MS 390 fine tuning questions

MS 390 (Denmark)

new problem: i have been talking with thainsaw mecanic,he said that it didnt gave any xtra horse/torq,with the muffler mod,thats what he said!! I will show him a diffrens then i have some new quistions
1.when the rev limiter is of,how many rpm Max?
2.is there anything else i should know before i give him my saw fore fine tuning?
I hope you can undestand what im writing,its a couple of years ago,im a little bit rusty with the writing

thanks fore your help

best regards Jan

windthrown

Hmmm, in Denmark the stock 390 mufler shoud be the standard open one, and not the stupid US one that has to be opened up. The Euro 390 has about 1/2 more HP than their USA cousins. I used the Euro model muffler pattern to open my last USA 310 muffler up, along with opening the H jet about a half turn more and it runs good, and is still pretty quiet.

The carb limiter caps are not a rev limiters. They actually allow you to run the saw leaner, and rev a lot higher, and self destruct as a result (they will overheat and the P&C will score). The maximum rev limit on the 390 is the same as the 310; 13,000 RPM. Stihl max rev limits are +/- 500, so you can safely go to 13,500 maximum RPM. I set my 310 at 13,250 RPM. The 290 has a max rev limit ot 12,500, so 13,000 is the max that you want to run one of them.

windthrown

After a muffler mod on a 290, 310 or 390, you need to turn the H screaw out about a half turn (more or less, depending on the muffler mod) more than the 3/4 turn that the limiter will let you set the H screw to. That is 1-1/4 turn total. The newer factory carb limiter will not let you do that, unless you remove the limiter cap on the H screw and cut the limiter tab off, and then put the limiter cap back in and adjust the H screw so that the saw burples, or 4-strokes at about 13,000 RPM with the bar and cahin in the saw, and no load (not cutting).

To remove the limiter tabs, I use a small picture hanging eye screw to screw into the limiter cap center holes, then twist the limiter cap until the tab lines up with the slot, and then pull the limiter stright up and out. Then cut the tab off with a box cutter, and put the limiter cap back in the carb. You need to put the trimmed limiter cap back in place to keep the carb H setting from losing its tune. The 1127 saw L settings are only 1/4 turn, and they allow you 3/4 of a turn with the limiter tab, so there is no need to trim the L limiter tab on these saws.

windthrown

Typical stock USA 1127 muffler (choked up tiny dual port):



My modified Eoro style 1127 muffler mod (opened up quad ports):



Note that the dual factory ports were both opened up and larger than Euro ports were added above in a similar location that the quad ported 'standard' muffler design has them.

Attached Thumbnails

   

windthrown

Here is the European or 'standard' 1127 quad port muffler:



This is the 'standard' muffler that the 1127 saws were designed to run with, and the muffler that the factory carb jets were originally designed to run with. This is the stock muffler model that is on the European saws, and the earlier model 029 Super saws.

The only other thing to do is open up the cover plate some to get a larger area for exhaust gasses to flow out from and that is it.

Attached Thumbnails

 

windthrown

Here is the stock USA 1127 muffler cover/deflector:



Here is my modified 1127 muffler cover/deflector:



This will continue to deflect the exhaust gasses toward the bar, like the factory cover does, but more forward.

Sorry I cannot photograph the carb limiter tabs and method to remove and trim them, as I do not have a macro lens on my digital camera. They are too small to film with the camera that I have.

Attached Thumbnails

   

MS 390 (Denmark)

that is hardcore instructions..... thanks alot man

best regards Jan

windthrown

No problem.

The above mods are designed within the parameters of the factory carburetor that comes on a stock 290/310/390. I have done several of them on 290s, opening them up even more, but more does not seem to always be better in these saws. This particular mod required that the factory H limiter tab be trimmed and a 1-1/4+ turn on the H screw from seating the jet to give the saw a good 4-stroke burple at an unloaded WOT max speed of 13,250 RPM with a 25 inch bar. The Stihl MS310 that was modified with the above muffler ran a 25 inch Stihl bar just fine, and kept up with my stock non-modified 361 cutting with the same bar and chain. The only difference was that the 361 has a max RPM of 14,000 RPM, and hence has a faster chain speed and 10% faster cutting time. The muffler mod gave the 310 about a 15% gain in cutting speed with a 25 inch bar that it came with. It does not bog down nearly as much as it did with that bar with the factory setting of 3/4 turn on the H screw and modified quad port muffler. A spark screen was used in all tests, and the above mods allow for the factory spark screen to remain effective. We have strict spark screen requirements here in the US west for fire control. This mod also does not bypass the silencer in the muffler, and leaves it quieter.

Note also that the previous owner of this saw was pushing this saw past its limits with the 25 inch bar that it had when I bought it. To compensate for that size bar which is really too large for a factory US model 310, he had tuned the H screw out ouly 1/2 turn to give it more speed (leaning it out). That resulted in a lean running condition, and fortunately the air filter was really clogged up with crap to make it run richer, which prevented the engine from overheating and scoring. He also did not have the adjustible bar oiler turned all the way up, and there was some bar burn. The fully open stock oiler should do fine oiling this bar now though. A lot of people do not realize that the 1127 saws have an adjustible oiler.

Overall this is a simple and easy mod to do on any 1127 saw, and costs next to nothing to do. A drill or Dremel tool is all that is required, along with a one edge razor plade and a small picture hanger eye screw. Also a tachometer is good to have to set the proper H setting, but you can also do that by ear. This mod will recover that 1/3 to 1/2 HP that the European cousin 1127 saws have from the factory, and then some. The mods will make for a saw that will give you the torque to cut with a pro saw like a 360 or 361/362, but it will not give you the higher RPM needed to cut with a 361 that revs higher. This mod will also make for a cooler running saw.

Note that you can also add a rim drive to these saws: the drum and rim are the same as the 360. Personally I do not like spur sprockets, and I swapped the spur sprocket with a 7 pin rim and drum. Now it is set to rip... for a cheap 'boat anchor' 1127, they are not bad saws with these mods. These saws have clam-shell engines, so the more radical typical porting options are not really an option. You can widen the engine ports, but that is about it. Lowering the squish to raise compression and turning down the piston is not really practical.

MS 390 (Denmark)

now I finished the muffler,just like yours,then starting on the chainsaw tomorrow morning,its my day off finding some horses

MS 390 (Denmark)

I got it tuned today and its works fine thanks yet again WINDTHROWN

KiwiBro

I'm not trying to kiss anyone's backside but far out this is good info!
Thanks so much for taking the time to explain it in such detail and such and easy to understand way, Scott.

I love your work and you are a credit to this site.

Talk about chalk and cheese compared to another site I posted on once asking for advice. That thread sank like a stone without a single reply. Whereas here at treeworld I get such awesome help. Bloody marvellous.

Cheers.

windthrown

Gee, the best Valentine's gift that I got this year. Don't tell the Canadians that are on my ignore list...

luckydave

Hi,
After reading all this info I wanted a ms361 but probably wont end up with one from the US because they are nearly all gone, but now maybe a ms390 muffler modded might be the go?
Luckydave.

windthrown

Maybe, maybe not. See my post on the 390 vibration issues on the 310/390 thread.

windthrown

Some update notes here after modding another 310 last week and some questions I have had regarding these mods (this works on the 290, 310 and 390 saws with the same muffler):

After you have modified the muffler and put it back on the saw, retune the carb. YOU MUST RETUNE THE CARB AFTER THE MUFFLER MOD OR YOU WILL LIKELY DESTROY THE ENGINE!

Tune the L screw first becasue it will affect the way that the saw runs at WOT. You do not need to pull the L side limiter on the carb or trim it. There is pleanty of movement on the L screw to adjust the saw as it is stock. Tune the L side first by setting the L screw out 1/4 turn from being lightly seated. Set the idle to 3300 RPM with a tach, and turn the L screw in or out to get the highest RPMs. Then adjust the idle to 3300 again and do it again, until you get the highest RPM from the L screw. The L screw is now set. Then turn the idle down to where the chain stops running with the brake off, and lower the idle another 1/8 to 1/4 turn.

Only the H limiter needs to be pulled and the tab trimmed. Some people have had issues with pulling the H limiter. You need to turn the limiter tab so that it lines up with the slot in the carb. It can be hard to see. Then lever it out with an eye screw, or whatever you have. Give it a good pull and it will come out. Make sure that you put the trimmed limiter back in the carb (in any position) or it may lose its tune. If you lose or break the limiter, get another one from a Stihl dealer. In general start with the tuning with the H screw turned out 1-1/4 turns from being lightly seated. The 1-1/4 turn replaces the stock 3/4 turn that is shown on the side of the saw. I use a sharpie pen to cross out the 3/4 H turn and mark in 1-1/4 so that anyone (including myself) tuning the saw will be aware of the new setting. I have found that almost all of my 1127 saws needed a 1/16 to 1/8 turn more than the 1-1/4 turn to get the optimum RPM. If in doubt, tune the saw richer and dumb down the RPM so that the saw does not run lean. By ear you will hear more burpling and 4-stroking when the saw is running WOT with a B&C out of the wood. That is a good sign. The WOT saw sound should clean up in the cut. Bury the saw in a cut and if it bogs some, lean the H screw out a tad a try again. Fiddle with the H screw to get a good clean WOT cut, and still get a WOT unloaded burple sound, and stay within the maximum RPM (13.5k). BTW: according to my Stihl tech buddies, all Stihl specs are +/-500 RPM, so the 13k RPM limit of the 310 and 390 is really 13.5k. The limit in the 290 is only 12.5k, meaning 13k max.

Another aspect of this mod will be that the saw may not start as fast as it did when it was stock. Stock these saw are *really* choked up. My stock 310 rolled over after only one pull on the starter fully choked, and one more pull at high idle. With the muffler mod, the saw starts in more normal Stihl tradition, and rumbles over after 2-3 pulls set to full choke and one or two more pulls at half choke.

windthrown

Here is a photo of a Stihl limiter pulled from a carb (courtesy of Therrin). Small little suckers. The tab on the left is what you want to trim off. Then put the limiter 'smooth barrel' back on the H screw and turn it out 1-1/4 turns from full stop as a base for tuning the saw now that the muffler is opened up. It should 4-stroke at WOT. Do some cuts and adjust for bar length, altitude, gas type and oil mix.

windthrown

Oh, and once a saw is detuned this way, if you take it into a Stihl shop in the states, they are required by law to re-tune it to factory specs. That means they would replace the limiter with a new one with the tab on it, and likely replace the muffler as well. Otherwise they are bound by law not to touch a modified saw like this. But if you are modifying saws, there is no need for you to take it into a Stihl dealer any more for work, is there?


Also in response to the 390 vs 361 debate above, the 390 and 310 are no longer available in the states. They have been replaced with the 311 and 391.