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The Anatomy of a Carb Sync



The Anatomy of a Carb Sync



How do you know which one to adjust?




The carburetor sync on a 2 stroke or a 4 stroke is one of the most important functions to keep up with for the health of your engine. Let’s take a look at performing a carb sync on a 912 series engine. The carb sync is nothing to be afraid of and with a few times at bat performing this function it will become fairly easy. First, why is it so important? The carb sync should be done anytime the carbs or throttle cables are removed or adjusted and at the 100 hour or Annual Condition Inspections. The reason for this is cables stretch, the pulley system wears, cables slip and parts wear and have more tolerances. The carbs are almost always out of sync at each 100 hours or the Annual. If you did a carb sync back at the last inspection then they may not be out of sync much, but they will be out and then your sync job should be easier. The sync instrument should also be used to set the idle sync if you change idle settings. Let’s start off with thinking of the engine as two engines, a left side and a right side. Two carbs controlling different sides of the engine. You don’t want one side trying to operate at 5000 rpm while the other side is trying to operate at 5100 rpm. These opposing rpms will cause excessive stress and wear on your engine over time and possible damage. You say there is a balance tube in between to help balance them out.The operative word in that sentence is “help”. The balance tube can correct and help with small differences between the two carbs, but it is not a cure all and it is there to help make the system run a little smoother than if there was no connection or correlation between the two carbs.


So which sync instrument to use? Well that is up to you, but here are a few considerations. You might use an electronic sync instrument like a CarbMate or a Syncromate or a set of gauges. Here are a few pros and cons of each sync instrument. The electronic instrument may have the capability to split hairs and give you a very fine adjustment, but they are harder to interpret as far as knowing which carb you want to adjust to achieve a specific goal to bring the two carb vacuums together. It takes more time and going back and forth to get this accuracy. You also need a power supply like your battery to attach electrical leads to operate the instrument. There is nothing wrong with this, it’s just different. The gauges (liquid filled are better for dampening and needle valves in line to assist for dampening needle pulsation) allow the user to see immediately which carb he needs to adjust and how much he may need to make this adjustment. This writers’ one thought here is; does the accuracy of an electronic device to split hairs that fine over a gauge really make a difference and can the carbs and engine really tell a difference? If you pay attention to detail and use good gauges you can be very accurate. The drawback to gauges may be not as an accurate setting as the electronic device. Picking one of these sync instruments is strictly up to the end user and their personal preference.


Let’s move on to the actual anatomy of the sync and what to look for. I would like this discussion to be on the use of the gauges because it will offer some visual numbers to work with. First the engine should be up to operating temperature. Safety first so put in place; wheel chocks, hearing protection, eye protection and a person at the controls for safety. Now you need to separate both carbs. You can use hose pinch pliers to clamp off the rubber hose between the carbs or just remove one side rubber hose off the air intake 90 degree nipple and plug you gauge into the rubber hose end and the other over the metal nipple. There are two small screws on top of each air intake you can screw your sync instruments into also, but you still need to address isolating the carbs. This writer prefers to slide the rubber hose back off one carb since it makes sure the carbs are fully isolated and no leak from the hose pinch pliers could occur. This is only what I prefer; it’s up to you to choose your method.


There are two syncs to perform, the mechanical sync and the pneumatic sync. The mechanical sync is shown in the Rotax Owners video (http://www.rotax-own...-exp-si-912-018) and described in the Rotax Line Maintenance manual and it’s quick and easy to perform. So now you’re all set in your safety gear so have your safety cockpit operator start the engine. (Don’t forget to advise them that if they see you spin more than three times in the prop to turn the engine off and make sure your cockpit manager likes you and don’t use your wife right after an argument. ohmy.gif)


Now we have the engine running and we take a look at our gauge set. If the needles are pulsating some then close the needle valves slightly until they stop and become smooth. The Rotax manual uses 2500 rpm for a sync reference for the higher rpm setting, but I will tell you from years of experience that if you do that they will be out of sync when you advance the throttle on up to 3500-4000 rpm. Just like many instruments or devices we use in life you are usually advised they are not accurate in the lower percentages or the extreme high percentages of their operating range. That puts 2500 rpm too low on the scale for fuel and air flow (needle position in the jet) to be accurate and we don’t fly near the idle side of that rpm, so why would you sync your carbs for the higher rpms at such a low rpm. Let’s mention here to that to adjust the higher rpms you adjust the Bowden cable screw either in or out which will add or subtract some rpm. You use the idle adjustment stop screw to affect the engine idle only. You do sometimes need to adjust the Bowden cable length to get the idle screw to have enough affect, but we can cross that bridge later.


Okay back to our running engine. Have your cockpit operator advance the throttle up 3500 rpm. All joking aside give that prop a wide berth. (My unlucky partner “Lefty”has a hard time holding two wrenches) So with the engine running at 3500 rpm we look at the gauges and see that the left side is at 5” of vacuum (more fuel)and the right side is at 6” of vacuum (less fuel). (Vacuum is expressed in inches of water “H2O or inches of mercury “Hg) The higher the vacuum in our case (6”) the harder the carb is trying to draw in air and fuel, leaner , less fuel. The lower the vacuum (5”) the more fuel it is receiving (richer). Keep this in your head about vacuum; the higher number is less fuel (leaner)and the lower the vacuum number, more fuel (richer). Now let’s go to the left side and loosen the Bowden adjustment nuts and screw it back out toward the cable and shorten the cable which pulls the throttle arm and reduces the rpm and fuel flow. Adjust it back until its 5” moves to 6” like the 6” on the right side.Now they should both be equal at 6” of vacuum at 3500+ rpm. If you went to adjust this left side and the adjustment was already way back and you didn't have enough adjustment there to pull it back any farther then you have two choices. Go to the other side and adjust that Bowden cable adjuster forward to lengthen it and lower the vacuum towards the left side. The other thing you may need to do is shut down the engine,screw the Bowden cable adjustment in towards the half way position and then loosen the cable at the throttle arm screw and shorten it by 1/16” to give you more room to adjust the Bowden cable adjuster farther back on that left side. Sometimes because of how these are setup you may need to adjust one side back a tad and adjust the other side forward a tad to make them equal and not run out of adjustment on either side.


Now pull the throttle back to idle and see where it is. If you have a 912ULS a good idle is around 1750-1850 rpm because of the vibration and hammering from the higher compression of this engine. Now if your idle is too high after you pulled the throttle back then look at the gauge and see which gauge has the lower vacuum number. Remember the lower the number the more fuel it is receiving. Let’s say the idle rpm is 1900 rpm and you want 1800 rpm. The right carb gauge is at 12” and the left carb is at 11”. The carb on the left side is getting more fuel and the rpm is too high. So that is the carb we want to reduce the rpm on and raise the vacuum to get to 12” like the right side. So you back out the idle stop screw and the 11” of vacuum raises to 12” of vacuum like the right side. If that made your idle rpm 1800 and you are happy then you’re done. If your idle rpms were still too high then back the idle stop screws out on both sides a little more until the idle rpm is where you want it and the vacuum on both sides is equal. Always double check your work. Run the engine back up to 3500+ rpm and see if the needles are still equal and if not then tweak the Bowden cable adjuster on the side you want to affect. Then back to idle to check that vacuum setting and the idle rpm. If you idle for a long time making an adjustment then run the engine up for a few seconds now and then to help keep it cleared out and from loading up at those low rpms. If your idle rpm was too low (1600 rpm) then screw the idle stop screw in more on the carb with the higher vacuum 12” down to 11” until the vacuum number lowers to match the other side of 11”and the idle comes up where you want it.


After you have doubled checked your work then shut down the engine and make sure all the nuts to the Bowden cable adjuster are snug. Remove the gauge set and connect the carb balance tube setup. Even after a sync the engine may be slightly rougher with the carbs separated, but should be a little smoother when it is reconnected.


Two last parting comments. The throttle in your cockpit at idle should have a stop on it and when you pull it back to its stop at idle then the idle stop screw on the carb should make contact at the same time. If you do not have a throttle stop for idle then you will most likely bend the idle stop lever on the carb.You will over power it and if you do or have the idle set too low then you stand a high much bigger chance of stalling your engine from low rpm and it won’t be when you want it to quit.


Second; You should balance the carbs at the high rpm and at idle. I have seen some back off the idle stop screw until it no longer functions and that means the carbs can only be synced at the higher rpms and not at idle. That means the engine is operating at idle at opposing rpms. If you thought it was important to sync your carbs at the higher rpms to keep them from opposing each other, reduce vibration and from hammering the engine why on earth would anyone not sync them at idle? This is a poor practice to get into. You spend a lot of time idling. Remember what our Dad’s told us; “If it’s worth doing it’s worth doing right”.


I know this was along article, but I thought it may be worth covering for some the Rotax owners.If you fell asleep half way through, print it out and take it to the airfield.








Your Rotax engine will give many hours of trouble free operation. Just follow the Rotax manuals and provide it with the prescribed on time maintenance, but not necessarily your neighbor’s advice.


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