Stihl 039 Rebuild

[a. palmer jr.]

Open that muffler up!
If you don't want a lot more noise, just open it right up around the outside of the baffle, and try and get a drill in to open up the holes in the baffle that lead to the inside of the muffler body.
Once you get the air flowing better, I will recommend Terry's 460 carb mod.
It really makes the 039 cut like a 64cc saw should cut!

I bought an aftermarket muffler from Huztl for one of the saws and it already had more holes drilled around the baffle compared to a stock one.

 

[Brewz]

Yeah the one with the 4 little holes is the stock muffler in Australia

Drill them out and then join them up making a slot top and bottom of the baffle

Then open up the front plate to stop restriction

Plenty of pictures on here and if you google it

 

[Terry Syd]

To be honest, I don't want to push it too hard as I don't think the saw was designed for super horsepower and eventually, something will become a weak link.

I noticed on a couple of seized 029 jugs I had that the seizure occurred in the same spot - where the boss was for the muffler bolt on the side closest to the chain. It didn't take me long to figure out why.

The manufacturing process created an air flow problem in that spot. It also allowed the heat from the muffler to expand the mounting boss to expand into the cylinder. So you ended up with a situation of reduced clearance and excessive heat in that spot.

On my AM 390 jug I drilled out the boss and opened it up with a Dremel bit so the air could flow through and the heat wouldn't expand the extra metal in that area. I also ground out any areas that would block the air flow to the front of the jug. I was looking for a more uniform expansion of the jug with good air flow. Even though I've bumped up the power, I've had not had any problems.

I should have mentioned this before - you have to take the jug off to do that mod. Oh well, these open port jugs tend to wear a groove in the rings that drops compression. The next time you change the rings you might want to do the mod.

 

[Brewz]

I would say that the reduced heat that comes with a good muffler mod helps regulate the heat generated by more power.
I find mine runs quite cool compared to when it was stock. I also clean it out very well after a day's work to stop buildup and help cooling.

When you say boss, are you talking about the extra chunk of casting for the third lower muffler bolt?

 

[Terry Syd]

If you check, the muffler bolt attaches to a cast extension from the cylinder. That extension not only blocks the flow of air, but it also conducts the heat from the muffler bolt back to the cylinder. It is something that the factory had to do in order to cast the cylinder.

I just drilled behind it and opened it up with the Dremel. I also thinned the heads of the bolts to allow more air flow past the cylinder.

If I ever have the cylinder off again, I'll take a picture of what I did.

 

[Brewz]

All good mate, I know exactly what you are talking about.

 

[a. palmer jr.]

That piston is probably still serviceable. Don't toss it.
As far as the muff mod, I've found about 1.25 open on the H and 1.0 on the L will put you in the ball park. Here's the mod I've finally settled on after trying a number of variations on different saws of this series.
Make sure you get all the chips/shavings out of the muffler before installing.

View attachment 461659

I kinda used your muffler for a pattern when opening up mine this morning. I used a hand held reamer to remove the burrs on the outside and the two in the middle. Still waiting on a clutch for mine but when it's done I'll check it out and report here. Probably be another week.

 

[a. palmer jr.]

There's a bunch of them on Ebay for around A$23, but here is one for A$21 - http://www.ebay.com.au/itm/Carburet...004712?hash=item3cfab1f668:g:YU4AAOSwv0tVZZMX

They're probably the same carb, just different sellers.

It was mentioned about modifying these carbs, I wonder what they would do on a 390 if they were used in stock condition.

 

[Brewz]

I don't thnk they would do anything on a stock saw.
You really need to open the muffler right up and maybe a light port job to get the extra air flow needed to take advantage of the bigger carb.

I have tested mine with both carbs and it ran 15% slower with the 460 carb but I recon it just needs fine tuning and a timing advance to fix the problem.

Still much work to be done to get a good measured gain.

If your saw is stock, don't bother with a bigger carb.
It will likely make it run worse and they are not a simple bolt on proposition. There are mods needed to be done to the carb just to get the saw to run, let alone run better

 

[fearofpavement]

Regarding modding the 1127 muffler, I don't connect the grooves in the muffler because leaving a bridge of metal allows some structure to remain. There's not that much material that needs to be removed to double or triple the outlet size, so be selective as to where you cut to retain the muffler's structural integrity.

And don't forget to trim the outlet cover a bit too.

 

[a. palmer jr.]

I'll probably leave the stock carb on the saw. I just did a slight muffler mod and no porting. The saw will probably be sold later anyway, just doing a little tinkering to see if I could make it do any better.

 

[Outdoorguy1979]

dud you have to replace the needle bearing in the crankshaft? and if you did how hard was it to do?

 

[a. palmer jr.]

dud you have to replace the needle bearing in the crankshaft? and if you did how hard was it to do?

I replaced the wrist pin bearing and the main bearings and seals. The main bearings required that I freeze the crankshaft for several hours then they slid on with the help of a pipe and small hammer. If the rod bearing had needed replaced I would probably be looking for another crankshaft..

 

[Outdoorguy1979]

Update I put the saw back together with new crank bearings and seals and the thing runs like a champ