Stihl FS90R carb repair - replace gasket/diaphragms (pictorial)

Discussion in 'Mechanic and Repair' started by Roger, Sep 26, 2010.

  1. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    I have used Stihl handhelds for many years. I used an FS85 string trimmer for many, many years, and replaced it about four years ago with an FS90R (4-Mix). I replaced gaskets and diaphragms in the FS85 after a few years of use. Likewise, I did the same task with an FC85 edger.

    I started having some unusual operational patterns with my FS90R a couple of weeks ago. It was demonstrating slight rpm variations when trimming, but typically only on the first job of the day. Also, sometimes it would choke out, that is act like it was depraved of fuel. The choking pattern was erratic. Some days it would never happen, other days it would happen a couple of times. It often happened when I was switching from right hand to left hand use.

    The trimmer is about four years old. I work solo, mowing/trimming about 1,100-1,200 yards per season. Typical use on one yard is 10-15 minutes of use. It has had other concentrated use (trimming down a bank, along a creek, etc.), but most has been routine turf trimming.

    The pattern was strange, not very well defined. I did a quick valve adjustment check, and found no change was needed. I checked the fuel pickup and found the filter to be clear of any debris. I checked fuel lines, and did not find any kinks, or other changes that might be cutting off fuel.

    Having run out of ideas for further diagnosis, I concluded the carb diaphragms were getting weak, and the carb was not pumping fuel as it should. I got the information off the carb, Zama C1Q-S110A. This was a number different than I found in charts on Internet (this is critical information). I ordered a kit from Power Equipment Warehouse. Lisa helped me get the right parts for my carb.

    I took the carb off and replaced the gaskets and diaphragms. What follows is a description of that task, and pics to support the replacement. If you follow the pics and descriptions, perhaps you will learn how to do the task yourself, or if you wish to farm out the work to others. Also, you will learn what I found, and the problem. Follow along ...

    The carb is removed from the machine. Remove the air filter cover, the air filter. Inside the air filter housing are two nights that hold a sandwich of the housing and the carb. Two posts come from the engine, and the nuts are at the end of the posts.

    The housing comes off, and the carb comes off over the two posts. Three hoses attach to the carb. Two are smaller lines going to/from the fuel tank. The third hose is a larger hose that runs from under the engine. I am supposing this to be a vacuum hose that drives the diaphragms to make the pumping operation work (somebody more skilled can confirm or deny my assumption).





  2. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    After the carb is off, the details of the model and number of the carb. On one side is the embossing of the model, C1Q. This is a family of Zama carbs used on many Stihl trimmers. The model number is etched on the opposite side. It may be very difficult to see. This number is usually not visible when the carb is mounted on the engine, but it easy to spot when the carb is removed. In this case, the number is S110A.

    Beware of this model number. One mechanic told me that FS90R machines have four different carbs. A chart I found on Internet showed the number to be S131. Clearly, that is wrong for my machine. An S131 uses a different repair kit, hence the importance of having the right number.

    The repair kit comes in a plastic bag. It includes the gaskets and diaphragms necessary to replace all of them, plus some internal parts. I chose not to replace the internal parts. The full repair kit is an RM-97, the 97 unique for this carb. There are many different RM kits.

    As a side note, other kits of just gaskets and diaphragms is available. These kits are a GND-xx number. I believe the xx is 55 for this particular model. This G & D kit would have been an alternative choice, but I ordered a full kit, a couple of dollars more. The RM kit was about $10.00. I did not use the metal parts for internal components in this repair.

    You can see the items included in the kit. The diaphragms are very fragile and need to be handled with care.




  3. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    The top and bottom ends of the carb needs to be removed. I chose to remove the top first, the end with the primer bulb. Four screws hold the top in place. When removed, the gasket and diaphragm are stuck together, looking like a single piece.

    The replacements are made, and the end is closed up with the four screws.




    Last edited: Sep 26, 2010
  4. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    The bottom end is now removed (two screws). The two screws sandwich a couple of items: (1) gasket and diaphragm, (2) cast aluminum plate with adjustment screw, (3) gasket and diaphragm.

    When the screws were removed and the housing with the vacuum feed is taken off, I discovered the source of my problem. Two pieces of debris were in lodged in the components. One was afixed to the diaphragm, the other caught in the spring. This would explain the erratic behavior, why the machine would run fine at times, other times would choke out. The location of the debris was impeding the flow, and the passageways were partially clogged. Apparently, when I switched the trimmer right hand to left, that motion was enough to move the debris into a place where flow was cut off, or at least choked down. It also explains why my action of vigorously shaking the machine would solve the problem sometimes, but not others. The shaking moved the debris.

    The debris was a red, rubbery like material. It could have been a shaving from a drill bit in a piece of plastic. It was not obvious the source. It was clearly not a fuel tank issue, because it was lodged in the fitting coming from the vacuum line, not either of the two smaller fuel lines. Yes, a mystery of what it is, and how it got there.

    The appropriate gaskets and diaphragms were changed out for new ones. I probably could have eliminated the replacements if I would have just taken out the debris, and closed it up again. But, "while you are in there" prevailed, and I had replacements at hand, so they were installed.

    The arrangement of gaskets and diaphragms are obvious. In other words, which one goes where, and how it is to be oriented is without question. There are holes and pegs that are used to index the casting into the right position. Small holes in the gaskets and diaphragms insure that the G & D are properly placed by using the pegs.

    All was closed up, the carb remounted on the engine. Be sure to get the fuel and vacuum hoses properly connected. Mark the two small ones before removal if you think you might get them mixed.

    It started second pull, and has been running top notch since the repair was done a week ago. It starts first pull, it winds up nicely, runs well at WOT, and has good power. All is well with my FS90R trimmer.

    Oh yes, I like the gas cap too! (sorry, just had to add that in the mix)

    I hope this was useful to somebody. And, maybe a skilled Stihl mechanic can tell me what I did wrong as well. Thanks.





    Last edited: Sep 26, 2010
  5. Roger

    Roger LawnSite Fanatic
    Messages: 5,943

    I realized I called the rebuild kit RM. It should have been RB-97. Sorry for the confusion.
  6. dKoester

    dKoester LawnSite Gold Member
    Messages: 3,394

    Thank You. I'm going to work on some of mine.
  7. Maico490

    Maico490 LawnSite Member
    Messages: 5

    Good thread. It takes a fair bit of time to put up a post like that.
    The debris, as far as I know, is excess crankcase sealant which has found it's way up through the impulse line. Stihl are well aware of this problem.
    As regards finding details on exact models of carbs Zama have a excellent website:
    Just go there and enter the model no of the carb in the "Product Look Up" link to get the full IPL or download the Quick kit Cross Reference
  8. ed2hess

    ed2hess LawnSite Fanatic
    Messages: 14,567

    Roger good post. Isn't there an awful lot of oil on the outsides of that unit? In the picture with the lines into the gas tank it is pretty oily. Looks like the kind of blow out you get when one of those two carb gaskets are bad. The ones between the filter housing and carb or the carb and the head gasket. Stihl don't have breather like Echo that leak do they?

    I was told that gu is the by-product of corn gas eating up the plastics in the gas tank and delivery system. Never seen quite that much but have seem
    somethign similar on several machines. On Echo those gas tank groments are getting ate up from inside.
  9. BigFish

    BigFish LawnSite Platinum Member
    Messages: 4,190

    Nope, it's what Maico said.

    Nice pics!
    Couple of things though, I usually ****** the carb/aircleaner/mtg. block area with contact or brake cleaner to eliminate all the loose crud before removing them.
    You also wanna check the check valves per the ZAMA tips site! Also it's not a bad idea to remove the hs/ls needle limiters and remove the needles. (New caps are avail.)
    Another thing is the welch plugs. Looked like the sealer on/around the plugs was deteriorating and curling up ( it usually does, thanks to ethanol ).
    I usually scrape the loose sealer out and reseal with " SealAll" which is Stihl recommended. Hopefully the plugs are tight.
    Also note that the mounting gasket isn't included with the carb kit.
    Last edited: Sep 27, 2010
  10. TriCityLawnCareLLC

    TriCityLawnCareLLC LawnSite Bronze Member
    from Ohio
    Messages: 1,025

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