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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.


On The Fence About Coolant



Waterless or 50/50?


Picking a coolant for some can put you on that proverbial fence line and you’re just not sure which way to fall. You need your cooling fluid to prevent high pressure and air or vapor within the system. You need it to transfer heat away from the engine,but it also helps distribute temps more evenly too. Your choices for the most part are a waterless coolant or a mix like 50/50. The waterless coolant most often talked about is Evans NPG (NPG =non-aqueous propylene glycol) waterless coolant (Rotax recommended). Evans NPG is basically non toxic,non-corrosive and can operate at zero pressure. Its boiling point at zero pressure is 375F and the freeze point is -40F and was originally developed with the race car in mind.


Now the next choice is an ethylene or propylene glycol diluted mix. Most store bought brands are 50/50, but some people buy the full strength or non-diluted coolant and then add distilled water to make a mix of 50/50 or a 60/40 mix. The 50/50 mix is good up to approximately 270F with the Rotax 1.2 bar radiator cap. If you have the old .9 bar (13 psi) cap you should replace it with the newer 1.2 bar (18 psi) cap (SB-914-029 & SB-912-043). The freeze point is approximately -34F. Making the coolant with too much anti-freeze(80/20) or too little (20/80 gives you all the wrong characteristics that you want in an engine coolant and can be detrimental to your engine. Having a mix too heavy with anti-freeze can cause loss of cooling and the freeze point will actually start back up. Having too much water and not enough anti-freeze will allow the coolant to heat too high and cause vapor areas within the engine which will also aid in detonation and metal fatigue and not protect well against freezing outside air temps.


So now you think Evans waterless coolant is the best thing since sliced bread. Don’t jump on that horse and race to buy Evans yet. Let’s take a look at the Rotax 912ULS (100 HP) engine as an example. The max oil temp is 266F, the CHT is 275F and the coolant exit temp is 248F. Your coolant temps are usually close to the CHT temps. Rotax has stated in SB-914-029 and SB-912-043 that if you use Evans NPG you can go up to the max temp of 266F, but if you use a 50/50 mix then your max must be lowered to 248F. They did this because the 50/50 mix boiling point of 270F is too close to the 266F max temp. You may get vapor spots within your cooling system and if that were to happen you will lose the cooling in that area and metal fatigue will set in and loss of cooling. If you use Evans to stop a boil over problem while idling then you may have another issue duringcruise. Even though Evans has that great boiling point it does have a drawback.Evans does not absorb and distribute heat as well as water. So your temperatures across the board (i.e. oil, CHT’s and coolant) will all be higher by about 25F-30F over the 50/50 mix. If you were already too high in temps then Evans will make it higher. If you have an open air engine that is exposed to the air and not cowed then Evans may work very well for you. If you have a tightly cowed engine then Evans may drive your temps up to red line and negate any benefits you might gain from the boil over protection at idle. For example I tried Evans NPG in my 2006 Flight Design CTSW because it was getting up to 250F+ in the summer. The problem then became that all my high temp alarms were going off on the oil and CHT’s because Evans raised those temps another 30F which put me over the 912ULS max allowable. I drained the Evans and went back to the 50/50 mix and then just unloaded my prop pitch to a little less course and the temps were all fine. If you have an engine heat issue then don’t throw coolant types or dilution strengths at the problem, but fix the cause (i.e. air flow, a lean fuel issue or reduce the prop pitch).


If you use Evans then you must drain and purge the system of all water with Evans Prep fluid which helps remove water from the system. This isn't hard and is quite easy. Now the other thing to consider with Evans is that if at any time your coolant level is low then you can never add water to the system and must add only Evans NPG and if you are not at your home port that could be a problem.


If you use a 50/50 mix then you can top off with justdistilled water. Do not use tap water in your coolant system.


One last commenton coolants, if you use a pure Dex-Cool coolant do not mix any other coolants with it unless it states that itis Dex-Cool compatible and then I would still think twice. Mixing some of these coolants with Dex-Cool can cause thickening of the fluid which will cause several problems. Do yourself a favor and just don’t go there, so pay attention to what you put in your cooling system or what you add to it later.




I hope you now will look at your Rotax cooling system and have a little more information to make that decision on which coolant type is right for you.





Enough with all this one paw typing, time to curl up on the couch.


Oils: A Fluid Discussion

What each Rotax 912 owner should consider

New, Rotax 912 series engine owners in variably ask what oil should be used. There is no perfect oil and that can make this discussion an arbitrary topic. Many owners just ask their neighbor what oil to use, some call their aircraft Mfg., some consult the Rotax Suitable Operating Fluid publication SI 912-016 R2 and others think all oils are created equal. In this article we will take a look at a few oil traits and additives. It is out of this writer’s and Rotax’s scope to possibly test or comment on all the oils on the market. It is not this writer’s position to have you purchase any certain brand of oil, but only to help you understand what’s in an oil and why we need to use a good motorcycle oil in the Rotax 912 series engine for its longevity and health. I have attached an oil study comparison by an independent tester commissioned by Amsoil dated March 2006 and written by David Leitten and I want to give him credit right up front. This is a very good article and you will walk away with a much better understanding of oil additives. My article will parallel and summarize his article to make it easier to follow. His article goes a long way in having you understand why motorcycle oil is different than car oil and what additives are important to you and your Rotax 912 engine. You will need to draw your own conclusions as to the oil that you want to use, but now you will be well armed with good information. Oil tests are usually conducted under the American Society of Testing Materials (ASTM) standards. Rotax highly recommends the use of motorcycle oils and here are just a few reasons. (Note: When I use the term motorcycle it is applicable to the Rotax 912 as a reference)


The difference

Here are a few reasons we do not want to use auto oil in our Rotax 912. Our engine speeds (RPM) are much higher as is compression (10.5:1 for the 912ULS), The horse power ratio is much higher, and as also engine temperatures. The one really big difference between the auto and the Rotax 912 is that the Rotax shares its engine oil with the gearbox, where an auto engine has separate fluids for these functions. This item is a big concern when choosing motorcycle oil over auto oil, as the properties need to be very different. We do not use our aircraft as often as our cars so those extended down times cause problems such as rusting and acidity. The attached article delves more deeply into these subjects.


Oil Properties

Viscosity is a measure of an oils thickness and it helps protect the engine as oil is non compressible. If your oil is too thin it can be pressed out of the way where the metal surfaces come into contact. Some oils do not keep their viscosity rating under high work loads and shear and you could lose some of the viscosity protection. An oil that is too thick causes excessive work and temperatures for the engine and could cause some problems on a cold morning start up.


Shear protection is a measure of an oils capacity to thin out or reduce its load-carrying capacity. The Rotax 912 operates at higher rpm temperatures than an auto and the gearbox shares the engine oil. This high mechanical rpm, higher engine temperatures and close tolerance operation can cause some oils to thin out and not protect your engine as well as some other oils with higher shear properties.


ZDDP (ZincDialkyl-Dithio-Phosphate) WOW that’s a big word. This is a zinc phosphorus compound. This is a very important additive for us compared to an auto.This additive was very important in older autos as a protection against metal surfaces coming into contact. You see, for most of the last century, the almost universal method to open and close engine valves was via flat tappets (solid or hydraulic lifters if you will) (like the Rotax 912), and the ZDDP additive was there to prevent or reduce wear between the lifters and the camshaft. The ZDDP in the minute amounts of oil that will get burned and exit through the exhaust system which will shorten the life of catalytic converters. Thus the EPA mandate to eliminate ZDDP from engine oil. The auto makers have responded by designing engines that utilize roller lifters or overhead camshafts, and have no need for the protection offered by ZDDP. The older auto owners can still purchase this additive from automotive stores. This additive was reduced to around 800 ppm. It coats the metal surfaces and when they come into contact acts like a sacrificial surface. Zinc is not the most important additive here, but the zinc phosphorus combination is.


The 4 ball wear test. This is test to see if the oil can protect parts from metal to metal contact and is spelled out very well in the attached article. Zinc levels in this test seem to play a big part in an oils ability to prevent metal to metal contact.


Gear performance test is a measure of an oil’s viscosity and how additives contribute to gear protection under extreme pressure, shock, sliding and shearing forces. This is very important to the Rotax 912 gearbox. If you have ever taken a gearbox apart for an inspection you can see the gear wear from the use of certain oils and the absence of wear with good oil.


Oxidation test is a test to see how well oil holds up under heat. If the oil starts to break down from heat it will cause oxidation to occur which shortens the life of the oil and causes more carbon accumulation. Increased levels of ZDDP help prevent oxidation.


Volatility: a.k.a. Evaporation of some of the oil components causes higher oil consumption and higher viscosity. High temperature is the main cause of this problem.


Oil Acidity: Oil base stocks, but mainly their detergent additives are designed to help reduce the amount of acid build up.


Foaming: This has been an ugly word in the early days with some of our oils and it was enough of a concern amongst other reasons that Rotax, several years back (SB-912-040 R1 dated 8-2003), increased the volume of oil in our oil tank to help combat this problem. Some oils are certainly up to the task more than others. Foaming is caused because of our gearbox and engine oil combination. Foam as we know has air bubbles and this prevents the oil from protecting our metal to metal surfaces. Foaming causes increased wear and oxidation. (Note: Your oil tank dipstick should have a squared top where you grab it. The old dip stick is round.)


Rust Protection: Many of us live in humid climates and moisture in our oil and on internal parts is inevitable. This of course is one reason Rotax wants you to obtain at least 212F oil temperature to boil off any moisture. Oil does not protect or prevent rust, and inhibitors to the oil must be added. Letting an engine set for extended down times allows rust to cause pitting on metal surfaces and in our bearings. If you are unable to fly for extended periods it is better to at least start the engine and let it run at operating temperatures for a while on the ground.


To add to your reading it should be noted that the base stock oil is just as important as any additive. Without a good base stock then the additives may not help much. You should be using a full synthetic or a semi synthetic oil and not using a straight mineral based oil. The straight mineral based oil will not provide the protection from all the items we have talked about above. It lacks tenacity and robustness against everything that wants to damage the metal parts in our high performance engine. If you use auto fuel then you can use the full or semi synthetic oil and if you use 100LL use only the semi synthetic. The full synthetic with 100LL can’t suspend the lead and it tends to fall out of solution and settle in places we don’t want it to.




The bottom line

Here are a few oil stand outs and this of course is not all inclusive because not all oils around the world can be tested for our purposes. You need to make up your own mind in choosing an oil that is right for you and this is just the opinion of this writer and IRMT mechanic. These listed oils have shown good characteristics in lab test and field use. These are all motorcycle oils.


Full Synthetic: (not in any order) Use with auto fuel only.


Mobile One Racing 4T 10-40W, Mobile 1 V-Twin 20-50W, Amsoil 10-40W or 20-50W


Semi Synthetic: Aero Shell Sport Plus 4 10-40W, Golden Spectro 4 10-40W or 20-50W


From all the research I have done, these oils contain good, high quality base stock oils and sufficient additives to support the engine.


Side note: I wanted to mention something I have heard and researched. AeroShell Sport Plus 4 was designed and is mfg. in England. Some oils are mfg. in different geographic locations and sometimes that changes their properties. So a one location mfg is a good thing. I understand that Aero Shell according to the Aero Shell Sport Plus 4 MSDS sheet has 1%-2% of ZDDP which translates to 1000ppm - 2000ppm.





I hope after reading these two articles that you will look at your Rotax 912 oil a bit more critically and evaluate whether you have a good oil or maybe make a change. But either way you now have more information to make that important decision.


What’s in an oil filter?

Funny you should ask

Rotax has a new addition to their parts inventory, the new Rotax oil filter and it’s manufactured by Mahle, part #825-010 (012) and made in Austria. Rotax took a look at all the different engine installations out there and I believe they thought it was time for achange and a change for the better it was. The new oil filter comes in an all white box verses the old filter box that was blue/white. It is slightly longerby 3.5mm or .138 in., but this is not where the changes stop by any means. They made some sweeping changes. Some of these changes are specifically designed with the mounting of the oil tank in mind. This is important because we have a dry sump system and not a wet sump. The white silicone drain back preventer (valve)is improved with little to no oil being allowed to drain out of the filter while the engine just sits. This was designed for the engine setups where the oil tank is mounted a little too low and prevents the oil from draining back into the tank and out of the filter and engine. Another change was the additionof a “one way check valve”. This feature is for those oil tanks mounted too high above the recommended oil tank location and keeps oil from draining out ofthe tank back down into the engine. The next change is a new bypass pressure spring. The old filter used a piece of spring steel. This new oil filter has a smooth round edge spring so it doesn’t have any sharp edges to wear through the filter housing. The old bypass pressure was 1.1 bar or 16 psi. This new filter is 1.2 bar or 18 psi. This will stop some of the oil filters from opening for a second or two at start up. The old oil filter could be pre-filled with oil which was easy, but this filter you can’t pre-fill with oil and you must turn the prop to put oil into the filter before your first start after an oil change.


So you see Rotax has made some fairly big changes to the inside of the oil filter to better help its users deal with their different engine applications. I have attached several pictures so you may see what is actually on the inside. I swore I wouldn’t cut open a perfectly good filter and wait for a used one, but curiosity got the better of me. For the photography critics out there, no I didn’t quit my day job. biggrin.gif


I hope this helps you to understand the new Rotax oil filter.


Be safe out there!






The Dog

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