Stihl FS80AV (4112 Series) Carburetor Rebuild

[Chop91]

Hello everyone, I will apologize in advance if my post is long winded. I recently inherited a Stihl 4112 series FS80AV brush cutter from my dad. He always took good care of his equipment including draining the gas tank and running it out of gas. I replaced the spark plug, fuel filter, gas lines and air filter with the hope of removing any potential issues early on. I can get the machine to start and idle but wants to stall when I throttle up. I attempted to adjust the carb per the Stihl repair manual with no success. Kind of an odd thing is, I found if i mess with the plunger on the bottom of the carb I can throttle up no problem but if i stop it goes back to wanting to stall. I'm guessing that a carb rebuild is in my future. Has anyone had any experience rebuilding a TK carburetor for this machine and are there any pit falls that I should be looking for as I do this? Any help that you can give me would be appreciated. Thanks

 

[Coupe]

Few other things I would try first like make sure the fuel filter in the tank is clean. The other
thing to do is were the filter is take it out of the line and plug a preassure tester in the same
line and put four or five pound of preassure in and see if it holds it. If not I would say your
diaphragms in the carb have gone hard if it has been sitting any length of time. It won't
do any harm giving the carb a full kit and a good clean probably overdue for one anyhow.
If that does not fix the problem you may have to dig deeper by testing for air leaks and
so on. I have a stihl hedgetrimmer that has been dropped in to me to be fixed doing the
same thing and will be doing it tomorrow so when I find out what is causing it will be able
to let you no. There will be some more guys on here that will have some more imput into
what else could possibly be causing it too.

 

[hacskaroly]

Has anyone had any experience rebuilding a TK carburetor for this machine and are there any pit falls that I should be looking for as I do this?

Welcome to the forums! Carb rebuilds are generally straight forward. Have a paper towel out and lay each part, gasket and diaphragm out as you take it apart in order and orientation, makes it much easier to put back together. Clean the metal parts with carb or brake cleaner. Watch you don't lose the needle spring as most repair kits don't come with one. If you take the high and low needles out, screw them in first noting how many turns so you can put it back to that as a ballpark to start with (adjust as needed when running). Like @Coupe stated, starting with a pressure test will let you know if you have issues before digging in too far. It does sound like you might have some crunchy diaphragms, not unexpected.

 

[Chop91]

Thank you, I will give a pressure test a try. Then a rebuild. Thanks for the help

 

[Chop91]

I should ask is there a specific pressure tester that you like to use or is there one that will work best for this application?

 

[hacskaroly]

I should ask is there a specific pressure tester that you like to use or is there one that will work best for this application?

I think the Mityvac 8500 has the most positive reviews from people here on the forum. I got a cheaper one and am not happy with it, will be getting a Mityvac!

 

[Coupe]

Just done one out here with the same problem would idle okay but as soon as I put the power on just died.And if I pumped the primmer bulb would power up but not great. I think you may have to richen it up as that's what I had to do.(running to lean) I dear say yours will have a carb
much the same anther one the chinese punish us with. Try that as you will need the right tools (screwdrivers) to do it. They are horrible carbs to get the engine to hit a nice note as always seem a bit surgey. Good luck with it as it may not be the cause of the problem as Hacskaroly has said as well if the diaphragms get hard that could be a problem. The way you test your diaphragm is it is the metering one that is usually the one that
will cause the problem so you will see a little ally button in the middle press it down and if it stays down and sounds a bit crackley its time for a new kit. Think I will stay with my little echo hedgetrimmer never given me a days problem yet.
On vacuum testers I have a Mityvac 8500 and swear by it they are fully rebuildable and very accurate .I brought a cheaper type and was no good whats so ever took it back to where I brought it they refunded my money and that's when I brought the 8500.

 

[Chop91]

Thank you everyone for your help. I will keep you posted on my progress.

 

[Chop91]

Just a follow up. I picked up a pressure tester from Amazon. The carburetor failed big time so I ordered a rebuild kit and will work on it as time permits later this week. Thanks again to everyone for your assistance.

 

[hacskaroly]

The carburetor failed big time so I ordered a rebuild kit and will work on it as time permits later this week.

Most carb kits you get will not have an extra spring in it, so be really careful when you replace the main needle that the spring does not roll off your work bench or go shooing off into the great workshop void. I have lost several springs and e-clips to the great void! I look forward to hear how it goes for you!

 

[edisto]

If you take the high and low needles out, screw them in first noting how many turns so you can put it back to that as a ballpark to start with (adjust as needed when running).

Definitely a good idea to take the needles out. I usually scribe the number of turns it takes on the metering plate the first time I take a carb apart.

 

[lehman live edge slab]

The tk series used on the older fs80/fs90 av and ave was a pain and very problematic overall Stihl actually made a conversion kit to adapt a standard carb to them that hasn’t been available for a long time. These old trimmers were actually made for Stihl in japan.
I’ll look in my books at home for a rebuild/tuning procedure on that carb this evening. I believe I have the actual shop repair manual for the 80 and 90 and it listed the update part number to convert them even. I also forgot but will check because I think I have an 80av powerhead only that may have a standard tillotson on it so would have all the parts for standard carb.

 

[Chop91]

I managed to pick up a repair manual for this machine with the factory carb settings which helped alot. The rebuild went well and it runs fantastic! If I can figure out how to attach it, I have a video of the initial start up. Thank you to everyone for all your help.

 

[bell7526]

I managed to pick up a repair manual for this machine with the factory carb settings which helped alot. The rebuild went well and it runs fantastic! If I can figure out how to attach it, I have a video of the initial start up. Thank you to everyone for all your help.View attachment 453046

Could you tell me where you were able to get a manual from? I am having trouble finding one.

I also have a FS80AV with a TK carburetor. I can get it to run for a couple seconds when I spray fuel directly into the carburetor. I don't have a clue what I am doing but would love to be able to use the old FS80AV.

It has been sitting since the mid nineties with a bag over the engine in a basement. I replaced the fuel lines, fuel filter, bulb, and spark plug.

 

[Unodog52]

I managed to pick up a repair manual for this machine with the factory carb settings which helped alot. The rebuild went well and it runs fantastic! If I can figure out how to attach it, I have a video of the initial start up. Thank you to everyone for all your help.View attachment 453046

Chop 91 hey thanks for the post would like to know where you got the manual need to know factory setting can’t get mine right have put rebuild kit in it will run for a while then die need help and thank you

 

[SimonHS]

This is just the user manual. @ray benson may be able to kindly supply the workshop manual?

https://www.stihlusa.com/webcontent/cmsfilelibrary/instructionmanuals/fs80_85_manual.pdf

 

[Chop91]

Could you tell me where you were able to get a manual from? I am having trouble finding one.

I also have a FS80AV with a TK carburetor. I can get it to run for a couple seconds when I spray fuel directly into the carburetor. I don't have a clue what I am doing but would love to be able to use the old FS80AV.

It has been sitting since the mid nineties with a bag over the engine in a basement. I replaced the fuel lines, fuel filter, bulb, and spark plug.

I picked mine up from Ebay but can send you a copy of the carb adjustments when I get home tonight

 

[lehman live edge slab]

This is just the user manual. @ray benson may be able to kindly supply the workshop manual?

https://www.stihlusa.com/webcontent/cmsfilelibrary/instructionmanuals/fs80_85_manual.pdf

View attachment 458081

This is for a newer fs 80 with regular carb I believe. The tk slide carb came on the old made in Japan Stihl brush cutters.

 

[SimonHS]

This is for a newer fs 80 with regular carb I believe. The tk slide carb came on the old made in Japan Stihl brush cutters.

Thanks. I did not realise that there was a different, earlier version.

 

[ray benson]

This is for a newer fs 80 with regular carb I believe. The tk slide carb came on the old made in Japan Stihl brush cutters.

Early FS 80 made in the late 1970's to mid 1980's.