do that much saw repair (yet)...but with that said, the only two major saw repairs I have performed were on this series of saw because they were run too lean and burned up. One was a 290 that had a scored cylinder. Other was a 310 that was completely destroyed internally. Piston was missing a "quarter sized" section of the intake skirt.....worn away, not broken. If that saw had run any more the piston would have rotated sideways around the piston pin in the cylinder....yes it was that bad!!! Sounds like you, Shawn, are in that small class of saw operators who actually pay attention to what is going on with their saws and make adjustments/changes before any major damage occurs! Cudos!!! Oh, and don't get me wrong. This series of Stihl chainsaw (series 1127) is actually a great design. My Dad has had an 029 for many many years. It is his "big" saw. He absolutely loves it. Great power, light weight, and always starts. He has cut countless truckloads of wood with it and wouldn't trade it for anything! When properly adjusted, they are fantastic saws!!!

 

[Shawn.54]

The over heating started when I started cutting full size trees for firewood cutting up tops was no problem but the saw did not run wide open for extended periods as soon as I noticed the heat I checked plug and readjusted carb when that didn’t help I had a experienced logger check it out and he said it wasn’t lean. Glad I did the mod I think it will prolong the saws life.
Especially since I have gained access to a lot of wood that is wider than my 20” bar.
Shawn