Rotax 912 Troubleshooting Help / Guide.


We accept no responsibility for the consequences of your troubleshooting endeavours as a result of following any of our suggestions. If you're not comfortable with what you are doing we strongly advise you to find a suitable qualified and experienced iRMT (Independant Rotax Maintenance Technician) / engineer to help. If you are within our operating radius then please give us a ring.

Contents ...

Introduction ...

Start by checking the basics ...

Your 912 won't even turn over! ...

Your 912 turns over but won't start ...

FUEL - Too little fuel? ...

FUEL - Too much fuel? ...

Source of ignition (spark)! ...

Getting any AIR? ...

Too little air? ...

Too much air? ...

Your 912 will start but runs roughly ...

Fuel Delivery / Carb Related Checks ...

Electrical / Ignition Related Checks ...

Exhaust Related Checks ...

Cold Starting Checks / Issues ...

Warm / Hot Starting Checks / Issues ...

Miscellaneous Tips / FAQ's ...

Vital Engine Statistics at a glance ...

More frequently used consumables ...

Introduction ...

Assuming that your 912 was installed correctly in the first place this page is intended to offer some basic troubleshooting guidance. Also below are some lists of things to check or consider that may have come to cause some of the more common problems relating to non-starting / poor starting, poor idling / rough running / inability to realise full power at WOT etc. It does assume a certain amount of basic knowledge and understanding of engine terminology and familiarisation with your 912's engine components. If you do have some knowledge of 912's, and are comfortable referencing the Line Maintenance / Heavy Maintenance Manuals etc for sourcing further more specific information, then you may find this page useful as a starting point for problem diagnosis. Failing that it can just serve as a list of items to keep an eye on / perhaps integrate into yor pre-flight checks. If in doubt always read the ROTAX manuals - they might not be perfect but they are rarely wrong! The IPC - Illustrated Parts Catalogue is a great resource for referencing correct torques, and checking where Loctites and other compounds are applicable etc.

This page will start with addressing non-starting problems and then moves on to look at poor / rough running.

Please note that this page assumes you haven't done something drastic like overspeeding or overheating your 912, or had a significant prop strike in which case you will more than likely need some professional assistance.

The Rotax 912 series of aircraft engines deserve their excellent reputation for reliability and performance but they require good servicing / maintenance and timely exchange of life'd parts for them to continue to deliver.

This document is a work in progress and is regularly updated so please feel free to check back for updates.

IMPORTANT ... Click HERE to ensure that you are looking at the latest version of this page!

We hope you find this resource helpful.

Start by checking the basics ...

We would suggest starting with some often overlooked and relatively easily checked basics ...

FUEL QUALITY ... Consider how old your fuel is and therefore it's quality. If for example it's been sat in the tank all winter since September and it is now April some evaporation will have occured. It will more than likely have degraded to some extent. The octane rating will have effectively dropped and should be changed for fresh fuel before you start drawing conclusions about problems you may or may not have with your engine.

FUEL CONTAMINATION ... Is your fuel contaminated with water? E5 (5% ethanol by volume) petrol is on sale in the UK and has been for some years. Ethanol can absorb upto 50% of it's own volume of water without visible seperation. The longer you leave fuel the more opportunity it has for the ethanol content to absorb water without you being able to see it. You wouldn't consider pouring water into your aircraft's tank when you fuel up with fresh fuel so why would you fly with fuel that has had a significant amount of time to absorb water?

FUEL FILTER ... How long has your fuel filter been fitted to your aircraft for? Note that "how long it's been fitted" is not the same question as "how many hours ago in flying hours" you renewed it. It might look clear whilst it in fact it may have mould growing on the filter mesh that could be impeding fuel flow. Ultraviolet and ethanol provides an great nurturing environment for mould growth. Rotax recommends a filter with a mesh size of 0.1mm. Are you certain that your fuel filter is not partly blocked and restricting fuel flow? Theoretically any mould growing on the mesh would only have to grow 0.1mm on the mesh to potentially fully block the filter. How long would it take to partially block your fuel filter? How much of a "partial" blockage do you think you would be happy to live with when you are climbing out on full throttle?

CORRECT FLUIDS ... Do you have all the correct fluids in your engine e.g. engine oil, coolant etc, in the correct volumes, and changed recently as per the ROTAX recommendations in the Line Maintenance Manual?

ENGINE MOUNTS ... Are your engine mounts in good shape? If you have four mounts holding your engine in place it only takes one of them to have deteriorated with age sufficiently for a situation to arise whereby your engine can effectively rock / vibrate on the remaining diagonally opposed pair of mounts. Have a good look at your mounts. Considering the wieght of your 912 engine and the torque that it generates on start-up and shut down (particularly the 100hp variants) do they look like they are up to the job? Is there visible cracking / deterioration of the rubber part of the mounts? Try and rock your engine in it's frame / mounts. Do they feel unduly soft? If you are not sure what should "feel" right for your aircraft and there is another example fo your aircraft in the hanger? You might try asking the owner if you can compare the two. Engine mounts are the shock absorbing interface between your engine and it's frame so they are by design intended to "give" a little - it's when they give too easily and fail to suppress the engine vibration then your 912 is likely to be susceptible to some vibration related issues that you might mistakenly interprete as rough running / engine problems. Your engine mounts are also responsible for keeping your engine securely attached to the airframe! Another reason why it makes good sense to monitor their condition on a regular basis and replace if neccessary.

PROPELLOR GEARBOX SHIMMING ... Is your propeller gearbox due for a service? If it is not correctly "shimmed up" there may be excessive gearset backlash which is not only going to result in premature wear of gearbox components but also in some avoidable vibration and possible starting / smooth running issues.

COMPRESSION GOOD / NORMAL? ... When you turn the prop to initially gurgle your 912 prior to turning it over does the compression for ALL 4 cylinders feel normal? You don't need to do a full differential compression test to ascertain whether the compression "feels" roughly normal unless it feels like one of the cylinders is way out in which case a differential compression test might be a good starting point for further investigation.

RECENT CHANGES? ... Finally ask yourself what has changed recently? Have you just had a service done / partial overhaul / full overhaul? Have you just changed your prop? Is it pitched correctly and thus presenting the engine with a well matched load? Recent work, however minor in scope might explain why something is it not as it should be. It's not unheard of for a partly failed component / hose connection / electrical connection to have been disturbed during routine maintenance and a fault introduced in this way. This is NOT a great reason for never doing any routine maintenance(!) but is worth considering as a part of your initial troubleshooting endeavours.

Your 912 won't even turn over!

Is your battery healthy and fully charged? If not consider if it's too old / past it's serviceable life? Has it simply discharged due to a lack of use. Has it suffered low temperature freeze damage over the winter months? Was it ever actually big enough in the first place? Consider replacing it with one with a higher cranking capability. Do you have loose or oxidized battery terminals / earthing connections? Do you have a faulty starter motor, solenoid start or start switch or corroded connections to these components?

Your 912 turns over but won't start ...

If the starter motor is turning is the engine realising proper cranking speed to start? If not suspect poor battery performance (see previous paragraph), wiring resistance (voltage drop) on starter circuit is too high (too fine a guage of cable) , a short circuit or poor / corroded contact? If you can hear the starter motor turning without the engine cranking you may have a faulty sprag clutch. If the starter turns, the engine fires briefly but then the engine stops dead again the sprag clutch may well be to blame and you should get this checked / repaired rather than keep trying to start a 912 with a failed / failing sprag clutch.

If the starter turns the engine at what feels like the correct cranking speed and it still won't start (and you definately have the mags on!) then your 912 is missing out on one of the fundamental ingredients that combustion engines need to run! They are of course fuel, a source of ignition (spark) and air. These all need to be available in the right quantites / ratios and at the right time. Before going any further you should check your choke cables for sticking / smooth operation / full movement /ability to close fully and therefore the fuel enrichment (choke) circuit is enagaging and disengaging correctly as you move the choke lever. If you are experiencing problems restarting your engine after your engine has run for a time and therefore warmed up then it could be because the fuel enrichment circuit (choke) is still partly engaged and therfore the mixture is too rich. For the choke circuit to be effective the throttle must be closed so you also need to check your throttle cables for sticking / smooth operation / full movement /ability to close fully before you try to start your 912. Your throttles have to be set to a position that equates to somewhere in the idle range when your throttle lever is fully closed or your 912 may fire but struggle to idle. Your throttle cables should be at least reasonably well mechanically balanced for your 912 to start (if not ideally pneumatically balanced but you need to have the engine running to do that!). So having established that you engine's control cables are in good shape and adjusted as they should be lets get back to checking that your 912 is getting the fundamental ingredients that it needs to run ... fuel, a source of ignition (spark) and air.

FUEL - Too little fuel?

Are you getting ANY fuel at all? The first and probably quickest place to check is the carb float bowls. Carefully remove them one at a time and check to see if there is any fuel in them and that it is up to the correct level in the bowl. One or more of them might be empty in which case you have identified that your problem is definitely at least fuel related in some way and your 912 is being starved of fuel. If this is the case you need to figure out if and where the fuel supply is being interrupted in it's journey from tank to carbs. Make sure that when you re-fit the carb bowls that the gasket / carb bowls re-seats properly and that you don't introduce a problem that wasn't previously there! Firstly check that you actually have sufficient fuel in the tank! Is any fuel actually getting as far as the pump? Is the fuel tank uplift tube, if you have one fitted, blocked? Is the fuel tank uplift tube reaching fuel at the bottom of the fuel tank properly? If not then fuel may be visible in the tank but not of sufficient quanitity to reach the end of the uplift tube. Is the fuel filter blocked? Fuel filters can appear to be clear but be blocked with hard to see mould that thrives on the alcohol content of our fuel - see comments in Inroduction section? Is the fuel tap turned to the on position? Is the fuel tap itself blocked? Is there an air lock in the tank because the breather pipe / fuel cap vent hole is blocked? If there is no mechanism by which air can replace fuel being taken out of the fuel tank then a partial vacuum in the fuel tank will form and your fuel pump will progressively find it harder to draw the fuel out of the tank until you arise at a situation whereby there is fuel in the tank but it wont come out. Do you have vapour lock in an engine that has recently run but won't restart? Are fuel lines lagged to prevent boiling of fuel in the fuel lines in a tightly cowled installation? Do you have a fuel return line to the tank correctly installed so that any boiled fuel can effectively be pushed out of the fuel lines and back into the tank? Is there a physical blockage in one of the fuel hoses somewhere? Has a piece of stray fuel hose rubber become lodged in the fuel line somewhere perhaps since a recent fuel line change? If fuel is getting as far as the pump then the fuel pump may have failed completely or working at reduced / insufficient pressure to cope with the demand. A good way to test for this is to remove the fuel line coming out of the pump and turn over the engine briefly (taking the precaution of removing one spark plug from each cylinder first for safety) to see if any fuel emerges. Check that fuel pressure is within the recommended / permitted range e.g. 0.15 - 0.4bar. T-connect a fuel pressure gauge temporarily into the fuel circuit to verify this if you don't have one permanently installed - Conair / SkyDrive do an excellent kit designed for just this purpose with some temporary transparent fuel hose for visual verification of the presence of fuel and/or bubbles in the fuel. Once you have ascertained that fuel is getting as far as your carb's fuel inlet and you are confident that the fuel is of a good quality, the correct pressure, and is not contaminated with water or ice you need to look more closely at the carb(s) itself. Other than checking the above items you are now probaly looking at striping the carbs for further investigation into the possibility of an internal blockage and there are many things to check on a Bing 64 carb - see carb related checks below for some of the things you should be checking for.

FUEL - Too much fuel?

Conversely upon closer examination you may find that your float bowls are over-full in which case your engine is probably receiving an over rich supply or may be just plain flooded. Have you inadvertantly flooded your cylinders with innapropriate use of the choke / fuel enrichment circuit? A hot 912 shouldn't need any choke to restart it. If it has been recently run and is still slightly warm then it's not a bad idea to attempt a restart with no choke and apply choke progressively until it fires. If this is the first start of the day and you have no reason to suspect that you have flooded it by inappropriate use of choke then again you could try checking the fuel level in the float bowls. Fuel coming freely out of the carb overflow / venting outlet when you spin the engine over is a clear indication that there is an excess fuel situation / float valve malfunction? One of your vitton tipped float valves may be damaged / worn and may not be seating correctly thus failing to close the fuel supply to the float bowl at the correct fuel height. In this case your carb's mechanism for fuel metering will not be working correctly. You can find details of how to test the functionality of float valve in the Heavy Maintenance manual. Your fuel pressure may be too high in which case, again, your float valve may not be able to close / seal properly and may be failing to regulate the level of fuel in the float bowl - use a fuel pressure gauge to verify the fuel pressure as fuel line enters the carb. Your carb breather / overflow pipes may very well be seeing varying pressure if they have some fuel in them and this can affect the fuel mixture causing further imbalance. This may have happened if your carbs are badly out of sync or because your 912 has taken a shaking during some unsuccessful starting attemps and the fuel in the float bowl has partly found it's way into the breather pipes and thus stopping them from venting freely. Your carbs are very sensitive to the pressure that the vent hole / pipe "sees". A positive pressure (compared with ambient air pressure) will cause your carbs to deliver a richer mixture and conversely a low pressure will cause them to deliver a leaner mix. It follows from this that for your carbs to stand any realistic chance of being balanced properly both carb vent holes (or the end of their respective vent tubes) need to "see" the same pressure otherwise cylinders 1 and 3 will be recieving a different fuel mixture to cylinders 2 and 4.

Persistent and prolonged attempts at starting an engine with no spark is likely to result in a flooded engine / cylinders. Overly wet spark plugs (see next section) should confirm whether you have flooded the cylinders. If you have a flooded engine sometimes the quickest way to give it a chance of restarting is to remove a plug from each cylinder and turn the engine over with no throttle and no choke just to vent the excess fuel out of the cylinders, then replace plugs and try starting it again.

Source of ignition (spark)!

The obvious and easy check for the presence of a spark is to remove the plugs and visually check for a spark. Before you do this, however, have a good look at the plugs. There are numerous resources on the web for helping you read the plugs for internal combustion engines so we won't go into finer details of reading your plugs here. Having said that if they are fouled, contaminated, badly worn or visibly broken then you should replace them with new ones. If their colour looks OK have they recieved an over zealous application of Wacker P12 thermal paste? If too much thermal paste has been previously applied it is possible for the excess to migrate up the threads and find it's way to the electrodes. Has a cheap possibly conductive "substitute" for Wacker P12 been used in place of Wacker P12? Computer CPU heatsink compound and other silicon or worse, silver-based (conductive) heat sink compounds are not a good substitute for Wacker P12. Having a conductive paste near the electrodes of your plugs is not going to result in a smooth outcome. Are the plug gaps set correctly? They should be set at between 0.6mm to 0.7mm. Too small a gap and the spark may be too small to reliably ignite the fuel mix. Too large a gap and the ignition coils may not be capable of generating sufficient voltage to bridge the plug gap. Visually checking for a spark is perhaps best done in a darker hanger / shade rather than in broad daylight as the spark is very faint due to the radio interference suppressing resistive plugs that are fitted to 912's as standard. Make sure that you use some reliable earthing leads to earth the -Ve body of the plugs to the engine / earth. Failure to earth your plugs could result in damage to your SMD / ignition modules (very expensive to replace). Remember your 912 must be turning over at a minimum of 220 RPM for a spark to be generated at all so if it is not turning over that fast then don't expect a spark to be present and your first course of action should be to focus on why your engine is not cranking up to speed e.g. check battery, starter motor circuit, starter motor earth etc - see previous paragraphs. If you are confident that your 912 is turning over at the correct cranking speed for starting but is still not generating a spark then you should conduct a thorough visual inspection of all ignition related cables including the shielded kill wires (p-wires) that go to your mag switches, the connectors that come out of your ignition modules / SMD's, ignition coils and the HT leads going to your plug caps.

Whilst we are on the subject of ignition modules / SMD's, it's worth noting that the SMD / ignition modules have various different modes of failure. Sometimes the wires coming out of the SMD / ignition modules into the 4 or 6 pin connector blocks (depending on the age of the engine) are fatigued and severed internally. Outwardly the insulation on the wires appears OK but internally vibration has taken it 's toll and the wires are severed. This kind of failure can lead to intermittent ignition problems. Give them a good wriggle / tug and feel for broken internal wires. A wire with it's insulation apparently intact but the internal wire cores servered will stretch as it doesn't have the strength of the internal metal strands to prevent stretching. Sometimes the wires are OK but the internal electronics of the SMD's fail. They can fail outright or more commonly their performance degrades gradually over a period of time. It is not unknown for them to fail to produce a spark at starter motor RPM but then happily produce a healthy spark once the engine has started on the other "good" circuit and engine revs have now increased from starter motor cranking speed to idling speed. Sometimes they can fail to work simply because they are not warm enough e.g. they dont work when engine is cold but work fine when engine has warmed them up a bit. If this is the case then once the engine has started and you do a mag check both ignition circuits will appear to work fine and the partial failure on one SMD goes undetected. If you were to try and start your 912 from cold on one mag only it would start and on the other mag it might fail to start e.g on this circuit the SMD / ignition modules has degraded. For this reason it is a good idea to periodically test your SMD / ignition modules by trying to start your engine from cold on one mag at a time with sole purpose of picking up on this type of failure before the failing SMD / ignition modules fails completely at take-off or at some other unwelcome moment.

When you are checking the condition of the HT leads that go from the ignition coils to plug caps look out for chaffing of the cable insulation (especially where the lower HT leads pass between the cylinders) to ensure that this hasn't affected the performance of it's insulation and that the HT voltage is not shorting out to earth before it even arrives at the plug cap. If you find that just a single plug is not firing this may very well provide the explanation. Are the plug caps gripping the plugs well or do they easily pull off - pull off torque is in Heavy Maintenance manual? Are the HT cables screwed onto the plug caps properly? Check that your spark plug cables go to the correct cylinders e.g. top plug lead for cylinder 2 hasn't been transposed with the top plug lead for cylinder 4 etc. It might seem like an obvious check to do but it does sometimes get overlooked!

If you can find nothing untoward with the above visual inspection then its time to grab a good quality multimeter and start testing individual components of the ignition system. Start at the charging coils and work your way through the system until you get to the plug caps. These are two charging coils, one for each ignition circuit, with red coloured wires coming out of them. They are mounted on the flywheel stator and they power the SMD / ignition modules which in turn discharge their stored energy to the ignition coils. The ignition coils are underneath the SMD / ignition modules. Note that the ignition system doesn't require power from your aircraft battery to work - it gets all the power it needs from the charging coils. Power from the battery is only required for powering the starter motor. Use your multimeter and test the output of the charging coils with the starter motor spinning and also check their impediance / test for continuity / check for a short to earth. If you find an open circuit on a charging coil or an ignition coil then it needs replacing. Note that there are two separate ignition circuits powered by two separate charging coils on the stator assembly behind the flywheel. You don't need to remove the flywheel to test the output and impedance of the charging / ignition coils. You can access the red wires by following them from where they emerge from the flywheel housing and trace them to their respective block connectors. If you don't have access to a multimeter it is still usually a fairly straight forward process identifying which component has failed in the ignition circuit by a process of switching components out from the good circuit until you find that when you switch a component over the problem follows to the other ignition circuit. You can test the funtionality of the ignition coils using an automotive spark tester - be sure to disconnect / isolate them from the SMD modules before using a spark tester. You don't want to damage the SMD modules whilst testing the ignition coils.

Getting any AIR?

This one tends to get taken for granted. Of course it's getting air! Well that's not necessarily true. Your 912 may be getting too much or too little air. Whilst mixture control is automatically regulated by Bing 64 carbs it is only able to do this within certain limits. Don't even consider changing the needle jet position if your 912 has ever run correctly with it in its current position and you've not moved it. It should be set on the third position down from the top if you generally fly within a few thousand feet of see level. If you have just purchased an aircraft and you suspect your engine is suffering from fuel mixture problems it might be worth checking that the previous owner didn't take it upon themselves to change this from its default position. If it is in the wrong position you may spend hours trying to "fix" other things in an attempt to compensate for this fundamental fault. To check what sort of mixture your 912 is running on a good starting point is to look at the colour of the spark plugs but be aware that 912's have a tendency to idle rich so if it was idling for an extended period of time the last time it ran then the plug colour will tell you that it's been running rich (which it probably was at idle) but wont give you a very accurate idea of what was happening at cruise power.

Too little air?

If you have K&N's air filters fitted you should be aware that it is possible to apply too much filter spray - they should only be coloured a very light pink colour on first application of new filter oil after a clean. The over generous application of air filter oil will not be helping your Bing 64 control the fuel mixture. Are your air filters are clogged and need cleaning? This probably wouldn't be responsible for your 912 suddenly not being able to start (least ways not from cold when a rich mixture is ideal) but might very well be responsible for degraded performance over time / fouled pugs or trouble restarting a hot engine.

Too much air?

Too much air? Carb rubbers can be leaking air into the induction system because they are split (and when running the amount they leak air through the split can vary as the carbs mounted on them bounce around slightly). Remove them and inspect thoroughly. They ususally split from the inside out. Carb rubbers can be over torqued to the inlet manifold and distorting thus failing to make a good seal. The carb to carb rubbers join may be over torqued on the older type fittings without the 8mm spacer. Inlet manifold o-rings may not be seated correctly, swollen, deteriorated or the wrong ones fitted - there are two different sizes. Occasionally the wrong sized ones are "forced" to fit causing induction leaks or swell with age and when replaced introduce an induction leak. Also check that your K&N air filters / airbox is mounted on the carb properly as their connection to the carb can leak air. You can test for induction leaks VERY judiciously with Easy Start with the engine at idle (IF it will idle).

Your 912 will start but runs roughly / exhibits erratic behaviour?

In addition to considering the items discussed above and checking items in the check lists below if you have a rough running engine you should perhaps first do a mag check to eliminate an electrical problem. If it does fail a mag test see the electrical check list. If it is not an electrical problem you should be consider that a rough running engine of "boxer" configuration is generally caused by different power outputs of different cylinders or sides of the engine. If cylinders 1 and 3 are collectively the source of the rough running then it is probably something to do with fuel delivery on that side of the engine so look for fuel delivery / carburetion issues on that side of the engine. If you suspect a carburetion issue you need to note what RPM the fault occurs at and therefore which fuel circuit of the carb is actively responsible for supplying the engine with fuel at the particular RPM you are having the problem at. Is the problem in the idle circuit for idling, in the needle jet / slider for midrange RPM or in the main jet for WOT. Equally the same applies with cylinders 2 and 4. If you are lucky enough to have CHT senders on each cylinder use them to identify which cylinder (s) you have a problem with by looking for spurious operating temperatures. In the absense of a full sets of CHT's and the problem is reproduceable on the ground then you could try measuring CHT with a laser temperature guage.

You might have an issue that is just affecting the one cylinder. If so you might want to run a differential compression test to identify if one of the valves is failing to close properly. An even more useful exercise would be to remove that cylinders spark plug(s), examining the plugs at the same time and then use a borescope via the spark plug hole to examine the inside of the cylinder. Check the exhaust valve for correct colour. Presence of hotspots on the exhaust valve indicate a failure to seal all the way around the valve seat. Your valve(s) may need lapping in again especially if you have been running your 912 on AVGAS and you have excessive lead deposits. Exhaust valve cooling depends on them closing fully so that combustion heat is conducted away from the valve to the valve seat. Irregular heat induced discolouration if your exhaust valves usually indicates that they are failing to close correctly. Whilst you have your borescope handy check for unusual cylinder wall scoring, evidence of overheating etc. Look at the piston crown for evidence of detonation / pre-ignition / excessive lead deposits. Remove rocker covers and check that hydraulic lifters are hard and adjusting up correctly. ONLY go as far as to remove cylinder head and cylinder if you find something untoward during your borescope examination that really warrants it. If you find yourself at this point now might be time to call for some assistance from someone with Heavy Maintenance knowledge.

Please see the following check lists below ordered by main engine component ...

Fuel Delivery / Carb Related Checks ...

  • Check carbs are synched / pneumatically balanced
  • Check for blocked fuel filter.
  • Is there a blockage behind the float valve?
  • Is the carb vent tube blocked?
  • Do you have a blocked idle jet? Your 912 is going to struggle starting with a blocked idle circuit.
  • Check for blocked fuel filter.
  • Check for water / debris in float bowl?
  • Running rich or lean? Needle position should be 3rd down from top. Higher up notch is leaner. Lower is richer. There is no reason to change it from it's default position unless you're routinely planning on flying over Everest.
  • Check Idle jet is not blocked.
  • 914 Only - Check carb bowl is not over torqued - classic mistake that distorts the carb bowl, affects it's ability to seal to the carb body, bends the float pins out of alignment preventing their free movement and also prevents the float valve vitton tip from seating correctly against the seat of the float valve.
  • Check diaphragm is seated correctly.
  • Check diaphragm has no holes in it - e.g. not perforated.
  • Check main carb spring is not stretched i.e. is the correct length and is the correct spring.
  • Check for induction leaks around carb sockets and manifold e.g. check carb socket to inlet manifold o-rings are providing a good seal.
  • Check carb sockets for external and more importantly internal splits. Check for de-lamination of rubber.
  • If carbs wiggle easily - suspect split carb socket.
  • Check for older style carb socket clips without the 0.8mm spacer as they may have been over tightened and distorted the sockets fit to carb thus creating an induction leak.
  • Check carb sockets have not been over torque to manifold.
  • Check floats not leaking - dry them out, weigh them, compare weight.
  • Check floats are not flaking and blocking jets.
  • Check floats are moving freely.
  • Check all jets are not blocked / partially blocked- clean with carb cleaner and compressed air.
  • Is there a blockage in the atomiser chamber between the main jet and needle jet?
  • Check all carb o-rings.
  • Check floats arm / hinge assemblies are set level, evenly / check the float arm height.
  • Check vitton tips of float valve are not worn and are seated correctly and thus preventing flooding of bowls.
  • Check for throttle and choke cable binding.
  • Check idle mixture screw is set to between 1.5 and 1.75 turns out from being fully screwed in.
  • Check for wear on main needle.
  • Is your fuel pump working correctly - use a Fuel Pressure Gauge to check fuel pressure with a mechanical gauge. Conair / SkyDrive do a very good mechanical gauge which can be T-connected into you fuel system for temporary diagnostics or permanently mounting.
  • Check that fuel pressure is not too high otherwise it is possible to over-ride the float valve and flood the carb.
  • Check that the float chamber overflow pipes / breather pipes are not blocked and emerge at the same place and are "seeing" the same air pressure.
  • Check that there is no residual fuel in the breather pipes preventing them from breathing freely.
  • Make sure that the choke assembly has not been mounted back to front.
  • Check throttle can open fully e.g. is not stuck up against the stops.
  • Is your fuel pump working correctly - use a Fuel Pressure Gauge to check fuel pressure with a mechanical gauge. Conair / SkyDrive do a very good mechanical gauge which can be T-connected into you fuel system for temporary diagnostics or permanently mounting.
  • High hours? - Maybe it's time to consider a complete carb overhaul - several components may be worn and rubber components well past their recommended 5 year life - o-rings, diaphragm, sockets etc if you've been operating "on condition". The cumulative effect of wear or sub-standard condition of several carb components may be conspiring to compromise your engine's performance.

    • Electrical / Ignition Related Checks ...

      • Check battery is charged / in good condition.
      • Check battery is a suitable size and can deliver correct cranking amps. Consider installing a more powerful battery e.g. Hawker Odyssey. Listen to cranking speed - does it sound low?
      • Check battery charging circuit / regulator working correctly. Measure regulator output.
      • Check the correct plug leads go to the correct plugs up and down.
      • Check correct plug gaps should be 0.6 to 0.7mm.
      • Check for spark plug contamination / fouling.
      • Visually check that a spark is actually being generated.
      • Check for damages wires going into SMD's connectors and indeed on any ignition components - external insulation may be OK but the internal multicore may be broken / vibration damaged.
      • Check that you actually have thermal paste applied to your plugs and that it is Wacker P12 not conductive anti-seize / silicon / silver based paste that may be shorting out your spark plugs.
      • Check that excessive paste hasn't been applied e.g. that no thermal paste is shorting out electrodes on spark plugs.
      • Check all electrical connections especially Earth connections.
      • Check all wires are tightened correctly.
      • Check for corroded contacts.
      • Check flywheel trigger gaps.

      Exhaust Related Checks ...

      • Check this is seated properly and that the springs are doing their job.
      • Check there is no obstruction in the exhaust.

      Cold Starting Checks / Issues ...

      • Poor quality fuel - see Introduction.
      • Poor carb setup - see Carbs Related Checks section.
      • Poor electrical / ignition setup - see Electrical / Ignition Related Checks section.
      • Over viscous engine oil - incorrect oil spec / grade.
      • Lead contamination of oil? Are you changing your oil regularly enough if using AVGAS?
      • Defective starter motor
      • Defective starting solenoid
      • Check for loose or damaged flywheel / incorrect installation of flywheel triggers.
      • Check flywheel trigger gaps.
      • Poor quality electrical connections including earth.
      • Starter motor powerful enough? Consider upgrading to the 900watt heavy duty starter motor - just make sure it will fit first!. It is vastly superior in performance to the standard 700watt motor. If you fly a lot in winter and are trying to start on a cold winter mornings it could make a lot of sense.
      • Worm / damaged sprag clutch.
      • Cabling to starter motor too long or fine a gauge? Either may be a cause for an excessive voltage drop in cable.
      • Mis-use of choke (fuel enrichment circuit).
      • Insufficient or excessive use of choke.
      • Mis-use of throttle when choke / fuel enrichment circuit is engaged. Don't use excessive throttle on full choke - too much throttle reduces the effectiveness of the choke.
      • Out of phase crank (after a prop strike).
      • Damaged engine or gearbox bearings.
      • Out of round crankshaft.
      • Out of round propeller shaft.
      • Damaged / badly worn reduction gear set.

      Warm / Hot Starting Checks / Issues ...

      • Poor quality fuel - see Introduction.
      • Poor carb setup - see Carbs Related Checks section.
      • Poor electrical / ignition setup - see Electrical / Ignition Related Checks section.
      • In a tightly cowled installation make sure all your fuel lines are fire sleeved - not only is this safer but also goes a long way to preventing vapour lock from overheating fuel lines. Also make sure all your oil lines are fire sleeved.
      • Make sure that your fuel system has a recirculation line / fuel vapour return line. This should go from mechanical pump back to tank or gascolator depending on installation and should be fitted with a 0.35mm restrictor. This recommended for 912 UL and 912 ULS but is not necessary on 914.
      • Don't use any choke on a hot start.
      • Use only partial choke on a warm start.
      • Fit some header wrap cloth to your exhaust in a cowled installation - obviously not necessary on an open installation on a trike. This will also go a long way to preserving all your hosing and wiring.

      Miscellaneous Tips / FAQ's ...

      • Engine powering off on a hard bounce? Check all your ignition wiring as you may have fatigued / severed cables that appear OK from the outside but are actually broken internally and go open circuit if they happen to get stretched on a hard / bumpy landing. Also continual bumps are not great for maintaining a consistent carb float level as all the fuel in your float bowl sloshes around. Some fuel will inevitably escape through the vent tube thus affect your float level.
      • Sporadic oil temp and CHT? Ensure that instrumentation that relies on a ground is actually correctly ground - double check those connections.
      • CHT too high? Is coolant getting around the system without restrictions. No tight bends, no reductions in cross sectional area of flow. Is your radiator big enough? If you have a tightly cowled engine are you sure there is sufficient airflow out of the radiator. Prop pitch set correct e.g. not too coarse. NOTE CHT sender indicates temperature of the cylinder head not the actual coolant.
      • Erroneous result from Differential Compression Test? - Try running engine upto operating temperature and then test it again. Some debris may have been temporarily preventing a valve from closing correctly.
      • Spark Plugs on cylinder 1 looks black / sooty? When checking spark plug colour / condition do this as quickly as possible after engine has been operating at cruise power rather than once it has been idling for any period of time. There is a tendency for Cylinder 1 to run rich and Cylinder 4 to run lean especially at idle.
      • Sporadically fluctuating oil pressure? - Consider installing the new type of oil pressure relief valve.
      • Can I use Friction Modifiers in my 912/914? Never us friction modifiers in 912 /914 Series engines. They will adversely affect the operation of both the slipper clutch (if you have one) / gearbox and sprag clutch. Once you have put these products into your engine they can be very hard to remove. They may still be there after you have flushed your oil.
      • Oil Level Check? When you check your engine oil level make sure the engine is level and after engine has been freshly gurgled.
      • Suspected blocked carb? Carb pulling the highest vacuum on a carb balance is the one that likely has the blockage so would be the first one to investigate.
      • Using AVGAS Regularly? If you flying habits mean that you use AVGAS regularly you should consider using a lead scavenger like Decalin. Also make sure that you use a semi-synthetic engine oil. A full synthetic does not suspend the lead as well and the lead will separate out of the oil and collect as residue in different areas of the engine.
      • What RPM should my 912 idle at? Once it has reached minimum operating temperature ideally at 1,800 RPM for the benefit of your gearbox. Below this there will be a lot more undesirable gearbox chatter and wear. But won't this result in a lot more tendency to float on? If set up correctly 912UL and 912ULS's should still idle smoothly down to 1,200 / 1,400 RPM. Some folks would argue that 1,400 RPM is OK to set your idle to for minimal float on / better short field performance. This is probably OK once you throttle right back for landing but outside this situation we would recommend using a little throttle to effectively elevate your idle to a minimum of 1,800 RPM whilst taxing or at the hold. Unless a customer asks for a specific RPM we generally set throttle of idle against the stops to 1,550 to 1,600 with the gearbox in mind. 1,200 RPM is perilously close to risking the engine stopping in a vital phase of flight in our opinion and more than 2,000RPM is venturing into the realms of not really idling any more! By virtue of their lower compression ratio 912UL tolerate a lower idle better than their ULS counterpart. The first warm up of the day your RPM should be around 2200 - 2400RPM. Testing your CHT Sender? Rather than test it resistance, try taking it out a putting it in a cup of freshly boiled water. If it reads 100C then it is probably OK.
      • Oil leaks from tappet covers / rocker covers? This may due to perished o-rings but can also be due to there not being a 0.2mm gap set between rocker covers when engine is cold. This gap is necessary to accommodate heat expansion that occurs when engine is at operating temperature. If the rocker covers are touching when they are cold then when the cylinders are up to temperature and have expanded the rocker covers will be stressed / distorted and a leak may result from this. If there is already a gap when cold then the o-rings / rocker cover seal with cylinder heads are the more likely culprit. It is also possible that what looks like a rocker cover leak is actually coming from the push rod covers.
      • Leaking coolant elbows? - Make sure that the o-rings are fitted correctly! The seal on the thread between the angled elbow and the flange often leaks and ideally should be set with Loctite 648. Some engines may have been "sealed" with a weaker Loctite and a leak has resulted. Unlikely to cause you a catastrophic loss of coolant (unless a poor repair attempt has already been made and the elbow thread has been striped) but when old coolant leaks and dries on your crankcase / other engine components etc it doesn't look pretty and is not so easy to remove.
      • 500RPM drop on mag check? Make sure you've not mixed your plugs leads up. Make sure the correct leads are going to the correct cylinder!

      Vital Engine Statistics at a glance ...

      • Fuel pressure minimum for 912 and 912ULS is 0.15bar - 2.2psi.
      • Fuel pressure maximum for 912 and 912ULS is 0.4bar - 5.8psi
      • Delivery pressure of a backup (electrical standby pump) must not exceed 0.3bar.
      • Max CHT for 912 is 150C for 912UL.
      • Max CHT for 912 is 135C for 912ULS.
      • Coolant flow rate at Max RPM 5800 is 60Litres / minute.
      • Coolant over flow bottle should have a 3mm vent hole.
      • Coolant Expansion tank must be full to the brim at all times.
      • Coolant overflow bottle should be approx. 1/3 full when engine is cold.
      • Oil Pressures

        912UL 912ULS 914
        Minimum Oil Pressure 0.8bar 0.8bar 0.8bar
        Maximum Oil Pressure 7.0bar 7.0bar 7.0bar
        Normal Working Range 2.0 - 5.0bar 2.0 - 5.0bar 2.0 - 5.0bar
      • Oil Temperatures

        912UL 912ULS 914
        Minimum Oil Temp 50C 50C 50C
        Maximum Oil Temp 140C 130C 130C
        Normal Working Range 90-110C 90-110C 90-110C
      • Max Oil pump suction pressure 0.3bar
      • Max Crankcase pressure at 5800 RPM 0.45Bar / 6.5PSI
      • Cylinder Firing order 1,4,2,3
      • No spark generated below 220RPM
      • 912 UL and 912ULS - Ignition SMD Circuit A generates spark for Cylinders 1 and 2 Top and 3 and 4 Bottom.
      • 912 UL and 912ULS - Ignition SMD Circuit B generates spark for Cylinders 3 and 4 Top and 1 and 2 Bottom.
      • Red wires from stator power SMD Ignition modules.
      • Yellow wires from stator are the 250W alternator output wires.
      • Flywheel Triggers assignment ... Red -Circuit A - Cylinders 1 and 2
      • Flywheel Triggers assignment ... Blue -Circuit A - Cylinders 3 and 4
      • Flywheel Triggers assignment... Green -Circuit B - Cylinders 3 and 4
      • Flywheel Triggers assignment... White -Circuit B - Cylinders 1 and 2
      • Propeller Gearbox Friction Torque should be in the range 30- 60Nm
      • Slipper Clutch Slipping Torque should be in the range 600- 800Nm
      • Propeller Gearbox gear reduction ratio for the 912UL series? 2.27:1
      • Propeller Gearbox gear reduction ratio for the 912ULS series? 2.43:1
      • Start up timing for the Rotax 912UL and 912ULS is 4 degrees BTDC (but not on the new soft start module) with first generation flywheel.
      • Start up timing for the Rotax 912UL and 912ULS is 3 degrees ATDC (but not on the new soft start module) with new generation flywheel. The new generation flywheel dramatically improves starting performance.
      • Timing during normal operation for the Rotax 912UL and 912ULS 26 degrees BTDC with both types of flywheel.

      More frequently used consumables ...

      • Plugs for 912UL NGK DCPR7E
      • Plugs for 912ULS NGK DCPR8E
      • Plugs for 914 X27EPR-U9 or DPR9EA-9
      • Spark Plug Heatsink Paste Wacker P12

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AeroFix Aviation Ltd
United Kingdom
Telephone: 07779 147 229