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Carb sync in simple terms

Safety Officer





This will get you through the basics and after you do a sync or two it will get easier and you will get better. This is not an official document and is only meant for your edification and will get you through a carb sync. Some experienced people that do carb syncs on a 912’s may vary some, but the basics ideas are the same.



Mechanical Synch of the Carburetors – Do this First


Bowden cables adjustment screws (cable housing adjuster) should be centered, but not mandatory. By having the Bowden cable adjustments in the middle of their adjustment range will give you a little more adjustment when you get to that part of the procedure.


Adjust throttle cables so that both carb throttle arms move simultaneously to both extremes (open and closed).


Ensure Idle Mixture Control Screw is properly set (on the bottom of each carb). Screw them all the way in then back out 1½ turns.


Reverse the throttle arm springs so they pull toward “closed” throttle just to facilitate mechanical sync.


Unscrew the Idle Stop Screw. Make sure the throttle is closed in the cockpit. With a .004” feeler gauge between the screw and the arm tighten the screw until it just touches. There should be minimal friction on the feeler gauge.


Remove the feeler gauge and screw the Idle Stop Screw in 1 turn.


Return springs to original configuration (i.e., pulling the throttle arms toward to the “open position”).


Connect the Pneumatic Synch Tool


It is imperative to have the engine up to the operating temperature. Remove one end of the rubber crossover compensating tube between the two intake manifolds or connect to the top of the intake manifolds or use the cross over tube rubber hose pinch off method shown in Rotax owner videos. All these attachment methods will work.


Hook up gauges or electronic sync tool. Secure one end of a gauge or electronic device to the rubber end of the compensating tube and the other to the gauge at the 90 degree fitting coming out of the intake manifold where the compensating tube hose was removed. Secure both attachments with a hose clamps to prevent leaks. Leaks will affect the results of your sync process. You can hook up your sync tool at the small screw on top of the intake manifold, but then you have to pinch off the rubber hose between the carbs on the cross over compensating line. I find this is just more work and you need to make sure the rubber hose on the compensating tubes are long enough (usually newer engines) and you don’t allow any leaks between the carbs. Many people have to cut the tube shorter and make the rubber hose longer to pinch it if you have an older engine with short rubber connecting tubes. I think it is easier and faster by just pulling the hose off as originally described and it facilitates complete carb separation.

Make sure you have good brakes, use wheel chocks and or if you have not so good brakes or no wheel chocks then tie your plane down and always have someone in the cockpit. Never do this alone while running an engine and no one in the cockpit..


“On Idle” Synch

NOTE* Many people like to do the high rpm or off idle adjustment first and then do the idle set.

I would do the high rpm sync first because it will affect the idle sync.



The gauge or carb with the lower vacuum number is getting more air/fuel and the gauge or carb with the higher vacuum number is getting less air/fuel.


Bring the cockpit throttle lever to idle stop. Ideal idle RPM is approximately 1700-1800 rpm depending on your specific aircraft and needs. Adjust idle setting to this value. The 912UL 80 HP can handle a little lower idle rpm over the 912ULS 100 HP due to compression ratio.


If the idle is high (say, 1900), adjust the carb with the lower vacuum number on the gauge because that is the one getting more air/fuel. Retard the Idle Set Screw until the vacuum numbers are the same. Example: If one gauge is at 15” of vacuum and one at 14” of vacuum retard the lower 14” until it drops to 15” to equal the other carb which will now lower your idle rpm.


If idle is low (say, 1600), adjust the carb with the higher value, it is getting less air/fuel. Advance the Idle Set Screw until the vacuum readings are the same. Example: If one carb is at 15” and one at 14” turn the idle stop screw in on the carb with 15” of vacuum to match the one with 14”. This will raise the overall idle rpm.


Needles should now match.



“Off Idle” or high rpm Synch


If while running the engine the gauge needles shake then you will need to install or close down the inline needle valves until the gauge needles stop shaking and pulsating. Now the needles will likely show different vacuum readings. Do this sync before the idle sync because this sync usually will affect your idle sync set point.


Run engine at approximately 3500 RPM. Do the “Off Idle” synchronizations at this rpm. I prefer 3500 rpm to start since that is closer to where you run the engine. You may find that setting the balance at lower rpms doesn’t stay very accurate and that as you move up in rpm to 4000-4500 rpm they are out of sync again. Set them at the higher rpm or at least double check the higher rpm when you think you are done. There is a good chance they won’t be in sync if you used a low (<2500) rpm to set them.


These adjustments will be made only at the brass Bowden cable adjustment for the higher rpm sync, where the cable housing meets the carb adjuster on top of the carb, do not touch the idle set screws.



The gauge or carb with the lower reading is getting more air/fuel and the gauge or carb with the higher reading is getting less air/fuel.


Compare vacuum gauge readings. Normally, but not always, adjust the carb (retard) with the LOWER vacuum reading (getting more fuel) back down towards the carb with the higher vacuum reading. Adjust the Bowden cable screw outward (toward reducing the air/fuel) so the throttle arm backs off (decreased RPM) to bring vacuum reading to a higher vacuum number.



When vacuum readings are equal, lock it down.

If vacuum readings are the same, bring throttle lever back to idle stop then advance it again to 3500 rpm. Verify vacuum readings are still the same for both carbs. If readings are different adjust the carb’s Bowden cable with the lowest gauge reading to match the higher gauge. Once the gauges are aligned move the throttle back to idle and back to 3500 rpm a time or two to make sure the carbs stay equal. Throttle to idle. Re-adjust the Idle Stop Screw if necessary at this point. You are now done.

Remove the gauges and hook the carb compensating tube back up. The engine will run slightly smoother after the compensating tube is reconnected and sometimes the idle will be slightly higher after the cross over balance tube is reconnected. If you know this then while doing the sync set the idle 50 rpm lower so when the balance tube is reconnected you be right on your personal target.


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