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 punnish 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 says 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 tha'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.