Rotax engine service information, problem solving Rotax  aircraft engines.

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Troubleshooting the Rotax ultralight aircraft engines. 

The primary two stroke engine used by current manufactures of ultralight aircraft, in both the United States and Canada is the Rotax engine. This engine is available in horsepower ranging from 9 to 75. Parts and service is available through a vast network of factory trained service centers, and the cost, although creeping up, is still within reason. Another factor contributing to its success is that it can be purchased as a complete propulsion package from one supplier.

Exhaust, carburetor, fuel pump, prop and reduction drive are all obtainable, in various exhaust configurations and gear drive combinations. These factors have all contributed to its acceptance by manufactures and pilots. But like any other product used in today's market, especially one where the use is so varied, problems have occurred.

Some of these problems can be traced directly to the manufacturer of the ultralight, others to the owner, and others to the engine. In an effort to make more pilots aware of some of the associated problems, and their possible solutions, the following is a synopsis of reported problems encountered by Rotax engine users, and some possible solutions.

These are general problems found in most cases with all the Rotax engines, in further reports we will deal with each engine separately.

Fuel Supply: The Mikuni fuel pump is the primary pump used in the ultralight industry, and with the Rotax engine.

The following is a list of items to be considered and adhered to:

  • When mounting the pump make sure that it is no more than 11 inches away from the impulse outlet on the engine.
  • Make sure that it is mounted away from any heat source, and that it is positioned so that the vent on the pump face is facing down.
  • The vacuum line used to connect the pump to the engine impulse outlet is of a much thicker wall thickness than regular gas line, and is not effected by heat.
  • The gas line and vacuum lines are generally exposed to sunlight in most older configuration of ultralights and it is suggested that they be checked on a regular basis for cracks, discoloring, or UV deterioration (Yearly replacement is a good preventative maintenance approach).
  • All lines should be carefully clamped, with clamps that do not pinch the line.
  • The fuel system should have some kind of a water trap, this can be as simple as a loop in the line used to join the two gas tank halves together on a root tube mounted tank.
  • A proper in line filter with a drain cock, or a pickup line located an inch from the bottom of the tank.
  • During initial installation of the tank it should be inspected to make sure that no plastic is present, inside, from the drilling of holes, or from the manufacturing process. More than one pilot has had to land with an engine out only to discover plastic lodged in his fuel pump.
  • A fuel filter should be installed in the fuel system prior to the pump to prevent any foreign material from entering it. If for any reason the filter is removed and is going to be reused make sure that the direction of flow is the same as when it was removed. Several pilot has reported re-installing their fuel filter backwards, depositing all the foreign material collected directly into their pump and carburetor. Most quality filters have arrows showing the direction of fuel flow.
  • When considering fuel tank location, try to make it as close to the engine as possible with the least amount of vertical draw. Although the Mikuni pump has been known to draw fuel vertically over 4 feet the recommended maximum distance is only 24 inches.
  • Make sure all tanks are properly vented. Many ultralight manufacturers use a standard marine fuel tank, with a vent on the handle (or gas cap) that can be screwed in to seal for transportation. The problem here is that the vent can vibrate closed, or can be inadvertently left closed. This allows the engine to run for a short period of time before it runs out of gas, just about the time your 50 feet in the air.
  • Another problem is with gas caps that have a small pin hole in them for a vent, this holes should be checked to make sure it hasn't become plugged. If you have one of these tanks a solution is to drill a small hole in the handle, at its highest point.
  • Another problem with some of these tanks is that they have a steel check valve located in the tank fuel outlet, in some cases this ball is to heavy for the Mikuni fuel pump to lift completely, causing a fuel starvation problem.
  • Make as few restriction in your fuel system as possible. Some pilots have installed electric fuel pumps into their systems, in addition to the Mikuni pump. There are several potential problems with this.
    • First the electric pump is capable of producing considerably more pressure than what the fuel system is set up for.
    • Secondly when the pump is turned off it is an added restriction.
  • One other commonly reported problem is in foreign material entering the system. This can be plastic from the manufacturing process, or simply cork lining used to seal the gas cap. Before installing any gas tank make sure that it is clean and free of any debris.

Fuel: The recommended fuel for the Rotax engine is PREMIUM UNLEADED, mixed with a GOOD QUALITY two stroke oil, the recommended mixture is 50 to 1. Fuel containing ethanol isn't recommended! Nor is the use of Pre-diluted oils recommended.

For best performance this fuel should be fresh, and under no circumstances should premixed fuel be stored in bulk for any length of time, as the oil tends to reduce the octane rating of the fuel in storage.

Spark Plugs: The proper spark plug for use in most of the Rotax engines is the NGK B8ES, or other brand name equivalent, (Champion spark plugs are not recommended, nor are they factory authorized for ultralight use.)

If using a spark plug with an BR/B in its heading, (usually used for ignition noise suppression) such as NGK BR8ES, a resistor cap should not be used. This is because of the double resistance supplied when using both a resistor plug and resistor cap. It is advisable with engines running in the inverted position to use a spark plug with an extended electrode. This aids in stopping spark plug fowling. Look for a BP in its heading (NGK BP 8ES).

When removing or installing the spark plug cap make sure that the cap is turned, in the rotation necessary to tighten the screw on cap on top of the spark plug, otherwise the improper removal of the cap results in the backing off of the screw on cap and the possible loss of spark to the engine, when the spark plug cap falls off.

The recommended gap on the spark plugs is .015 of an inch. These plugs are capable of running for over 20 hours trouble free on a points and condenser engine and up to 50 hours trouble free on a dual CDI ignition engine (if proper fuel and oil are used and the engine is in proper tune). They should be checked on a regular basis however, and replaced at the first sign of a miss or after severe flooding of the engine.

If using a cylinder head temp gauge, removal of the ring on the bottom of the spark plug is recommended, since the gauge sending unit replaces it. (Cylinder head temperatures should range in the 300 to 400 degree F range.)

In practice, a properly tuned engine should show a nice chocolate, or brown color on the spark plug, white indicates an engine running hot, while black indicates one running to rich. Any sign of aluminium on the spark plug indicates a problem of a very serious nature, and requires your immediate attention!

Ignition: The most widely reported problem with the Rotax ignition system is in the failure of the ignition coils, and pickups on the Dual CDI engines. Coil failure and the wearing of the point arms, causing improper timing on the points equipped engines.

The only solution for the coil failures is to replace the coil, it is also a good idea to have an extra one on hand. If ordering an EXTRA coil for the CDI engines order the PTO side coil, it will fit on both sides. The problem with the wear on the point arms generally occurs in the first 10 to 15 hours of operation and if the owners manual recommendation for all engines is followed, the owner will have had his timing checked during this period of time.

On the subject of the owners manual, this is a very complete and concise document, and if read and followed, will eliminate most of the problems associated with some of the most widely reported engine problems.

A video put out by Ultralight Flyer web video magazine which is approximately two hours in length, would be an investment I would recommend. It is available either on the web, in video format, or on DVD.  This video takes you through the complete tear down and reassembly of your Rotax engine, showing proper torquing procedures, how to time your engine, etc. etc. etc., and is very professionally done.

One other problem that has cropped up from time to time is the incorrect installation of the ground and magneto wires on the two poles on the ignition coil, on the points equipped engines. On Rotax engines these wires are locate on opposite sides of the coil, for each cylinder. They are numbered 1 and 15.P On air cooled Rotax engines/B the coil should have the ground wire going to 15 on the PTO side and the magneto points wire going to the 1, while the recoil (mag side) side cylinder should have the ground wire going to the 1 and the magneto point wire going to the 15.P The ignition suppressor box on the air cooled Rotax engines is wired black to black, and red to red.

ON THE LIQUID COOLED 532 ENGINE, the coils are wired PTO side 1 to ground, 15 to points, Recoil (Mag side) 15 to ground 1 to points. P On the 532 the ignition suppressor box is wired BLACK TO RED, AND RED TO BLACK.

During regular preflight it is recommended that the pilot check the ground wires, and the bolts retaining the coils to the engine. The pilot should be looking to see whether the bolts have loosed off, which could result in a coil falling off, or ground wire loosing ground, or to see if the ground wire, where it enters the connector used to retain it to the bolt, is broken or frayed. This is caused by the air turbulence around the wire causing bending it back and forth, which over a period of time results in failure of the end where it connects to the wire.

If a coil is removed for any reason, or the retaining bolts are found to be loose, they must be LOCTITE (using purple 222 loctite) back in. To simply retighten them is asking for problems, since these are self threading bolts, and generally pull threads when they back out.

Another reported problem encountered by pilots with the ignition system on the Rotax engine is in the connecting of a tachometer to the two black leads used to shut the engine off. When connecting a tachometer to the Rotax engine connect it to the two green leads coming from the magneto. These leads are usually connected to two yellow leads, making for a yellow/green, yellow/green combination. To connect the tachometer simply separate the two green leads from their two yellow counterparts and connect the tachometer leads, to two greens.

Under no circumstances should a tachometer be connected to the two black leads. Another problem encountered with the ignition system is with the ignition suppressor box. This box is generally located on the Rotax starter recoil, is a brass color, with two leads, a brown and black, coming out and joining into the ignition system.

When this box fails it can cause a complete loss of spark to one or both coils. A way to quickly check the box is to reverse the leads, if there is spark one way, and not the other then the box is still good. If the box does happen to fail on you, it can be simply disconnected from the system and your spark will return. It is suggested however that you replace the box, at the earliest possible time as it does play an important role in extending the life of your engine.

The final problem encountered in the ignition system of the Rotax engine is in the use of an ignition switch which simply joins the two black leads from the magneto together to shut the engine off.

This is not the recommended switch. In all applications the proper switch is referred to as a four pole switch. This switch allows both black leads to be fed into one side of the switch, while the other side goes to ground. When the switch is operated to shut the engine off both coils are grounded, ensuring a complete ignition shut off.

Carburetor: In most applications the Rotax engine uses a 36mm Bing carburetor. In adapting the Bing for ultralight use several problems have been encountered.

One problem that has been reported is the failure of the retaining clip of the jet needle, located in the carb slide, due to vibration and fatigue . This failure allows the jet needle to fall down into the main jet cutting of the supply of fuel to the engine.

The engine still runs but at an idle. This should be an area of regular inspection.

There are two vent tubes located on the side of the Bing carb. Do not connect any additional hoses to these vents, or vent them outside an enclosed engine installation. By venting outside the engine cowling you create two different air pressure areas, that of the outside air, and that of the air inside the engine cowling, which can effect you fuel mixture.

Recently Rotax has introduced a modification to the vent tubes which requires the installation of a brass fitting, joining the two tubes together. This fitting is equipped with vent holes, and provides for even pressure distribution to both sides of the float chamber. Make sure that these two vent or balance tubes are not pinched off in any way, as it will effect the carb mixture.

For a short period of time Rotax shipped out carburetors which had two pins in the slide throat area. It was found that these pins had a tendency to break off and enter the engine possibly causing piston damage. These carburetors are on recall, and if your craft is equipped with one it is recommended that you contact your nearest authorized Rotax engine service center for a replacement carb body.

Identification of this problem carb can be done by simply looking at the side of the carb opposite the idle adjustment screw, if two pins can be visibly seen then your body needs updating, if a solid block is found you are okay.

Another problem encountered on early model carburetors equipped with a plunger on the side of the carb, was the bending of the float arms.

Because the plunger, when used presses down on one side of the float assembly it was found that over a period of time bending of the float arm occurred resulting in improper fuel levels in the float bowl chamber. If your craft is equipped with this plunger a simple cure is not to use it.

Another problem area is in the top cap on the Bing carb, the one that screws down onto the top of the carb body and retains the throttle cable. In what looks to be the center of the cap is located a screw in fitting, which can be used to adjust the length of throttle cable. In fact the screw in fitting is not located in the center but is off to one side. If installed incorrectly this can result in wear on the throttle cable where it passes through the bottom of the fitting, as the cable now pulls at an angle and rubs against the outside of the fitting during throttle operation.

One other problem on early model Bing carburetors and intake manifolds was that the two aluminium protrusions, which are coupled together with a rubber intake manifold flange, were smooth. This allowed the carb body to rotate in flight, in some cases causing engines to leak excessive amounts of fuel and or simply quit.

Later model engines were kernelled on both the intake and carb sides to prevent this from happening. Also make sure that the rubber intake manifold flange is installed with the arrow on its side pointing towards the engine, for proper fit.

Joining the carburetor to the intake manifold, on the 532 Rotax engine is a Mikuni rubber intake manifold adapter. Two problem areas have been reported with this rubber intake manifold.

The first area is that directly under the clamp used to tighten the rubber manifold to the carb body. It is difficult to locate, since the pilot must remove the carb, and clamp and closing inspect the area. What has been found is separation or tearing of the rubber. This generally causes erratic running, hard starting, and possible engine failure caused by the leaning of the mixture.

The other area of separation is located around the bolt holes used to connect the rubber flange to the intake manifold. Although his area is easier to inspect, it still must be looked at very closely to find the separation. This separation has been reported to be caused by the use of regular unleaded fuel with an ethanol additive which has acted on the rubber, an additional factor is the lack of support for the carb body and breather. In most applications these are all supported by this rubber intake manifold.

To help eliminate this problem it is suggested that some way of supporting the carb/breather be found, and that the pilot use premium unleaded fuel.

Most other Rotax engines use a rubber intake tube to join the carb body to the intake manifold, deterioration of this rubber tube due to exposure to UV, and gas has also been reported, and is an area that pilots should check during regular preflight.

Another reported problem especially in pusher configurations is the air cleaner falling off and coming in contact with the prop. A simple solution is to install a bracket, or clamp, to prevent the airfilter from coming in contact with the prop if it were to come off.

Air Cleaners: The type of air cleaner used on the Rotax engine greatly effects its performance. In fact the engine will not run properly with some air filters. Many pilots have reported being unable to develop full power after installing the ROTAX AIR BOX silencer kit. This kit when used on the Rotax engine requires the rejetting of the engine. My personal recommendation is not to use either the silencer kit or after muffler, on any ultralight where performance is marginal, since the use of either reduces performance.

Several ultralight manufacturers have shipped their kits out using a sponge air filter. This filter is not recommended for most ultralight applications as it is prone to absorb water, especially during early morning, or after rain.

Most current manufacturers are recommending the use of the K& N filter. This filter when maintained correctly has proven to be one of the best on the market.

TUF Click here for K & N Airfilter Service information

Exhaust Systems: Various ultralight pilots have complained about problems with the Rotax exhaust systems. Most have complained about the side mount exhaust splitting at the elbow, and directly along the welded seam.

Others have complained about the baffle inside the exhaust breaking away and clanging around inside the exhaust canister.

Others have complained about the spring retention brackets wearing through, or the springs breaking because of the angle that they have to be mounted to reach the brackets.

The solution to these problems are varied but in most cases are not the fault of the exhaust system.

The first problem can be solved by simply rewelding the exhaust, we have gas welded, arc, welded and tig welded exhaust systems. Most are still flying today with this repair.

The problem is generally caused by rust forming at the exhaust manifold and muffler joint, thereby making this a solid joint rather than a flex joint. The solution is to coat the ball socket with an asbestos silicone available through your nearest Rotax engine service centre.

Another cause is the way the exhaust is mounted, in many cases it is mounted solid, and this causes excessive vibrations and wear leading to breakage, change your mount so the system is rubber mounted and allowed some flexibility.

The baffle problem has been eliminated, in most recent exhaust models, but was a problem in early models, the solution is to open up the canister and reweld the baffle back in, or buy a new exhaust.

The third problem with the exhaust spring mounting tabs is to reweld the tabs so that the springs pull straight not at an angle, and also so that the distance between the two tabs is just enough to tension the springs.

It is also recommended that these springs be safety wired, and that a bead of high temp silicone be run along the spring coils, this prevents a harmonic vibration from setting up and also aids in preventing the spring from separating, if it breaks.

Recoil: Another commonly reported problem is with the Rotax recoil. Pilots have reported the recoil handle, and rope backing out of the recoil housing during flight.

In a pusher configuration this can lead to the hand becoming engaged in the prop. This problem although widely reported is not a problem that happens overnight, in most cases the pilot has noticed for some time that the recoil rope is not going back up immediately after starting the engine, or that he has to pull the handle back and forth several times to get it to go back in.

Usually a good cleaning of the recoil mechanism will cure this problem. A stronger spring is also available, and is recommended if your recoil rope is a longer length than normal, to allow for in cabin starting.

One other reported problem with the recoil is the breaking of the housing, where the rope enters the housing, and in a circle about 4 inches in diameter in the center of the housing.

Both can be identified prior to complete destruction by looking for cracks in these areas. Some recoils can be rewelded while others must be replaced.

Hey, guys and gals if you have had a problem with your ultralight, or engine please drop me a line, include photos or pictures when possible, you could end up saving someone some money, or even possibly their life.

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