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Author Topic: Moded muffler on a 390  (Read 3316 times)

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Offline timberfaller390

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Moded muffler on a 390
« on: January 02, 2010, 11:24:51 pm »
I just got done doing a muffler mod on my 390. I know alot of ya'll will say it was an act of foolishness but it has improved the sound and hopefully the performance. I plan on putting er through er paces tomorrow so I'll let ya'll know how it goes
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Offline nmurph

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Re: Moded muffler on a 390
« Reply #1 on: January 02, 2010, 11:42:43 pm »
anyone that thinks a MM on this series of saw is foolish needs to run one bf and after bf they run their pie hole. it will really wake you saw up. you will be eating unsuspecting 361's for breakfast as long as you stick to bucking. you still can't do anything for the 1.5lb weight disadvantage.

Offline ErikC

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Re: Moded muffler on a 390
« Reply #2 on: January 03, 2010, 12:29:07 am »
  It's pretty easy to do a simple muffler job. I replaced the burned out muffler on my 039 a while ago, and until I opened it up a little I was disappointed with my repair! I wouldn't buy aftermarket parts though, for the 039. Just me.
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Offline windthrown

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Re: Moded muffler on a 390
« Reply #3 on: January 03, 2010, 03:43:12 am »
Make sure that you retune the H screw on the carb to richen up the mix. O/w you will run lean and score your P&C. I modded a 290 I had and it ran great with an opened up muffler. Those 290/310/390 mufflers are really choked up from the factory and only have tiny holes in them. You can get a good power boost from a MM.
Stihls: 440R, 361, 360, 310, 260, 211, 020T. Husky: 372xt.
I ship Stihl saws down under: message me for details.

Offline boisblancboy

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Re: Moded muffler on a 390
« Reply #4 on: January 03, 2010, 11:58:52 am »
Im just curious, does anyone have any pictures of what to do exactly on a muffler mod for the 390?

Also after the mod, how do you know exactly where to adjust the H screw for the carb?

Offline Rocky_J

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Re: Moded muffler on a 390
« Reply #5 on: January 03, 2010, 02:16:43 pm »
Don't overcomplicate it. Look at the exhaust outlet and observe how big the opening is. Then make it about twice as big (a good, basic target for that saw).

Assuming the saw is properly tuned before you start, you will turn the H screw about 1/8-1/4 turn counterclockwise. But read this before you do that so you might have a little bit of a clue as to what you're doing and why.
http://www.madsens1.com/saw%20carb%20tune.htm

Offline windthrown

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Re: Moded muffler on a 390
« Reply #6 on: January 03, 2010, 06:59:41 pm »
I use a tach as well as burple tuning myself. Make sure a B&C are on the saw, preferably the one you are going to use on it. Make sure that the filters are clean. Set the L and H screw to the factory settings; trurn them in until they *just* seat, and back them out as indicated on the side of the saw. Start and let the saw warm up, and adjust the L & idle (L/A) screws. Set the idle at 3300, adjust the L screw for max RPM at idle, adjust the idle to 3300 again, and re-adjust again as needed to get the highest idle speed out of the L screw and being set at 3300. Now, these saws are different than the Stihl pro saws in that the factory book says to lower the idle at this point to 2700 by turning the L screw out, then adjust the idle screw to stop the chain from running. Check the throttle response at this point and adjust the L screw a tad either way to clean up any hesitation.

Now adjust the H screw. Run the saw at WOT unloaded for a moment and read the high setting. The 290 is 12.5k +/- 500 RPM, and the 310 and 390 are 13k +/- 500 RPM. The saw should still burple (4-stroke) at WOT unloaded. If it runs clean and sounds good, it is set too lean. Running lean will burn your saw up and score the P&C. Now cut some cookies in some good size wood. The saw sound should clean up in the wood, and hold a good speed. If the revs drop too low, lean the H screw (turn it in) just a tad (1/16 of a turn at most). Now do the same cuts and repeat as needed to get a good cutting speed. Now check the unloaded WOT revs with a tach. You should be under the max revs at that point. If in question, set the saw a tad richer than leaner. You can also read your plugs to check your carb settings. Do a full cut at WOT and cut the saw from full throttle. Let the saw cool and pull the plug. A medium chocolate brown color is what you want. Light tan and white are indications that the saw is running lean and it will run too hot. Run too hot and you will score your engines. With a muffler mod you should get better 'in the cut' engine speeds, and the saw will run cooler. You can set them at the top end of the factory rev limits. 

The 290/310/390 are very cheap and easy to muffler mod. I sold my modded muffler for the 290 and the ex got that saw with a 210 that I gave her. I do not have photos of them. But if you pull the outer baffle off the 390 muffler you will see a depressed area and outside of that two small holes. One hole is smaller than the other, and they are oval. They are rediculously small. You can go to several layers of mods on these saws.

Basically the depressed rectangular area is where an inner baffle is inside the muffler. Outside of that is an area where the factory holes are, in a raised L shaped area that is under the spark screen. You can drill holes along the L shaped area to open up the muffler. I did my first one that way, and drilled 3 3/16 in holes, one above the right factory hole, one between the factory holes, and one to the left of the factory holes. That alone will give you good results with moderate gains and still keep the saw fairly quiet. You can take that to the extreme and drill and Dremel out any area in the whole L shaped area. The next stage is to start drilling in the depressed area. I found that drilling 3 x 3/16 inch holes in a triange in that area had very good results. You can take that to the extreme as well, and drill and file out part or the entire depressed area, but the saw will be LOUD! Now, once you have drilled the muffler out in any of those ways, you also need to file down/open up the baffle cover plate to allow more air to flow out of the muffler. I filed off the high raised baffle port area back to the obvious angle on it. That more than doubles the exit port area of the muffler, and leaves good back pressure, as well as allows for a spark screen to be used. 

Personally my favorite mod on the 290 was drilling 3 x 3/16 in holes in the muffler L shaped area (one above the right hole, one between them, and one to the left of the left hole), and 3 x 3/16 in holes in the depressed area. Then I filed off the cover plate to the angled area, and that was it. The H screw is marked/mapped on the air filter cover of the saw. The newer 390s have the limiter caps, so you have to remove them on both the L and H screws, make the adjustments, and replace the limiter caps back into place. They help keep the carb from wandering out of tune.
Stihls: 440R, 361, 360, 310, 260, 211, 020T. Husky: 372xt.
I ship Stihl saws down under: message me for details.