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Lockwood vacum carb. Sync tool

in 912 / 914 Technical Questions 10 years ago
Can anyone explain why there are two
Different orifice sizes on the vacum set
I purchased from lockwood ?

Thanks Bob

  • Re: Lockwood vacum carb. Sync tool

    by » 10 years ago

    Hi Bob,

    The orifice in a line in a vacuum gauge set is usually there to dampen the pulses from the engine pulsation so the needle doesn't bounce around. The actual size wouldn't make a difference.


    Seems like they should be the same. Lockwood may be the only one to answer this question. They may have run out of one size and used another? Have you ask them?

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Guilherme Tomazini
      
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  • Re: Lockwood vacum carb. Sync tool

    by » 10 years ago

    Thanks Roger! No I haven't gotten back to lockwood. I can't sync the carbs after three hours . I suspect the throttle cables I ordered two new ones . When I put my gauges away I noticed that the orifices are totally different . I am pulling
    20 inches on 2&4 and 12 inches on 1&3.
    It's driving me crazy. Appreciate any help.
    Low vacum side is dumping some fuel overboard via the vent line . I have the old style carbs spring set to idle .

    Bob
      
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  • Re: Lockwood vacum carb. Sync tool

    by » 10 years ago

    Hi Bob,

    Is this 8" vacuum spread only at idle or also at the higher rpm set?

    If it is the cables with an 8" spread you should notice the throttle arms are not opening at the same rate and the same amount. See if they are advancing equally. This could be a bad cable setup or just one cable hanging up. If you have a throttle yoke where the main cockpit cable goes in a yoke and then two cables exit look there and look for any sharp bends in the cable with the 20" vacuum carb. If they are opening equally and you still have an 8" spread then I believe you have a jet obstruction or hose feed obstruction in the one with 20". That's a lot of vacuum. This is the carb and cable I would focus on. The other vacuum seems to be more normal.


    If you put your hand on the high vacuum carb throttle arm can you gently pull out any slack cable or is the cable taught at full extension?


    p.s.
    For those who aren't sure what the gauge numbers mean here is something to remember. The gauge with the higher vacuum is receiving less fuel than one with a lower vacuum number. So a 15" vacuum gauge is receiving less fuel than one with 12" vacuum. Large discrepancies in vacuum may mean a couple of things: (common issues to check)

    You didn't do a good mechanical sync.
    The cable lengths are way off at the Bowden cable adjuster.
    The cables are not advancing equally.
    There is a fuel obstruction in the carb at a jet (more common with the smaller orifice idle jet) or an obstruction in the fuel flow at a fitting.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Lockwood vacum carb. Sync tool

    by » 10 years ago

    Hi Roger , again thank you. It appears throttle arms are opening equally but one
    Throttle cable has slop in it . I realize it's
    Pulling a lot of vacum , lockwood recommends 11" at 2500 rpm and. 5 inches at idle. Since I last wrote I was able to get them down to 15 and 17 inches at 2500 rpm adjusting the Bowden fittings . I have been making large adjustments with the throttle arm.
    I am not satisfied with the throttle cable
    Condition themselves . One is sloppy with a mild kink in view between throttle arm and Bowden adjustment. . The two new cables arrived so I might go that route. Roger I have hours into a simple
    mechanical adjustment. When I start from ground zero, back off idle stops center up threads on Bowden adjustment
    Vacum goes high. It's runs well at 15. /17
    but I need them closer . As a side note I
    am doing this alone. Aircraft is rans S7S.

    Thanks, Bob
      
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  • Re: Lockwood vacum carb. Sync tool

    by » 10 years ago

    Hi Bob,

    The actual vacuum numbers mean nothing. There are factors that influence the vacuum readings. Things like field elevation, rpms during the sync, OAT's (cold versus hot), single or dual air filter, dirty or clean air filter., ect... Some gauges are off at least 1" of vacuum between the other. If you use gauges hook them both up to a vacuum source at the same time together and see if they both read the same.
    Lockwood are the only ones I know that doesn't sync at idle. They back the idle stop screws completely back out of play and only adjust the bowden cable and use 2500 rpm. You can't get idle and the higher rpms to both come to sync that way. Something has to be off.
    No one can give you an exact vacuum number at an exact rpm. I would sync them at idle and at 3400-3500 rpm. 2500 is just too low and too close into the idle circuit and is a poor representation of the higher rpm that you may use in cruise. Try this test. Sync the carbs at 2500 once then advance the throttle up to 3500-4000 rpm. My money says the gauge needles split. Not all throttle setups are the same or even good and some do have variances whether some want them or not.
    I would bet I do at least 60+ carb syncs a year. Once you understand what affects the gauges and what each is telling you and you keep up on your own engine the carb syncs are a piece of cake and usually only take 5-15 minutes. Once in a great while I get a PITA sync that takes 30-40 minutes, but that is usually the throttle system that needs help not the carbs. If you have a poor in sync throttle setup then you may only get close because the throttle setup never goes back to the same place twice in a row. Then you either fix the system or live with the slight inaccuracy.
    Depending on rpm the idle vacuum is usually between 10"-15" and the 3400-3500 rpm vacuum is around 5". These are just close estimates and your actual may be the same or different. You don't care, but you do care if they are equal and you should be able to get them right on.


    If you want you are welcome to give me a call.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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