New ms390

[backofbeyond]

I bought a lightly used stihl ms390 yesterday for 200$. I got a wheel barrow thrown in with it! That is the factory bar so I figure it is lightly used. Compression is about 155psi but i was having trouble stabilizing the saw with the gauge in a vice so it could be slightly more? I took the muffler off tonight and drilled a hole in the area of the baffle with a 5/16 drill bit and opened the exhaust hole up a similar amount. I put the spark arrestor back in because i might use it in a small alaskan mill and i dont want fire hazard. I consider this a mild muffler modification, does anyone think it was not enough or too much? Ill retune the saw tomorrow. I took the limiter caps off tonight. ill post pics.

 

[a. palmer jr.]

That's probably enough. Just richen up the carb a bit and try it out!

 

[blsnelling]

Trim the caps and put them back on.

 

[a. palmer jr.]

Might be good idea. Most of the used ones I get have already been thrown away along with the little trim ring..

 

[backofbeyond]

Might be good idea. Most of the used ones I get have already been thrown away along with the little trim ring..

What do you mean by little trim ring?

 

[a. palmer jr.]

On the Walbro carbs the limiters sit inside a little chrome ring with two holes in it. That ring will snap off with a screwdriver if you don't wish to re-use the limiters. Possibly you might have a Zama carburetor and it's made differently..

 

[backofbeyond]

On the Walbro carbs the limiters sit inside a little chrome ring with two holes in it. That ring will snap off with a screwdriver if you don't wish to re-use the limiters. Possibly you might have a Zama carburetor and it's made differently..

I will look for that and I appreciate the advise.

 

[Red Amor]

 

[Red Amor]

Gday
This is a photo of the mods I made on the muffler on my ms310
Ive doubled the size of every hole on the thing and opened the gills with a large screw driver
the saw once retuned screamed like a banshee and held better reves in the cut and was less likely to bog down
I did the same mode to my friends 390 and woke it well up very happy with the results
I hope this helps

 

[a. palmer jr.]

That doesn't look like the muffler on any of my 1127s. Mine are all one piece with the little baffle on the front..I try not to overdo the muffler mods on my saws because of my ears. I bought some ear protection after the fact..

 

[backofbeyond]

Yea, I was trying not to go to far at first. I think i increased the exhaust by about 60% percent.

 

[fearofpavement]

For an MS390, you didn't open it enough in my opinion. The one hole in the center is ok. However, where the two slots are outside that area, that could be enlarged (longer and wider slot area) and I leave a little in between them to maintain strength. Then on the other side of the depressed area, you can also put a couple of slots in there. Drilling out the center is where a lot of additional noise comes from as it's a straight shot to the cylinder exhaust port. If you open up the outside area of the baffle (outside the depression) it won't add that much noise. These saws have significantly more power output when the exhausts aren't the limiting factor. I think some people go way too crazy on the mod. I have tried different variations and came up with a solution that works well on all the 1127 models. Being the 039/390 has the largest engine in that series, it benefits the most from opening a restricted muffler. I believe there is enough perforation in the baffle itself and don't recommend messing with that. Also, OP, your muffler is one of the very restricted ones as most muffs I've seen on the 390 have four slots from the factory. (two on either side of the depressed area). I'll see if I can find a pic of "my recipe" so you can compare to what you have. (your cover is ok now in my opinion)

 

[a. palmer jr.]

Also, if you buy an aftermarket muffler some of the work is already done for you. I guess the mufflers for the US saws are more restricted than those meant for other countries.

 

[backofbeyond]

I think i will open up the slots some more and i kind of wish I had keep everything outside the baffle depression considering what fearofpavement said about the baffle not being terribly restrictive itself. post pics if you have them.

 

[a. palmer jr.]

There's an old thread on muffler mods for this saw with pictures. I followed fearofpavement's model on mine and it did well.

 

[backofbeyond]

I added some 5/32 holes outside the baffle channel. As has been mentioned before, this really just equals the holes that are on the cheap asian replacements on ebay. I had previously added a 5/16 hole in the middle of the baffle channel which is pictured in the first post on this thread. I believe I have added 100-120% of factory exhaust flow on this saw. I am at about a turn and a half(1 1/2) from closed on the high prior to these latest holes and figure I will have to go maybe another 1/4 turn when I retune again. Any opinions on typical number of turns on the high screw? Im aware of the typical speach on burble at high rpms, and in and out of wood, in the cut and the typical videos to watch. Im just wondering if anybody remembers about where they were at as a reference?

 

[a. palmer jr.]

I think I have mine at about one turn but it has a Zama on it right now..