Anyone sell a ready-made modded muffler for a Makita 7900?

[Tree_Frog]

Just bought a new Makita 7900 PHO for $731 shipped and was wondering if anyone makes any ready-made modded mufflers? I'm a little concerned about splitting the muffler and gutting/modding it, since I don't have a backup in case I screw up. And, if you haven't looked, OEM mufflers aren't cheap.

 

[JimmyT]

Tree_Frog I have the older model 2010 Makita DCS7901 6.3 HP @ 13,500 rpms. It runs so well in stock form I never modified the non cat muffler but I did modify one of my 5105S saws. I used a 5100 non cat muffler then cut an exhaust gill in front of the muffler.

 

[huskihl]

It can’t breathe without removing the SLR tube from the muffler. Opening it up is the only way to remove it

 

[Fatherwheels]

You need to open the muffler, find some pliers with a lip / thin edge of some sort that will go under the
bent down folded edge on the muffler, you need a flat surface on the other prong of the pliers
so you don't damage the edge around the muffler, then gently prize back the folded edge that
holds the two muffler halves together, you only need to do two sides as with a bit of wiggling
the front muffler half will slide out from under the back folded edge with two sides straightened.

Then you will need to cut out the inside of the muffler, the tube like contraption that is in there,
some times there is a cage in there with holes drilled in it, its very obvious what this cage is
when you see it, I enlarged the holes in the cage, and I think I added a few towards the front
end of the cage.
I filed the muffler outlet to a bigger size, so the original spark screen fitted back without leaving
any gap for a spark to get by.
Its not too difficult to do, it makes the saw breath, you can add more fuel via the jets then too,
as you don't want to be running lean, it also helps cool down the saw, which was my reason for doing the mod.

 

[Fatherwheels]

Make sure you clean any filings or dust from the muffler before assembling it again,
that stuff will score your piston.

 

[Fatherwheels]

Here is the muffler with the birdcage I mention, you can see it from the inlet.

 

[Fatherwheels]

Here is some pictures of the SLR type muffler and how to mod it.
COPY--https://firewoodhoardersclubDOTcom/forums/threads/gutting-the-dolmar-7910-muffler.19771/
Remove the DOT and put in a . in the above address, for some reason posting the full address does not work here.

You can also buy a muffler to experiment on, its new though, dont see any second had ones,
https://store.chainsawr.com/collect...-assembly-new-pn-038-174-200-dolmar-bulky-bin

 

[JimmyT]

On the 5105 there is a upper and lower perforated baffle and that is really not where the restriction is. The main restriction is the small 3/8 inch tube like round port about 2 inches above the upper baffle that runs up to the final outlet port. When you cut a gill in the front of the muffler the exhaust is pretty much bypassing the small round hole and you quadruple exhaust flow. There is a fine mechanical line here because the German Design Engineers created this restriction to create back pressure to make the engine scavenge for remaining unburned fuel in the crankcase to produce a cleaner burning more efficient engine. All I can say for sure is don't modify your muffler until your warranty has expired.

 

[Fatherwheels]

On the 5105 there is a upper and lower birdcage and that is really not where the restriction is. The main restriction is the 3/8 inch metal tube right above the upper birdcage that runs up to the outlet port. When you cut a gill in the front of the muffler the exhaust is pretty much bypassing the small round tube and you quadruple exhaust flow.

I think @huskihl is reffering to the SLR type obstacle in the 7900 saws.

 

[huskihl]

I think @huskihl is reffering to the SLR type obstacle in the 7900 saws.

Yes. Like this...

 

[Tree_Frog]

But I guess nobody is making off the shelf modded mufflers? I would think this would be a pretty successful venture.

 

[JimmyT]

Technically against EPA regulations which carries a hefty fine.

 

[Fatherwheels]

But I guess nobody is making off the shelf modded mufflers? I would think this would be a pretty successful venture.

Not that am aware of, I bought a secondhnd muffler off fleabay, enlarged and added more holes, easy enough.
I see them new for 70.00 usd.

 

[JimmyT]

You need to open the muffler, find some pliers with a lip / thin edge of some sort that will go under the
bent down folded edge on the muffler, you need a flat surface on the other prong of the pliers
so you don't damage the edge around the muffler, then gently prize back the folded edge that
holds the two muffler halves together, you only need to do two sides as with a bit of wiggling
the front muffler half will slide out from under the back folded edge with two sides straightened.

Then you will need to cut out the inside of the muffler, the tube like contraption that is in there,
some times there is a cage in there with holes drilled in it, its very obvious what this cage is
when you see it, I enlarged the holes in the cage, and I think I added a few towards the front
end of the cage.
I filed the muffler outlet to a bigger size, so the original spark screen fitted back without leaving
any gap for a spark to get by.
Its not too difficult to do, it makes the saw breath, you can add more fuel via the jets then too,
as you don't want to be running lean, it also helps cool down the saw, which was my reason for doing the mod.

Whinbush, on the 7900 when you put the two halves back together can you crimp it up tight enough so the muffler doesn't rattle or do you braze it up with a torch.The 5100 non cat muffler is really simple inside compared to the 5105 stainless steel cat version.

 

[KarlD]

Whinbush, on the 7900 when you put the two halves back together can you crimp it up tight enough so the muffler doesn't rattle or do you braze it up with a torch.The 5100 non cat muffler is really simple inside compared to the 5105 stainless steel cat version.

I did mine a few weeks ago and it looks perfect and does not rattle at all. I just folded the edges back, no brazing. I don’t think it leaks along the edges either. It is definitely lounder but also a lot more gutsy

 

[JimmyT]

I did mine a few weeks ago and it looks perfect and does not rattle at all. I just folded the edges back, no brazing. I don’t think it leaks along the edges either. It is definitely lounder but also a lot more gutsy

Karl, sounds like you are a good sheet metal man, increased the saws performance and saved money to boot.

 

[KarlD]

Karl, sounds like you are a good sheet metal man, increased the saws performance and saved money to boot.

Tbh, I’m relatively sh*t at stuff like the mm, but is is EXCEPTIONALLY easy and the only thing to consider is to get the right tools for the job. Doing it with the wrong tools will = needing to buy a new muffler lol

 

[Tree_Frog]

...The only thing to consider is to get the right tools for the job. Doing it with the wrong tools will = needing to buy a new muffler lol

Which tools did you use?

 

[KarlD]

I used a narrow (5mm) wood chisel to slightly prise the fold open; I then used a gas torch to heat up the fold and then a flat head screw driver to open up the fold wide enough (about 45 degrees) to be able to grab it with a pair of pliers and open it up the full 180 degrees

 

[Fatherwheels]

,Whinbush, on the 7900 when you put the two halves back together can you crimp it up tight enough so the muffler doesn't rattle or do you braze it up with a torch.The 5100 non cat muffler is really simple inside compared to the 5105 stainless steel cat version.

Just fold the edges back to where you got them, no leaks at all.