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