OK Guys, Dolmar 5105 muffler mod!!!

[Ambull]

OK, so I claimed that I could remove the cat from a 5105. I dry tested the stock saw first, and even with the H screw all the way out, the saw was going well into the 13800 rev limit. So I took apart the muffler, and the inside looked like this:

I got a weight on the muffler before and after the mod. It lost only an ounce.

Before:

And after:

Busting it out wasn't too bad. I used a drill, a flat head screwdriver, and a pair of needle nose pliers. Took about 45 minutes maybe.

Here are most of the cat pieces:

I decided to not reinstall the cover or the screen, so I probably saved another 3 ounces or so. The saw was noticeably lighter afterwards.

So I then cut off the trim tabs. Both carb settings were wide open, and the saw was too lean. I ran it, and it sounds much better with the muff mod. I set the H screw so that it just barely hits the rev limiter.

Any advice on max RPM would be appreciated. Should I set it richer?

Thanks for looking.

 

[mdavlee]

I would go rich enough to get it off the limiter by 3-500 rpms.

 

[Brian VT]

Any chance you could video your tuning so I might hear what it sounds like when it hits the limiter ?
As you turn it in from rich, does it go from rich 4-stroking to clean and then to rev-limited 4-stroking ?

 

[Warroad50]

I removed the cat from my Dolmar 420 same as what you did. Sounds better starts easier runs a bit stronger. Really did not have to adjust the carb on mine.

 

[Ambull]

I removed the cat from my Dolmar 420 same as what you did. Sounds better starts easier runs a bit stronger. Really did not have to adjust the carb on mine.

Before I cut the trim tabs, I could hit the rev limiter way before full throttle, even with the cat in there. I would guess that making the fuel richer adds quite a bit more power. There is so much friggin snow on the ground, and it was like 15 degrees today, so I can't tach the saw in the cut until it thaws out a bit.

 

[Ambull]

Any chance you could video your tuning so I might hear what it sounds like when it hits the limiter ?
As you turn it in from rich, does it go from rich 4-stroking to clean and then to rev-limited 4-stroking ?

As the Snellerizer told me, it is very hard to hear the difference between rev limiting and 4 stroking. The mic on the camera would probably make it even harder. It is really easy to see it on the tach, as all of a sudden the tach drops down to about 4800 RPM and the saw is clearly still running at 13800. Maybe I will try some runs tomorrow with it just below rev limit, and just above. I'll get back on this.

 

[Ambull]

I would go rich enough to get it off the limiter by 3-500 rpms.

OK thanks for the advice

 

[Warroad50]

Before I cut the trim tabs, I could hit the rev limiter way before full throttle, even with the cat in there. I would guess that making the fuel richer adds quite a bit more power. There is so much friggin snow on the ground, and it was like 15 degrees today, so I can't tach the saw in the cut until it thaws out a bit.

Ive got 30" plus on the ground where I'm at the high for today was only 3 and heading for a low of -20 below tonight. Went out on Saturday and cut when it was about 15 degrees out not to bad. Cut some ash by down by the river.

 

[Ambull]

Ive got 30" plus on the ground where I'm at the high for today was only 3 and heading for a low of -20 below tonight. Went out on Saturday and cut when it was about 15 degrees out not to bad. Cut some ash by down by the river.

You guys up there are a whole different breed.... lol.

 

[Andyshine77]

Any chance you could video your tuning so I might hear what it sounds like when it hits the limiter ?
As you turn it in from rich, does it go from rich 4-stroking to clean and then to rev-limited 4-stroking ?

Not a 5100 but it should do. The trick is to start way too rich than lean it out.

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[Brian VT]

Thanks Andy.
At the end, was that the rev limiter or 4-stroking ?
To me it sounded like you leaned it until it was bouncing off the limiter when not cutting. Might just be my speaker or the mic.
It sounds more like a "burble" at the beginning and a "crackle" at the end.
I can certainly see why you need to tune these in the wood. The change in the sound is much more noticeable than on the ground or bench.

 

[Andyshine77]

Yes at the end it was hitting the limiter, and yes the best way to tune them is in the wood.

 

[MCW]

it is very hard to hear the difference between rev limiting and 4 stroking.

On my modified 7900's it is quite easy to hear the difference between the limiter and 4 stroking. Maybe not on other saws?

 

[SawTroll]

I would go rich enough to get it off the limiter by 3-500 rpms.

If that is for starting the fine tuning it makes sense, but it hardly is where you want to end up!

 

[porsche965]

I bought a 5100S non-cat muffler for my 5105 and ended at 14,100 running just a tad rich but really makes good power in the cut.
What I noticed was that the non-cat holds it's power in 16" hardwood for 5 cuts back to back without a rest, but the cat muff. seemed to slow down, or actually feel like it lost power after the 2 WOT cut. ?? Heat??
Sticking with the non cat of course.

 

[Ambull]

If that is for starting the fine tuning it makes sense, but it hardly is where you want to end up!

I was sort of thinking the same thing. Only way to know for sure is to tach it in the cut, I guess. It should be able to get to at least 13800 I would think.

I had read about the trim tabs on the carb, but I was amazed at just how lean the thing was when I really got into it. And that was with running both settings on the max tab. I would advise everyone to cut those things off, and punch the cat out of the muffler.

 

[SawTroll]

Yes at the end it was hitting the limiter, and yes the best way to tune them is in the wood.

I admit that I am sceptical to that, as you will not know where you really are compared to the recommended max (= how lean the carb is set), and all wood is not the same - or am I missing something? :confusedn:

 

[SawTroll]

I bought a 5100S non-cat muffler for my 5105 and ended at 14,100 running just a tad rich but really makes good power in the cut.

To relate to that, I started the run-in on my 5100 at 14,200 and hit the 14,500 limiter at about the time it was run in - like cuttinscott told me I would. I then adjusted it down to about 14,400 (a very minor adjustment on the H screw) - that was the highest firm reading I could get on the tach.

 

[Andyshine77]

I admit that I am sceptical to that, as you will not know where you really are compared to the recommended max (= how lean the carb is set), and all wood is not the same - or am I missing something? :confusedn:

It's actually the only true way to tune a rev limited saw. As long as the saw has enough fuel, max recommended rpm means nothing to me, epically when it comes to ported saws.

Like I said, tune the saw rich than lean it out until it runs clean in the cut, most times you're still a little rich on the H. The most important thing is to start out rich. Also you can hear the difference between the limiter and 4 stroking.