Author Topic: Stihl Yard Boss MM55 Carb  (Read 455 times)

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Offline qlopp

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Stihl Yard Boss MM55 Carb
« on: April 07, 2016, 01:40:27 pm »
I must have missed the specific complete tiller ipl?

IPL is Stihl terminology for the parts breakdown booklet, correct?

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But I could come up with the powerhead ipl as well as the Kombo tiller head parts?

The MM55 (at least the version I bought) is sold as a tiller, not a Kombi head setup.  The attachments that I'm aware of do not require switching out gearheads, only swapping different types of blades/wheels onto the tiller shafts.  The engine is much like the Kombi system engine though.

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The powerhead is the same as the 38, 45, 46, & 55 series weed whackers

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Do you have engine troubles or tine/gearbox troubles?

Everything else has been perfect but I have had carburetor problems from day one.  I rarely use this tiller, but every time I start it in the spring I need to take apart the carb and play with it.  The basic issue is that it is getting unmetered air from somewhere.  Sometimes I have to run it partially choked just to get a quick job done.  Other times it miraculously works great.  I have given up on fixing the original carb and have ordered one to arrive tomorrow.  I don't know this forum's etiquette on posting links so if you search ebay for this item# 161884709935 (CARBURETOR REPLACES STIHL 4601 120 0600 FOR MM55 MM55C TILLER ZAMA C1Q-S202A) you can see pics of the exact carb I'm getting.  This supposedly aftermarket replacement carb has squared 90 plastic fuel hose fittings rather than the radiused brass pipes on the original, but otherwise it looks the same as my Zama C1Q with casting# 126.  It's cheap enough to try.  If I get the tiller running properly I would like to service the filters, plug, and fuel lines.

The odd thing about this carb, and one reason I'd like to read more info on this specific unit (still haven't found an exact source), is the large plastic air nipple cast into the black plastic housing.  Other more popular Zama C1Qs are virtually identical to mine but they lack this fresh air vent or whatever it is.  Since I have an air leak and the primer bulb isn't drawing a full charge of gas into itself, I must assume the air leak that is leaning out the engine has to do with a passage inside the plastic block or between it and the main casting.  Gaskets and close inspection have revealed no issues but I'm not a small engine expert.

I will post back when I get the new carb on to update its running condition, though I feel the real test is to see what happens next spring since it is always problematic after winter storage.

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Offline 660magnum

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Re: Stihl Yard Boss MM55 Carb
« Reply #1 on: April 07, 2016, 05:00:10 pm »
The vent you are talking about just connects to inside the air filter. Normally there is just a hole in the carburetor cover for this function.

If the regulator diaphragm vent hole gets plugged, the engine will stop. A rotor tiller works in such a dusty environment at times, this was added for reliability. Some Stihl chainsaws have this.

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Offline qlopp

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Re: Stihl Yard Boss MM55 Carb
« Reply #2 on: April 07, 2016, 07:30:14 pm »
The vent you are talking about just connects to inside the air filter. Normally there is just a hole in the carburetor cover for this function.

If the regulator diaphragm vent hole gets plugged, the engine will stop. A rotor tiller works in such a dusty environment at times, this was added for reliability. Some Stihl chainsaws have this.

Thanks.  What would be the downside from strictly a performance point of view (not durability) if the same shape of C1Q but without the plastic vent nipple was bolted up to my engine?  Or maybe it wouldn't even fit at all?  Just curious in case the aftermarket knock-off doesn't work.

Offline 660magnum

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Re: Stihl Yard Boss MM55 Carb
« Reply #3 on: April 07, 2016, 08:08:39 pm »
I noticed the tiller powerhead has a different airfilter than the weed whacker.

On my 026 chainsaw, I put a regular carb on the saw and plugged the hole in the air filter.

I've used the replacement Chinese carbs for the 27.2cc weed whackers and they've all worked fine.
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Offline qlopp

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Re: Stihl Yard Boss MM55 Carb
« Reply #4 on: April 08, 2016, 12:18:04 am »
I noticed the tiller powerhead has a different airfilter than the weed whacker.

On my 026 chainsaw, I put a regular carb on the saw and plugged the hole in the air filter.

I've used the replacement Chinese carbs for the 27.2cc weed whackers and they've all worked fine.

Great info, thanks.  I'll report back here by Monday on how it went, unless I am told it should go in another thread or PM instead.  Sorry to hijack this MediaCAT thread if that is what the consensus is.

Offline qlopp

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Re: Stihl Yard Boss MM55 Carb
« Reply #5 on: April 11, 2016, 03:14:29 pm »
The carb arrived ready to bolt on other than needing to swap over the throttle stop screw.  The threads between screw and casting were not tight enough to ensure the setting would stay put so for now I used a drop of blue Locktite on the threads.  There is a hollow "hole" cast above the threaded hole that I presume is used for squirting a liquid sealant (paint drop, glue, whatever) to lock the screw in place.

It started right up cold but died for a while with the adjustments I guessed at.  I got it to operating temp and set the screws and all is good now.  It even has better top end and better throttle response than ever before.  Like I said, the original carb was always junk but the last time I checked at the local Stihl dealer years ago it would have cost >$150 so I kept putting the project off.  If anyone else is hesitant to use a "Proven Part" carb which looks different than his original Zama but matches the application, I'd say go for it.  Mine had the area of the lower cover sanded off where "Stihl" had been cast so I'm guessing this is the same carb that Stihl would sell over the counter these days, only obsfucated for exclusivity and warranty reasons.

Final note on the old carb, there was an essentially unfixable problem in the plastic "vent" housing that caused an air leak in the primer and feed circuit as well as it would cause gas to puke out into the air cleaner housing.  It was a mess.

Thanks for the great help on this matter!

A couple pics of the new carb, hopefully the ink and casting numbers are legible.



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