Author Topic: Stihl 4140 family Weed Whackers FS-45,55, etc  (Read 1846 times)

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Offline 660magnum

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #40 on: May 16, 2014, 09:09:23 pm »
I never really used my FS45 that much as I liked my straight shaft Echos.

Now my FS45 has a FS90R straight shaft business end so I'll use it more. I did the ditch by the high way and around all the trees with it already this year.

The old FS45 curved assembly is standing in the corner waiting for me to put it in the barn. I'll have to look and see if it has been modified. I don't know just how much the trimmer was actually used but it was in the sun a lot for the covers are dull.
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Offline 660magnum

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #41 on: May 20, 2014, 09:27:24 pm »
I changed over the starter cover on the old parts FS55R I was playing with. I discovered that you can not install a old rewind spring in a 4140 series rewind housing. I bought a new rewind spring < $10 and it comes in a little plastic cup. You place this plastic cup / spring over the install position in the starter cover and push the new spring into place in the starter cover. Slick as can be. I then wound the old spring into this little cup (no problem) for the next time I need a starter spring. I put in a new 3mm starter cord, started up the FS55R and did the ditch out by the road again.

Found a few mistakes in the Service manual - one being no mention of the rewind spring cup for re-winding the spring. You cannot get a new cup anyway without buying a new spring.

Then there is a wrong screw part number listed to hold the gas tank on. The correct screw is listed there as well.

Another being the typical Stihl retainer spring/pawl guide for the starter pulley is shown aimed in the wrong direction in the service manual.  Installed as shown in the service manual and the pawl will not come out when you pull on the handle. The way shown in the IPL is the correct way.
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Offline 660magnum

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #42 on: May 23, 2014, 07:54:50 pm »
This one Stihl FS55R had been running "funny" with the mixture being unstable. I suspected a old stiff metering valve diaphragm

Monday of last week I went to start this Stihl FS55R weed whacker. With the choke on and second pull it sputtered. Flipped the choke off and from then on it was flooded. Then I noticed gas pouring out the air filter. So the metering valve is stuck open?

I was going to get a diaphragm kit for it but worked on a couple others instead.

I got to thinking that I had a bad one of these carbs that I had put new diaphragms in several years ago so I found it and stole the metering valve diaphragm. I didn't take the carb  off the FS55R, I just changed the metering diaphragm with the carburetor mounted on the weed whacker. The prize FS55R started right up but soon I noticed the clutch wouldn't release. It's a chore to get to the clutch! I shut the Stihl down and turned the cutter head. It wasn't stuck so bad that it would turn the engine through compression. I turned the cutter the other way and the piece of sand, or whatever it was, fell out and the clutch worked perfect again. Thank goodness.

I then did my weed whacking, cleaned it up and put everything away.
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Offline 660magnum

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #43 on: May 23, 2014, 08:46:00 pm »
I was doing the ditch out by the road on Wednesday with one of my all stock Stihl FS55R's and when I got done, the neighbor across the street came over. He was talking how he didn't really care for his Stihl FS45 curved shaft whacker. I showed him my former FS45 that I put the FS90R assembly on. First thing he said was that it was too heavy.

That assembly weighs close to one pound more than the FS55R assembly. A complete FS90R Four Mix weighs more than two pounds more than a complete FS55R. So keep that in mind.
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Offline 3000 FPS

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #44 on: August 23, 2014, 12:51:13 am »
Thought I would add this here.
I worked on a Stihl weed wacker today.   It is a FS100RX.   This trimmer is a 4 stroke engine with valves, rocker arms, and push rods, but it runs on 2cycle fuel and it has no crankcase for holding oil.   When I got it started it sounds like a 4 stroke engine.   
My big question is does this engine have transfers that feed up into the intake valve to direct the fuel charge, and what other 2 cycle characteristics does this engine design use.   Like how do they oil the top end of the valve train. 
PP 505, 475, 445.

Offline 660magnum

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #45 on: August 23, 2014, 01:38:05 am »
Look in your SBC mail for the 4180 service manual.

The thing oils by blow by and the intake runner is like a B&S 4S with oil in the crankcase?
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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #46 on: August 23, 2014, 01:47:51 am »
The four stroke Stihls are rather popular in this part of the USA. Two neighbors have them and the other has a Stihl FS45. And from the early part of this thread, I have a gaggle of FS55's

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Offline 3000 FPS

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #47 on: August 23, 2014, 01:48:56 am »
Very good Jim thanks.   Looks like some good reading I appreciate it.
PP 505, 475, 445.

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #48 on: August 23, 2014, 01:50:41 am »
This FS 100 is not the first 4 stroke weed wacker I have come across but it is the first one I have worked on with no oil in the crankcase.
PP 505, 475, 445.

Offline 660magnum

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Re: Stihl 4140 family Weed Whackers FS-45,55, etc
« Reply #49 on: August 23, 2014, 03:21:22 am »
The owner's manual which Stihl calls the Instruction manual is downloadable from Stihl

It takes on the everyday service and operation including valve adjustment.

The cutting head is the 27-2 the same as the FS55R or you can use the blades.

The head ratio is the same as the FS55R but none of the gearbox parts interchange though the guard is the same.

The diameter of the tube and the square connectors on the flex and solid shafts are the same but the lengths are 3" different.

The retaining hole in the tube for the motor end is 90 degrees out of phase between the FS55 types and the FS90 types.
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