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011 carb rebuild fail

Discussion in 'Technical Area' started by KSWR, Feb 9, 2020.

  1. KSWR

    KSWR Active OPE Member

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    WT 29,Cleaned all ports twice with spray carb cleaner, installed rebuild kit, set float lever with W gauge, re-installed, idles ok, set H and L screws to 1-1/4 turns out, H screw adjustment does not make any difference, r&r'd again, cleaned H passage again, re-installed no difference, anyone have another carb WT29,WT563,C1S-S1,1120-120-0601,1120-120-0605 ? I did swap carb from a good saw, saw runs ok then, did not see any for a reasonable price on ebay, or should I watch for a parts saw ?
     
  2. hotshot

    hotshot Well-Known OPE Member

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    If the high needle has no impact on running, then check your welch plug for leaks. Try to spin or move it around in its seat with a sharp scribe.

    If the nozzle check valve assembly failed your pressure/vacuum test, then you can get a replacement Walbro 86-578-1 for $3.

    You never mentioned the 8# hold pressure test either, it should remain there all day.

    That carb is too basic to be a fail.
     
  3. AVB

    AVB Well-Known OPE Member

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    Or if it is even there. I have Zama that they simply fall out during the ultrasonic cleanings.
     
  4. KSWR

    KSWR Active OPE Member

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    Thanks for the quick reply,Where is the nozzle check valve?
     
  5. AVB

    AVB Well-Known OPE Member

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    It is part of the main nozzle that supplies fuel into the carburetor throat. To it is not as simple to test as many thinks. And I don't have a WT-29 to physically look at currently. I haven't seen one for a couple years.

    The pressure/vac test mention is done to test the metering needle and fuel pump side of the carburetor by pressurizing to 8 psi and looking leakage. The only to test the nozzle is access it tip in the carburetor throat and apply a very small amount of pressure and see if it leaks backwards into the carburetor. The reason you can't reliable test the metering side of the carburetor is that the mixture screw(s) are open to the carburetor throat without check valves. Now some carburetors do a have separate check valve for mixture screws and it is check by light vacuum applied to it.

    Of all things don't wire probe or use high pressure air as on this carburetor or you will damage the check valve(s).

    In the WT service manual below on page 3 has the main nozzle circled.
    https://www.walbro.com/wp-content/uploads/2016/01/WTseries.pdf

    Personally I don't recommend using carburetor cleaner on these cubes. It is best to clean them using ultrasonics as it gets deep into the small internal passages that can be plugged which sounds like the case here.
     
  6. KSWR

    KSWR Active OPE Member

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    So is the main nozzle under the large welch plug or the small one?
     
  7. KSWR

    KSWR Active OPE Member

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    welch plug has been replaced and is not loose, still not to clear on how to do the press vac test and how it affects the hi side circuit.
     
  8. hotshot

    hotshot Well-Known OPE Member

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    Screw/seat the low speed adjuster in all the way, then use a short piece tubing & your mouth to blue/suck air through the high side metering inlet hole in the metering inlet chamber. The high speed screw should be backed off a couple of turns.

    The nozzle should pass air into the venturi, but NOT allow any air flow backwards.

    An alternate method is to pull the high speed screw out & block that threaded hole with your thumb.

    That carb should be an independent feed circuit to the low/high sides. If it’s not, then you’ll need to block it’s low side metering inlet hole too ...

    Stihl has a free carb service manual that is an excellent read, explaining how to test.

    Not sure about your “vac” term above, but NEVER pull a vacuum on the FUEL inlet fitting as you can ruin the soft metering inlet needle tip.
     
    Last edited: Feb 15, 2020
  9. KSWR

    KSWR Active OPE Member

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    Thanks I'll try that next time I'm out in the shop.
     
  10. KSWR

    KSWR Active OPE Member

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    It seems like it does allow air flow both ways, hopefully they're not obsolete, I'll try to find one, thanks again
     
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