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Hi Rotax-Owner.

I am attempting to synchronize the carbs on my RV-12 again (912 ULS). I decided the CarbMate is not that great and I picked up a set of mechanical dial gauges. I like them a lot more. However, I'm experiencing a rather frustrating thing that I also experienced with my CarbMate, which is that over the range of the throttle, the synchronization changes. If I set it up so that I'm synced at 2500 RPM, it's off when I run it up to cruise RPM. If I set it up so that I'm synced at cruise, it's off at 2500. I can feel it, and so I believe the tools, and as I said, both the CarbMate and the mechanical dial gauges agree that this is happening.

The carbs were recently overhauled (using 889 534 overhaul kit), as well as new choke innards, new carb pistons, the works. Everything including the kitchen sink was replaced in them. In hindsight, I would have just bought new carbs to begin with, but it is what it is.

After finding this potentially dubious behavior to begin with, I took the carbs off twice now and reinspected them to make sure everything is assembled correctly. I have found no issues, and I've been very careful.

The air filters were recently cleaned and re-oiled. The carb sockets are brand new and tightened properly down to the spacers, and they are snug. The carbs are fully seated in the sockets.

One thing that seems a bit awry is that when I perform the carb sync procedure in accordance with the RV-12 MM, after doing the mechanical synchronization and starting the engine, the full idle position is about 2500 rpm cold and goes up to about 2600 once warm. This is waaaay high obviously. The RV-12 MM says not to back the idle screws off more than 1/2 turn to achieve on-idle synchronization. It's a bit ambiguous in the wording whether it is acceptable to go past 1/2 turn to set the idle speed after the idle is synchronized, but it needs a full turn to be close to 1800 ish, and a little further to get it where I want my idle stop to be, around 1650. Not sure if this is fairly normal on RV-12, but I would not expect it to be so far off to start with.

The idle stops do not appear to be bent / damaged.

Any ideas as to what might be going on here? The RV-12 throttle setup uses a different spring than the default Rotax throttle return spring, and it springs to full open and is pulled to idle by the throttle cable. The RV-12 throttle arm setup is a bit different than the default 912 setup.

I expect that perhaps the first thing anyone reading this will suggest is the carbs themselves. I've been pretty thorough with those, I do not think the issue is there. Could be, but I don't think so.

Another possibility is that this behavior is completely within "normal" and I'm being too picky? It's not grossly off. But it is noticeable.

Unfortunately, I do not know whether this was occurring before the carbs were worked on.

Your thoughts are appreciated.


LSRM-A
Rotax iRMT Heavy Maintenance
RV-12 SLSA Owner

  • Re: Carb sync varies over throttle range

    by » 5 weeks ago


    I suspect that if you correct that high idle it will solve your issue or at least unmask the cause of your wandering synchronization. The bottom line is will the fully warmed engine smoothly decelerate from WOT to a set idle speed (at or above 1400 rpm) without stopping. I use up to a full turn on either or both carbs for the mechanical setting if needed to assure the desired idle speed and successful mechanical synchronization. Reference para 12-20-00 pg 32 in the Rotax 912 MML (mechanical synchronization process) which states: “If a setting of more than 1/2 turn is required, repeat mechanical synchronization to prevent too high a load on the idle stops. If the idle speed is too high, the maximum the idle screw can be unscrewed is complete turn.”  But my airplane was certified E-AB. From your post, I infer this aircraft you reference may be a different certification and guidance from Van’s may be required.


  • Re: Carb sync varies over throttle range

    by » 5 weeks ago


    Jim Isaacs wrote:

    I suspect that if you correct that high idle it will solve your issue or at least unmask the cause of your wandering synchronization. The bottom line is will the fully warmed engine smoothly decelerate from WOT to a set idle speed (at or above 1400 rpm) without stopping. I use up to a full turn on either or both carbs for the mechanical setting if needed to assure the desired idle speed and successful mechanical synchronization. Reference para 12-20-00 pg 32 in the Rotax 912 MML (mechanical synchronization process) which states: “If a setting of more than 1/2 turn is required, repeat mechanical synchronization to prevent too high a load on the idle stops. If the idle speed is too high, the maximum the idle screw can be unscrewed is complete turn.”  But my airplane was certified E-AB. From your post, I infer this aircraft you reference may be a different certification and guidance from Van’s may be required.

    The airplane is an SLSA, so definitely tighter rules on what can be done to it than an E-AB.

    The engine runs okay in the full range from idle to cruise RPM, but it is only smooth in whatever one spot the synchronization converges. Beyond that in either direction, it's off. I can control where the convergence occurs, but I currently cannot get them to stay in sync throughout the range.

    My gut is telling me something is wrong.


    LSRM-A
    Rotax iRMT Heavy Maintenance
    RV-12 SLSA Owner


  • Re: Carb sync varies over throttle range

    by » 5 weeks ago


    Hi Eriselle,

    Do your mechanical balance first. Take your time and try to get the feeler gauge measurements the same.

    The mechanical balance should get your idle speed & carb balance close. Then move on to the penuematic.

    While doing the pneumatic balance, adjust the carb that is furthest from the optimum idle speed (try not to adjust both carbs).

    Once you have both carbs balanced at fast idle (2000-2500 rpm), you can then carefully adjust the idle speed up/down - I usually do this at the cockpit throttle, so that both cables are being adjusted at the same time. You may have to adjust the idle screws to accommodate significant changes. Note; at your preferred idle speed, the idle screws should NOT be touching the stops. Make sure the stop to idle screw clearances are the same both sides (feeler gauge).

    Once you have idle speed adjusted, check for balance up through the rpm range to high speed cruise (5500 rpm or Max static rpm). SLOWLY move the throttle and watch the carb balance - out of balance indication, is likly to be a sticking throttle cable. You can try cleaning the inner cable (most cables should not be lubricated) however in most instances the cure is a new cable.

    Note: Winter engine idle speed will usually be slightly faster than summer, due to ambient temps. In our summer, it's not uncommon for engines to stop as the aircraft flares for landing - solution is a small adjustment at the throttle to raise idle speed.

    Good luck😈

     


  • Re: Carb sync varies over throttle range

    by » 5 weeks ago


    I had this problem too. I solved it by fixing the control cable housings on the throttle side.
    I also fixed the carburetor control arm at exactly the same angle.
    I also installed an idle stop on the throttle in the cockpit.

    Maybe this can help you

    Jacques


  • Re: Carb sync varies over throttle range

    by » 5 weeks ago


    Yes control cables are very important to synchronous operation across the entire rpm range, are they the exact same length? Also during the mechanical synch did you check to see both carbs come off the idle stops at the same time and at the same distance from the idle stop (gap check)?


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