Inside the 441 M-Tronic Carb.

[eiklj]

I've had this saw on my bench for awhile now and finally got most of the parts in to fix it. It's in the shop because the owner dropped a tree on it and busted up the tank housing and broke a chunk off the left case half.
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While it's on the bench, I thought I'd tear into the electronic carb to see what it looks like. I thought I'd take some pics along the way. There is nothing wrong with the carb (yet), I was just curious.
Is there a way to make my pictures smaller?

 

[eiklj]

Carb teardown

Here I've pulled the carb. Note the micro switch that is tripped by the choke lever.
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[eiklj]

Here I pulled the diaphragm off. Note the factory-set idle speed screw. They also have a little plastic piece and Torx screw I assume for tamper resistance to keep idiots like me from doing what I'm doing.
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[eiklj]

Here I pulled off the metering plunger assembly the controls the fuel into the carb.
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These 2 holes line up with...
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These 2 holes.
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[eiklj]

Another HD carb for comparison:
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M-Tronic on the left.

M-Tronic on the right.

M-Tronic on the right.

 

[eiklj]

A couple more...

Here's a couple more comparison pics.
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M-Tronic both on the right.

I have no idea what the other Walbro HD carb came from, I want to say a cut-off saw or something because of the larger bore diameter.
More pics coming...

 

[eiklj]

Here's what controls the fuel flow. Just 2 wires going into it. I want to find out if it's maybe a 5v system and exactly how it's controlling the fuel flow. Maybe a PWM signal? (Pulse Width Modulated) which is quite common in electro-hydraulic control systems that use a small input voltage to control hydraulic flow??? I don't really know, just guessing.
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[eiklj]

And finally a couple pics of the coil assembly. Obviously some circuitry going on inside that thing. I assume a pick-up that measures engine speed?
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jason

 

[eiklj]

Gotta get back to work. I'll check back later this evening. Hope this was of value.

 

[Saw Dr.]

I have no idea what the other Walbro HD carb came from, I want to say a cut-off saw or something because of the larger bore diameter.

Believe it or not, HD-5 is an 029 carb.

 

[Chris-PA]

Very cool - repped!

There are only two wires that go to the carb, running to the the fuel control and the switch, and also providing power. Clearly they are not using the chassis as a ground/return. The wires run to the fuel control first, so the switch is not opening the circuit. I expect that they are doing PWM to the fuel control, and that they can detect a dead short on the output from the closing of the micro switch. So they know rpm from the ignition and they can detect choke - that's it. There may be a temperature sensor in the module, but somehow I doubt it. They just lean the mixture periodically and watch what happens to the rpm, and from that they determine which way to adjust the mixture.

It's a very simple system, but it provides feedback and corrects the inability of a standard carb to provide a fixed fual/air mixture with varying load, as well as compensating for atmospheric and other changes.

 

[young]

great info.

 

[WoodChuck'r]

Sweet thread!


Good **** tearing that carb down - lots to learn for sure! :msp_thumbup:

 

[Gologit]

Good thread. Don't make the pictures smaller. They're fine the way they are.

 

[Stihlman441]

Rep sent
Thanks for sharing great thread.

Cheers
Andrew

 

[PJF1313]

Sooo... that's what's in the "M" saws!



Thanks ALOT for your time and effort to show US minions how it "ticks"!

 

[eiklj]

Thanks for the comments and feedback. I'm always glad to pass on any info I can. I've learned so much on this site and feel it's my duty to share when I can.
I can post more pics if anybody wants to see something in more detail. (not just the carb) I still have everything tore apart on the bench and probly will for the week. I'm ready to split the case so's I can swap out the broken half.
je

 

[Miles86]

Thank you-great photos.

I now go to my Stihl dealer and order a spare carb for my MS440.

 

[Chris-PA]

Thank you-great photos.

I now go to my Stihl dealer and order a spare carb for my MS440.

Why?

 

[belgian]

Why?

Probably because industrial electronics have an average lifetime of let's say 7 years....just guessing. He must have seen $$$$ in the future..., can't really blame him.