Chainsaw Repair

At the request of a member on another site I posted this procedure for doing a MM for my 361.  Kevin asked me to put it up over here also, so here ya go!  This could be used on a variety of saws as well, with some slight changes depending on the saw.  I'm sure others have advice to add as well, I welcome any input. Thanks to Kevin, aka Cut4fun, Bill Gaiche from the Forestry Forum and anyone else who helped me along for their info when I did mine.  I'm including removing the limit caps on the carb as part of the process, as it is a must to be able to tune your saw once mods have been made.

Starting with the air filter cover removed you need to get to the limit caps on the H and L needles.

 
Remove the 2 hex nuts under the filter to get the air box off, which will give you access to the boot covering the adjustment screws.

 
Pulling the carb out a little on its mounting bolts will give you the room needed to pry off the boot, but be carful not to damage it!

 
Now you have access to the limit caps on the carb.
 

 
The limit caps are the red plastic pieces seen in the photo.
 
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Note the grooves at around 5 o'clock in each, these grooves are how the caps go in to the carb body.  Turning both the H and L out (counterclockwise) as far as they will go will align a tab on each of the red caps with the groove in the carb body.  Once aligned, the caps can then be pulled out of the carb body.  I have heard of using a drywall screw to screw in to the caps and pull, however I got them out using a screwdriver and needle nose pliers.  When you get each out, there is a tab on each which simply needs to be trimmed off with a utility knife.  After trimming, they go back in, which will help keep the carb screws from vibrating out of later on.  You now have a fully adjustable carb!  Put everything back together on the carb side of the saw and then off to the exhaust side.
Now for the muffler.  I did my mod to on the recoil side of the saw as the factory exhaust is on the clutch side and there is more room. I started by drilling 2 3/8" holes.  I don't recall exactly but I left somewhere around 1/4" between them.
 

 
Initially I had only used these 2 hoes as my mod and had good results, but later I went back with a Dremel and a milling bit and cut out the material between the holes, then a grinding bit to clean up the edges.



Next I used a Husky 288 deflector and 288 spark screen, Bailey's part #s HVP 503 58 99 01 and HVP 503 40 56 01, respectively.  Also used a pair of sheet metal screw to attach. The screen isn't necessary, and can be omitted if you choose.



I drilled pilot holes for the self tapping screws and then drove them in to attach the screen and deflector over the new exhaust port. Before buttoning it back up use a shop vac to suck out any metal shavings that may be in the muffler, don't want them finding their way back into the engine. Reattach the muffler, making sure to put the heat shield and gasket back on as well. Here it is all done. 

   



 

Once back together you will need to return the carb to account to the changes, which will include opening up the H for sure and probably some with the L and idle as well.  I like Madsen's site for info on tuning, which even has audio files so you can hear a saw from lean to rich.
http://madsens1.com/saw_carb_tune.htm

All in all, the saw runs better, cooler, and has more torque and power.  Years later and it still looks great inside looking through the exhaust port.



 
May be hard to see in the photos, but there is a nice film of oil clearly visible on the piston.  I keep mine set on the rich side, and have had no problems doing so at all.

Nice thread on how to muffler mod a stihl 361. 

Thanks so much for sharing this over here too.

im getting  ready to do a muffer mod on a ms 390  so any help  wood  be good

also  if  any 1 help on the tuning  of corburator  ms 390 460 mag 

I discovered what I call the 2-4 break in the way a 2 cycle runs 60 years ago on model airplane engines. But I also noticed the same thing on gasoline two strokes.

I think the 2 - 4 mixture difference was more pronounced and noticeable (easy to identify) back then with the old chainsaws?

With the model airplane engine, if you got it too lean and let it go into the air, you were stuck with that running condition until it run out of fuel unless you deliberately crashed the plane to stop the engine. With the model engines of the early 50's, one lean run and the engine was cooked for good. The engines would unload when in the air and would go leaner. So you had to anticipate how much it was going to unload and set the mixture so the engine didn't go too lean. Some Italian engine brands were reactive in flight and if they started to go lean, they would "Run Away" and go leaner on their own until death.

Things have changed over the years and due to metallurgy advancement, newer two stroke model airplane engines will open up the clearances when hot and can run as lean as you like as long as the fire doesn't go out. These same engines, if you run them in four stroke, will beat out the rod journals and cause a slung rod.

Some newer chainsaws will four stroke above 15000 with little modification