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Tuning a Stihl carburetor

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Eddie in MI

06-21-2005 15:50:01

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I have a Stihl FS 76 trimmer. I would like to throroughly clean and reset the carb on this machine. I know the basics of setting the jets, but does anyone have any pointers on disassembly/reassembly or cleaning solution? any tips or tricks?

The machine simply isn't running up to par and I was going to go over it myself. Don't want to take it to the shop; I'd rather learn to do as much as I can for myself (plus, I don't like the local Stihl dealer...). I've already got a new spark plug in the machine; I've gone over the electric components with electric motor cleaner and I have also cleaned out the muffler and replaced the spark arrestor.

Just want to get this thing in tip-top shape.

TIA,
Ed

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thejdman01

06-22-2005 16:58:21

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Re: Tuning a Stihl carburetor in reply to Eddie in MI, 06-21-2005 15:50:01

general iead of MOST sthils is turn it in clockwise (tighten all the way) that means snug not truck lug night tight both mixture screws and back out 1 turn a piece usually good place to start. better yet count the number of turns before dismantling it and put them bbakc there as a starting place.

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caseman2

06-22-2005 04:24:58

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Re: Tuning a Stihl carburetor in reply to Eddie in MI, 06-21-2005 15:50:01

I would pay more attention to the exhaust system than the carb. The muffler and the exhaust ports give more trouble than the carb.

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echomasterservicetech

06-22-2005 02:52:30

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Re: Tuning a Stihl carburetor in reply to Eddie in MI, 06-21-2005 15:50:01

Basic carb rebuilds can be accomplished by simply disassembling slowly and carefully seperate both sides. I seldom installed carb kit, I found most machines ran poorly as a result of bad fuel. Seperate pump side slowly and you will be able to reuse gaskets,pay extreme attention to their orientation as many times in WALBRO and ZAMA carbs gaskets are not arranged in a common manner.To rough in adjustment simply turn either one idle mixture screw or two in all the way until lightly seated,then turn out one and one half turns, as stated earlier you will see an H and a L on two screw models.H adjusts WOT start unit and run at WOT, slightly turn screw in until machine just starts too miss and then turn out till it starts too miss,now turn in equal distance between these two points and repeat on L at idle speed.When carb is open look for signs of SHELLACING this is caused by fuel being left in machine to long.Fuel shelf life is just over thirty days before it begins to decay,it is not the fuel of yesterday. By adding a fuel stabilizer a lot of this can be avoided.One last thing when evaluating gaskets and such, if machine has not had much use in a long time,replace gaskets.

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Rod (NH)

06-21-2005 17:10:28

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Re: Tuning a Stihl carburetor in reply to Eddie in MI, 06-21-2005 15:50:01

Hi Eddie,

You can download an official Stihl carburetor service manual here. Most of the site is for Stihl chainsaws but if you scroll down to the carburetor section, I believe that the Stihl carburetor manual also covers the Stihl carbs used on many trimmers, including the FS76. The basic carb mixture adjustments, if any, should be spelled out in the booklet that came with the trimmer. Depending on model, you may not have many adjustments since a lot of small engines now have fixed jets or limiter devices to restrict changes by owners.

Rod

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06-21-2005 16:27:24

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Re: Tuning a Stihl carburetor in reply to Eddie in MI, 06-21-2005 15:50:01

If it runs, you have done about all that needs to be done, excpet for replacing fuel filter and air filter. What do you mean by not running up par. It might need a little carb tuning, but be careful, too much turning can lean engine and burn up the engine from lack of lubrication!

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Eddie in MI

06-21-2005 17:55:16

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Yeah, that's why I though I'd ask before I did in reply to JT, 06-21-2005 16:27:24

I know that I don't want it to lean out...

It's been a while since the machine has had a good "going-thru." I was just going to make sure that everything was nice and clean.

Rod: thanks for the link. I'm definitely gonna check that out.

As far as not running up top par, the machine idles nicely and it runs well at WOT, but it seems to get to full acceleration slowly, with some hesitation. Any ideas?

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Rod (NH)

06-21-2005 19:56:01

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Re: Yeah, that's why I though I'd ask before I did in reply to Eddie in MI, 06-21-2005 17:55:16

As I remember, hesitation on acceleration indicates a main mixture that is slightly lean. If you have a high speed needle adjustment (usually indicated with an "H") and it is not already up against a limit stop, I'd try opening it a little. Say 1/8 turn or so to see if that helps. I'd avoid taking the carb apart if possible. If you do decide to take it apart, I'd advise having a rebuild kit available before you do.

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Eddie in MI

06-21-2005 21:36:07

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Thanks! ntxt in reply to Rod (NH), 06-21-2005 19:56:01

ntxt

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