Stihl FS85 will not idle, rather stops

Discussion in 'Mechanic and Repair' started by Roger, Jun 8, 2007.

  1. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    I have an Stihl FS85 that has given me excellent service over the five years of use. It is my only trimmer, and gets used about 2-3 hours per day.

    The past few day, it has been stopping if I leave it idling for about 30 seconds. It usually happens as I walk between certain parts of the yard, such as end-to-end on a driveway, or other movements between trimming tasks. I carry it with one hand, and after idling for a short time (30 seconds or so), then it will stop. It will tend to slow and sputter in the last few seconds, before stopping.

    Sometimes getting it started is a challenge. I have noticed the primer bulb does not have fuel. I usually need to push the primer bulb a few times to get it started again, along with some choke.

    If I stop the engine right after use, wait for 30 seconds, and attempt to restart, it fires right away. In other words, rather than keeping it running (at least my intention) while walking from one area to another, if I STOP the engine, and then refire when ready for use, it starts right away.

    When running at WOT and working, it runs fine. It does not appear to be short of power.

    Believing it was getting fuel starved, I thought perhaps the pick-up body inside the tank, with the filter, was partially clogged. I bought a new one today and installed, but I found no change in behavior.

    I am unsure how the fuel is pumped (pressured, sucked???) from the tank. Is it possible that when running at WOT, the method to get fuel from the tank to the carb is working well, but when running at idle, there is not enough "power?" to get the fuel from the tank to the carb? It has all the marks of this pattern.

    I have never rebuilt a carb off a 2 cycle like this one, so am unfamiliar with the innards. Is there a diaphragm inside that works to get the supply of fuel to the carb? Perhaps it is failing ....?

    Anybody have any suggestions (other than "go buy a new trimmer"), or observations I could use to solve this problem?
  2. ed2hess

    ed2hess LawnSite Fanatic
    Messages: 14,586

    The unit has one adjusting screw on the carb besides the idle screw try adjusting on that first. Got to watch the adj. screw works backwards of normal logic CW turns it out and increases the amount of fuel...I think. I am an echo guy but have two of these and they are the best units I have but nobody likes to use them. The carb has a diaphram like you thought and that pumps the gas....the tab is alum. so it could be worn. The carb is not real hard to rebuilt just have to watch parts when you take it apart. The kit runs around 10 dollars. I got one in shop for repair right now and just started taking it apart....these are units worth repairing.
  3. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Messages: 3,716

    infamous bad zama carbs
  4. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    Having used the trimmer in a wimpy status the past couple of days, I believe I need to do something to resolve the problem. I can get by with keeping the rpms moderately high during the no-trim intervals, but clearly the engine is being starved from enough fuel.

    I need to either, (1) Rebuild the carb, or (2) Replace it.

    I made a call this morning to the dealer, and found no rebuild kits in stock (actually two places). One told me the rebuild kit was over $50, but I think that is wrong. It should be about $10-$15, right? Also, I learned the kit contained two diaphragms, one a "pumping diaphragm." Most likely, this is the culprit. But, nobody had these in stock either.

    I asked for a new carb, but was told I needed the model number of the one on my FS85 trimmer. And, I learned the only way to learn the number is to take it off. Is this right?

    I would like to get my parts, organize my workbench, make the repair (rebuild, or new install), and get back in service. This is my only trimmer, and I need it daily. I do not wish to tear down, build back up the bad unit, and return it to work.

    I have done some checking online, and find repair kits on e-bay (other places too?) for FS85. The kit seem to be applicable for several Zima carbs, ones used by Stihl for these trimmers.

    I am looking for advice here on the best path to take. Is my only choice to tear it apart, get the numbers, and start the ordering process? Or, can I order a kit, with reasonable assurance the kit will work when I attempt to make the repair?

    These are popular trimmers and surely somebody else has gone through this process. If the repair seems to a long, drawn-out process, then perhaps I should be a new trimmer, and get on with my work, leaving the repair to off-season. That is very drastic. But, I am coming upon the season when I use the unit as an extended reach hedge clipper, in addition to turf trimming duties.

  5. topsites

    topsites LawnSite Fanatic
    Messages: 21,653

    My fs-100 does it too, very aggravating.

    Have you tried spraying it out real good with some carb cleaner?
    Also, of course pull the screen from the exhaust...
    Put a new spark plug and air filter in it, too, gap to .020.
    Maybe apply a dab of die-electric grease to the tip of the spark plug, where the wire connects.

    If you have compressed air (but watch your eyes and nose) I would also blow out the clutch area real good.
    On that note, pull the plastic cover off and blow all that out as well, especially around the carb linkage area.
    If you can get to it, blow some air through the magneto parts of the coil.

    If you can, do a compression test, should read 120+ psi.
    Just to eliminate the bs stuff, you do run super in it?
  6. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    Not having much response, I decided to get a carb rebuild kit. After finding a couple of dealers with no parts, I found a couple that claimed to have what I needed.

    However, each party required that I bring the old carb for identification codes. They are not visible without having the carb in hand, and able to turn it to look at various parts.

    I took it off this morning, and went to a dealer who claimed to have rebuild kits. After much searching, the conclusion was that they did NOT have a rebuild kit. However, the parts man suggested that I simply buy a "gasket and diaphragm kit." He said installing the new parts in the second kit solves most problems.

    So I bought it, $10.69. This evening I began to work in the unknown. I have never touched one of these carbs for repair. The kit had two diaphragms and two gaskets. Pretty quickly, I realized what could be taken off, and found the place where one set of a gasket and one diaphragm would fit. The second set was under another plate on the opposite side of the carb.

    Within a few minutes I had both parts installed and was unsure if I did was right -- all so easy, so simple!

    I reinstalled the carb on the FS85, and attempted to fire it up. Quickly it came to life, with good response on the trigger. The idle was set too high, and I made that adjustment. After stopping and starting it a couple of times, and a trial run around a bed in the yard, and I convinced myself the trend looked promising. I let it get hot, and then let it idle for awhile. It did not stop, like before.

    I think the problem is fixed!

    I will post some images of the repair at the end. I hope this helps eliminate any apprehensions for some who may be reluctant to attempt such a repair. It is very easy to do. Is somebody wants more detailed description and more images, PM me with your e-mail address.

    The key is to get the right parts (nothing profound here!). For your information, I learned:

    carb identification: Zama, model C1Q (embossed on the side of the carb), with sub id as S45 (etched near one flange on the carb, not near the embossed portion). So, the formal identification: C1Q S45.

    The gasket and diaphragm kit: Stens 615-108.

    The complete rebuild kit is available for this series of carbs: Zama RB66 series (I am not sure what else is in the kit besides the four items I have described above. These kits are available on e-bay for about $13, plus shipping).

    Some pics:





  7. dutch1

    dutch1 LawnSite Silver Member
    from Jayhawk
    Messages: 2,249


    Other items in the Zama rb-66 include fuel inlet needle, metering lever, metering lever pin, welch plug(s) and fuel strainer. I have seen kits with/without the metering lever spring. When installing a new needle/metering lever, it is helpful to have the proper Zama tool to set the metering lever tip to the proper height.

  8. Roger

    Roger LawnSite Fanatic
    Messages: 5,943


    Whew! I'm glad the dealer did NOT have the full kit. I would not have known what to do with all the other parts, and probably would have done more harm than good. Thanks for the information.

    For those of you who have done these kinds of repairs, and those which are more complicated, my simple explanation undoubtedly sounds like kids-play. However, I made the post for those who have questions and may be reluctant to make the repair. Maybe my explanations and pics will help, or maybe it will encourage somebody to get involved in a project they should have left alone. I did not find other "how to do" threads on these carb repairs.

    I am just hopeful that this repair will get my trimmer back into service and running well. I don't have time for spending on these tasks, as interesting and entertaining as they might be. They produce no revenue!

    BTW, I have much to report on the LawnBoy, but will wait until I get a bit more experience with changes that have been made (another interesting, entertaining project that produces no revenue).

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