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Thread: CarbMate P-sync tool description

  1. #1
    Birdseyeview's Avatar
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    Default CarbMate P-sync tool description

    Does anyone have a good description of how the CarbMate tool works on a 912? I got nothing that describes the functioning of the tool when I purchased it from ACS and can't find any descriptions on line. I've watched a few P-sync videos on Youtube and Rotax's video, none of which describe the CarbMate's operation and why the various lights come on or what to specifically do about them when they aren't the center green LED. A few turns of the bowden cable clamps in one direction or the other don't seem to move the LED indications in the right direction (so far). If I knew what the LED numbers meant I could increase one carb 's throttle or decrease the other carb's throttle accordingly to get me to the center green LED. I had expected that the Mech sync should have gotten me a little closer to the optimal P-sync with only a few bowden cable turns needed. Frankly I'm confused and a little frustrated because the P-sync is taking forever to accomplish and I need some expert clarification.

    Before I attempted the P-sync I had throughly cleaned the carbs again, checked the float levels, completed a mech sync per Rotax instructions, and performed a differential compression test before the P-sync attempt and all cylinders came in at 79/80 psi. I'm burning 100LL and the engine starts good and idles well at 1800-2000 rpm, in spite of some mild carb vibrations, which I hope can be reduced or eliminated with a good P-sync. No carb venting fuel at any speed nor with the CarbMate hooked up.

    When I hook up the CarbMate to my 912 the "REF" side is plumbed to the right manifold (via the cross-over tube) and the "CARB2" side is plumbed to the left manifold fitting. At an engine speed of 2500 rpm the CarbMate readout lights up on the "REF" side as a red -4 LED. When I increase idle speed to 3500 rpm the LEDs starts to move towards the center green light indicating that the pressure/vacuum difference between the two carbs does actually decrease as the engine speed goes up.

    When I disconnect the "REF" hose I can blow into it in order to get the same -4 red LED light seen during the test. When I suck into the "CARB2" hose it goes to the other side of the LED line where the plus numbered LEDs are at. Does red -4 LED mean that the right side (REF) manifold of my engine is positively pressurized or does it just mean that my left side (CARB2) has way less vacuum? I'm guessing that each manifold is actually seeing only a vacuum of some varying level. I'm also guessing that the positive pressure I'm seeing on the "REF" side means that the right hand carb's throttle needs to be increased to increase its vacuum level (and thus lowering the false positive indication. I assume vacuum level in a given manifold goes up as the throttle is increased. Are all my guesses and assumptions good ones or am I still confused? Any opinions on where should I go from here? I really appreciate any inputs.

    IMG_8462.jpg
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  2. #2
    Senior Member jiott's Avatar
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    Default Re: CarbMate P-sync tool description

    Larry, I don't have a Carbmate, but when I use the standard gauge setup for p sync I just push or pull a little bit on one throttle arm and watch which way the gauges move. This tells me what direction of adjustment needs to be made on THAT cable to bring the gauges into sync. I think you could do the same with a Carbmate and watch the LED lights.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  3. #3
    Birdseyeview's Avatar
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    Default Re: CarbMate P-sync tool description

    Jim,

    I wish it was that easy but on my installation the two throttle cables are each .090" diameter and tied together tightly where the two separate cables join the push pull rod (a McFarland friction lock throttle assembly). As a result, a little push or pull on one cable is transmitted completely to the other cable and there is no independent movement, not even a little. I'm completely forced to use the individual bowden cable clamps to adjust only one or the other carb at a time. That's why I'd like to understand better what the CarbMate is telling me so I can move the bowden cable in the correct direction. I hope there is another option that I may have overlooked.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  4. #4
    Senior Member rv9ralph's Avatar
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    Default Re: CarbMate P-sync tool description

    I used a CarbMate to sync my carbs 3-5 years ago, I will try to remember the process. As you stated, first do the mechanical sync. Then connect the CarbMate using the crossover tube as the connection point with one CarbMate to the crossover and the other to the connection that would normally be connected to the crossover.

    The led lights on the CarbMate show that the vacuum is more or less to one side or the other. The "Ref" is the carb that you decide not to adjust. The other carb is adjusted by turning the adjustment at the end of the cable housing on the carb. By turning the adjustment at the end of the cable you will slightly change the vacuum until the carbs are in such. Start at around 2500 rpm with the sensitivity set to where the faceplate recommends. Then check at higher rpms until you get it the best you can.

    I did the sync alone and did not feel comfortable doing the adjustments with the engine running and shut down between adjustments.

    That is about it, good luck.

    Ralph

  5. #5
    Birdseyeview's Avatar
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    Default Re: CarbMate P-sync tool description

    Quote 1: "The "Ref" is the carb that you decide not to adjust.

    Quote 2: The other carb is adjusted by turning the adjustment at the end of the cable housing on the carb. By turning the adjustment at the end of the cable you will slightly change the vacuum until the carbs are in sync. "

    Ralph - Thanks - Your 1st quote above is very helpful. I couldn't find a definition of what the "REF" side was until reading your definition - it now makes perfect sense for that piece of my puzzle.

    Relative to your 2nd quote - do you have a suggestion which way I should adjust "CARB2"? Should I increase or decrease the throttle setting at that bowden cable? Or should I just experiment with both directions until I stumble into the right direction? I like knowing how deep the water is before I dive in. Example: If it should require me to increase the throttle setting of "CARB2" I can do that but I'm nervous that the P-sync may be far enough out that when I do get it balanced that both throttle levers may not hit their full throttle stops at the same time. Is that a problem at the full throttle setting or is it a logical consequence of a normal P-sync and therefore considered as OK as long as the carbs are sync'd? On the other hand, if I need to decrease the throttle settings on "CARB2" there may not be enough butterfly valve angle left to go closed before it stops rotating internally. I guess in that case I may need to start over with the mech sync or move to adjust the "REF" side carb to compensate? Or could I switch the "REF" and "CARB2" by reconnecting the hoses to the opposite sides of the crossover tube. If I did that I'd probably have to redo the mech sync first and start again at the beginning.

    I may be making a mountain out of a molehill but from my questions above you can see that this all gets complicated fast and I hope to document the definitions and rules for all this when I finally get it all figured out. Any further thoughts, recollections, or discussion would be helpful to clarify all this.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  6. #6
    Senior Member rv9ralph's Avatar
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    Default Re: CarbMate P-sync tool description

    OK, so think of it this way...

    The mechanical sync sets all of the adjustments on the carb, needle valve, throttle stops, etc. the same

    The CarbMate sync process sets the throttle cable so both carbs operate the same... in sync.

    Hope that helps.


    Ralph

  7. #7
    Birdseyeview's Avatar
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    Default Re: CarbMate P-sync tool description

    Ralph,

    Your last response tells me that the throttle stops aren't ultimately that important as long as the two carbs are in sync as much as possible over most of the engine speed range. That last suggestion on how to think about all this is very helpful so thanks for the clarification and pardon my anal line of questions. I'm new at this sync business and I sometimes tend to overthink these things when all I should do is focus on the right end result.

    Pardon one more question from me - If I can't get the carbs sync'd across the entire speed range is it more important to get them sync'd at the low end, say at idle, or the higher end of the engine speed range, say at cruise?
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  8. #8
    Senior Member rv9ralph's Avatar
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    Default Re: CarbMate P-sync tool description

    Pardon one more question from me - If I can't get the carbs sync'd across the entire speed range is it more important to get them sync'd at the low end, say at idle, or the higher end of the engine speed range, say at cruise?
    Yes!

    Slightly out if sync at low end will result in vibration that will make you think the engine might shake its self loose. Vibration at cruise will not be so violent but is not good for anything attached to the engine. (The whole plane.)

    Try to get the best of both.

    All the settings are important, as I said, the mechanical sets all the parameters on the carbs the same. For some that is enough. Using vacuum to further sync the carbs (with the CarbMate or vacuum gauges) sets the throttle cables to a point that is equal for both carbs. The throttle should then move the carbs in sync.

    Remember... on the 9xx series of Rotax engines, you have two 2 cylinder engines operating side by side, insync with each other. When out of sync, you get vibration.

    Ralph

  9. #9
    Senior Member jrevens's Avatar
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    Default Re: CarbMate P-sync tool description

    Hi Larry,

    https://www.youtube.com/watch?v=4L76WW7wHYM

    My suggestion is to check balance at idle and at 3000 rpm, period. I like to do the adjustment with the engine running, taking extreme care to remain clear of the prop of course. You adjust with the nuts shown below, to move the sheath back & forth as necessary.
    Attached Images Attached Images
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  10. #10
    Birdseyeview's Avatar
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    Default Re: CarbMate P-sync tool description

    John, I appreciate your suggestion about your rpm choices during the P-sync. From your response it sounds like you are also using a CarbMate. When you get a "REF" or "CARB2" reading that needs adjustment what logic do you apply to determine what direction the applicable throttle cable is to be moved?

    I've also been trying the P-sync while the engine is running and my Kitfox 6 has the short cowl (same as the series 5) which puts me even closer to the spinning prop. I get very nervous during that operation and have been trying to rush through it, which may be why I've had some trouble with the P-sync. I plan to try again soon, take my time and be more methodical about it.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

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