Fs90 Valves? Worth repairing?

Discussion in 'Mechanic and Repair' started by KenFres, Jan 2, 2019.

  1. KenFres

    KenFres LawnSite Member
    Messages: 6


    I'm a small one man band running on the smell of an oily rag and a small number of properties on the weekend. I recently bought a very cheap, used and 'probably dead' fs90r in the hope that I could reduce use on my fs94r- I'm wondering if it is repairable. I make do with my own maintenance with no training (another reason I love this website!) but have never had to take a 4mix apart or do valve work.

    Pulling on the cord I end up with a hard stop. On partial disassembly, I can spin the flywheel and the exhaust valve will move - when the piston gets to the top, it stops. When I reverse the flywheel, the piston doesn't drop, but with a scrench in the spark plug port I can push it down and repeat the process. I can't see much through the exhaust or spark plug ports on a 4mix!

    Can you give me any guidance? Would this be a bent or dropped valve? A crankcase problem? Would it be worth pulling off the valve covers or is it complete disassembly or just not worth doing?

    I'd hate to have to take the hit on the cost of this with nothing to show for it.
  2. BigFish

    BigFish LawnSite Fanatic
    Messages: 5,152

    Guess what.

    Look, if the piston ain't goin down when ya rotate the crank, it aint good.
    " What we have here, is failure to cominicate". As in the crank ain't talkin to the piston, right?
    So, it would seem ya got more than valve problems. All ya can do is tear it apart and see, right?
    It'l be fun, trust me.
  3. Walker56

    Walker56 LawnSite Fanatic
    Messages: 13,719

    Here is the service manual for the FS90. No sense tearing in to it without a reference guide

    Attached Files:

    Doc8406, That Guy Gary and hort101 like this.
  4. That Guy Gary

    That Guy Gary LawnSite Silver Member
    Messages: 2,693

    Go over that service manual then tear it down, these engines are really simple and easy to repair.

    I think it's more likely you've got a busted cam wheel than a valve, and they are cheap and easy to replace.
  5. ArTurf

    ArTurf LawnSite Fanatic
    Male, from Ark
    Messages: 6,077

    Sounds like it has dropped the piston in addition to valve issues. Sounds like a lot messed up inside, Ehhhhh
    BigFish likes this.
  6. ed2hess

    ed2hess LawnSite Fanatic
    Messages: 15,840

    I had similar situation an 4-mix it was parts from broken gear cam. I think you can pull one cover off to check...I couldn't get all the broken parts out without complete tear down.
    That Guy Gary, hort101 and Walker56 like this.
  7. OP

    KenFres LawnSite Member
    Messages: 6

    Thanks, that's all helpful and makes sense, but not what I hoped to hear.

    I've removed the shaft and throttle. The muffler, carb (probably not needed?) and spark plug are easy to remove. On a quick look through the manual (very much appreciated Walker!), much looks easy. I really do hope this is fun and not just frsutrating!

    But what is mechanically difficult to do, or are there tricky bits to watch out for?

    • Page 40 - Removing a flywheel has defeated me in the past as I have no special tools. Any tips on how to remove it? There is a youtube video "Stihl FS55 RC clutch drum, clutch, and flywheel removal" - is that what you would do without a puller?
    • Page 41-42 Replacing oil seals- I really don't know what is going on in those pages and it refers to special tools. Will it be clearer when I get to those step?
    • Page 43 - Lower half of the crankcase - do I need any gaskets or sealer to put this back together or simply use existing parts?
    Edit- And where would you start? Valve covers and the gear cover, then clutch flywheel etc?
  8. OP

    KenFres LawnSite Member
    Messages: 6

    Hmm, BigFish suggested this for flywheel removal on another post about a redmax:

    "It's been a while since I worked on one, but I'm thinkin a two jaw puller will work.
    Or, simply unscrew the nut till flush with the crank end and while prying up on the rotor, smack the nut squarely with a hammer, and it should pop loose. ( the method I used )"

    BigFish, how do you pry up - screwdriver? plates like in the video?
  9. BigFish

    BigFish LawnSite Fanatic
    Messages: 5,152

    You should start by tossin' it in the trash, and buyin' a new motor or a fleabay unit.

    You do realize what it means when the piston ain't movin' down when ya rotate the flwhl. like ya described ,right? It means $$$$ in parts.

    What would you use to pry up on sumpin?? I use a big screwdriver.

    Look, you need to study the manual. It pretty much covers everything ya asked about.

    From what I can tell, this project seems a bit much fer yer abilities...just sayin'.
    Last edited: Jan 3, 2019
    That Guy Gary likes this.
  10. That Guy Gary

    That Guy Gary LawnSite Silver Member
    Messages: 2,693

    Remove the valve cover first, you should be able to inspect them while turning the motor as much as you can to see if your problem is there.

    I wouldn't remove the flywheel unless you need to replace something attached to it. You can pull out the piston with it still attached to inspect it.

    If the valves look like they are working correctly I would check the cam wheel next. Remove the starter cup and take the cover off.

    To brake the engine when you remove the starter cup stuff as much rope as you can in the cylinder through the spark plug hole when the piston is at the bottom.

    I wouldn't worry about oil seals at this point either unless it has a leak.

    You'll need some liquid gasket maker to put it back together. You also need a feeler gauge to adjust the valves that is thinner than anything in the sets you can buy at an auto parts store. Get this from a dealer, it'll come with a new valve cover gasket if you need it as well. Cost is 2 bucks and if you intend to own 4 mix equipment you'll need one for proper maintenance.

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