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vacuum leak likely locations on sthihl 4mix?

Discussion in 'Ask the Experts' started by r7000, Jul 28, 2022.

  1. r7000

    r7000 Active OPE Member

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    I have a KM90r weed trimmer that is breaking my balls.
    I assume the 4mix engine in it is the same as many others... when I bought a replacement carb off amazon it says:

    Hipa FS90R Carburetor C1Q-S174 for STIHL FS90 FS110 HT101 FS130 FS100 FS110R KM110R KM130 HL100 HT100 FS100RX FS130R FS87 FS100R KM90 KM90R KM100 KM110 KM130R FC90 FC95 FC100 FC110 HL90 Trimmer

    so i assume the intake and most of the engine is also the same, the only thing changing is bore diameter for the bigger stuff.


    My KM90R started with being able to start, but wouldn't stay running for more than 10 seconds at idle, and would have no power under throttle, and would die out after greater than half throttle. If you feather throttled you could get the rpms up past ~3000 into the full rpm range.

    trimmer is personal use, not abused, got 10 years or whatever on it. has very good compression. The exhaust screen was clean.
    • I put in a new cam wheel and resealed the cam cover, neither of which looked to be the problem.
    • started up seemed better at first but then quickly went back to the same problem and was worse.
    • I then bought the amazon carb, did not help. Nor did a new NGK spark plug.
    • got the valve cover + washer + feeler gauge kit. Pretty sure at this point my original valve cover gasket was a problem. I am sure now the valve cover is not the source of the vacuum leak. Valve lash is also set, and I don't believe was ever a problem.
    • It now won't start on first few pulls, but it will start, then will go to fast high idle and stay like that fairly good, such that the weed trimmer head is spinning fast enough and you could use the trimmer just don't give it any throttle it won't accelerate and will die.
    being a "four stroke" it's running up in rpm just like a vacuum leak would on a v-8 for example. The large hose at the bottom center of carb looks in good shape but I can't see where it connects to the crankcase.... where else would be a likely source of a vacuum leak that I am missing?

    for those of you knowledgeable on these units what are the common problem items... in regards to vacuum leaks? don't tell me crank seal :(


     
  2. huskihl

    huskihl Muh fingers look really big

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    I would think any air leak that would matter in a four cycle engine would be in the intake between the carburetor and head. But I am not familiar with how the fuel gets into the bottom end of the 4 mix engines to lubricate them. @lehman live edge slab might know something about them
     
  3. Stump Shot

    Stump Shot Disciple of Monkey's GoldMember

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    I won't tell you "crank seal", however if one looks at the seal area and has oily junk all around, you can pretty much be able to tell yourself at that point.
    This is the four mix death rattle...
     
  4. Stump Shot

    Stump Shot Disciple of Monkey's GoldMember

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    In a normal four stroke engine, yes. These little goomers the charge goes through the case just like a two stroke for lubrication, therefor it can't suffer leaks like a two stroke, this as best I could ascertain from wrenching on Stihl back pack blowers and having to junk a few.
     
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  5. lehman live edge slab

    lehman live edge slab Live Action

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    Well I haven’t done much on them other than set the valves but when I was researching them after i ended up with some to just look over and set valves just because I wanted to know how they operate. They do rely on a sealed case because your still using piston motion on the case side for pressure and vacuum. They in a sense added a valve train to a 2 stroke saw engine because it runs on a similar principle. Piston goes up draws gas into case where oil lubes the bearings then on the down stroke it pushes the charge up a port sealed by a reed up to the valves where the next down stroke and intake valve opening allows the charge into cylinder. So I’d think any leaks at crank seals or that reed not sealing off the charge from case would affect running. I also wouldn’t 100% trust an aftermarket carb if that was one off Amazon and it’s still on the trimmer. I do have a friend that I’ll put a text into for some ideas and also check my 4-mix service manuals after work today.
     
  6. r7000

    r7000 Active OPE Member

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    my problem was my new carb was out of adjustment, in addition to me working in the dark late evening in haste. Next day when I focused on the trimmer I got it running well with the new amazon cheapo carb adjusted.

    Then for kicks I put the original carb back on, what is supposed to be 3/4 turn open on both the L and H circuits. The L is open about 3/4 turn and idle is great. The H i had to open 2.5 turns to get it to run acceptable, and gonna get as much use out of the original carb now as I can I guess.
     
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