Carb sync varies over throttle range

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.

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.

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😈

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

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)?