|Adjustments you can do to make your engine safer and more reliable.
Working on ultralight aircraft on a daily basis gives you some insight
into the problems associated with the sport. One of the major problems is "lack of
knowledge" about "ultralights." This includes engines. Rotax supplies about
80% of the worlds ultralight aircraft engines.
They come from Rotax complete with carb(s),
exhaust, and gear box, ready to bolt into or onto an ultralight. Up until this point
Rotax has complete control of the "quality of the product." Having been to the
plant in Austria and the distribution centers in Canada and the U.S. I can verify that the
facilities and people involved are # 1 in the industry.
The problems start once the engine is taken out of the box! They start with the new engine
owner and his or her lack of experience with engines. So lets start here and see if we can
help get this new engine, up flying and performing to the best of its abilities. We will
take it from the point in time that the engine is installed "DRY"on the
aircraft. We will deal basically with two different engines air cooled and liquid cooled.
Inverted installation problem with the Rotax liquid cooled engines.
Okay here we go. If your engine is liquid cooled and is in an inverted installation you in
all likelihood have had to drain the oil from the Rotary valve tank and relocate the tank
so that it still sits above the engine.
While this may not seem like a big problem - it can be a very expensive
one if done incorrectly. Rotax liquid cooled engines are designed to
work with the spark plugs facing UP. When you place the plugs facing
down, the Rotary valve chamber inside the engine develops a problem.
It becomes "air locked." Why?
Because the system was developed to work upright - a plate was added to
prevent the oil in the chamber from sloshing around - which could have
resulted in lack of lubrication.
To prevent this a baffle plate was added. When the engine is turned upside
down this baffle plate traps air in the top of the chamber - which can result in damage to
the rotary valve gear if the engine is started. This damage will result in the necessity
to completely tear down the engine!
So to prevent this damage you MUST fill your rotary valve tank while the
engine is sitting with the spark plugs facing up, then turn the engine over and install
Coolant: In order to run a liquid cooled engine we have to install
coolant. Rotax recommends on the two stroke engines that a 50/50 antifreeze and water
mixture be used. The antifreeze MUST be one that is aluminum compatible
and the water used must be distilled. In summer applications I recommend
a 95 % water - 5% antifreeze solution - this is considerably more efficient especially in
You will notice that there is an air bleeder on the one end of the cylinder head this vent
goes back into the rad system. Failure to connect it could result in air being trapped in
the front cylinder resulting in piston seizure.
The radiator reservoir must have an area in it to allow for expansion of the coolant.
It is also recommended that a thermostat be installed at this time - when supplied from
Rotax the engine DOES NOT come with a thermostat.
Checking fan tension on two stroke air-cooled Rotax engines:
It is very
difficult and in some cases nearly impossible to check fan belt tension on some aircraft.
A neat little trick I use is to remove one of the little
mouse ears from the fan cover. I recommend the carb side so that your not
burning yourself on the exhaust checking the fan belt tension. With the mouse ear removed
and the engine turned off you can check the tension using your finger anytime - quickly
Gear Box: Whether you are running an A/B/C or E gear box it requires oil. The
proper way to fill the box is with the engine sitting on the plane LEVEL. There are two
screws on the side of the box the lower screw is your upper fluid level.
Remove the filler cap install 90 weight gear oil until the oil runs our the screw hole,
wait until it stops draining and then reinstall the screw.
Inverted installation problem with the Rotax dual carb 503 engines.
If you are using a dual carb 503 in an inverted installation you must turn the aluminum
intake manifolds over - once again the engine was designed to run spark plugs up.
503 dual carb the inlets - when the engine is upright have a slight upward incline. When
you turn the engine over this incline is NOW down, which necessitates the removal of the
intake, turning it 180 degrees and reinstalling it.
Final Adjustments - Carbs
Okay the next thing we have to adjust is the carbs (dual carb models only). For correct
jetting refer to the Bing Jet chart. For correct air screw
mixture setting refer to the
Bing Jet chart.
Before doing this procedure on dual carb models make sure that the two carbs are level
with each other, you can do this by running a straight edge over the top of both carbs, it
should lay flat on both carbs at the same time!
The idle on all of the Rotax, two stroke engines is 2,000 rpm with the proper prop load
installed. To achieve this back your idle adjustment screws out until they are no longer
touching the carb slide. Now adjust each in an equal amount of turns - I suggest 3 turns
in. This way both slides are at the same location in both carbs at idle.
Next have someone apply throttle SLOWLY while you watch the slides. The slides should BOTH
start up at the same time. If not adjust using the two adjusters located at the top of the
Once you have them starting up at the same time open them all the way up and make
sure that the slide is completely clear at its upper most throttle position. You now know
you have both carbs starting at the same time and that they are both opening up to full
Final Adjustments - Oil pump
To bleed your oil pump - fill your oil tank - which should be 3/4 of an
inch above the pump at the highest angle of attack that you can reach. The pump is GRAVITY
fed from the tank - thus you must always have gravity working for your.
bleeder screw until air stops coming out of the hole and no air can be seen in the line.
Re-install the bleeder screw and cranks the engine over with the ignition turned off until
no air can be seen in the lines going to the cylinders.
To adjust the oil pump: With the throttle
at the idle position the line on the arm of
the oil pump should align with the line on the oil pump body. You adjust this using the
|The line on the arm and the line
on the housing should align at an idle. To adjust use
your oil pump arm cable adjuster.
For break in when using an oil pump it is recommended that in addition to the pump
you use a 100 to 1 gas to oil mixture.
Spark Plugs: Yes I know they are brand new but they still have to be
adjusted. The recommended gap is .015 thou to .019 thou - set them at .015 and they will
wear out to .019. If you are using a spark plug that has an aluminum screw on insert on
the top of the spark plug make sure that it is tight.
These have a habit of vibrating off
- resulting in loss of ignition. They also wear and can cause "arcing" between
the cap and plug. The Rotax supplies plug has a steel cap that does not wear or come loose
- it does cost more.
Starting the engine:
With the engine, fuel pump, carbs, gear box installed and the tach and ignition
wiring connected, the next step is to put the proper prop load on the engine, so that we
can break the engine in. Now the maximum rpm is determined by which engine you have and
how it is equipped. You can get this info from your manual or from this site.
To break the
engine in we need for it to be able to pull about 300 rpm LESS than what it would pull if
it were flying straight and level under full power.
e.g. - A 582 should pull straight and level under full power 6500 rpm - thus for break-in
- with the plane tied down we should be able to pull 6200 rpm.
If you have a ground adjustable prop installed, and the plane securely tied down - start
the engine up let it warm up for a few minutes - then apply a quick full throttle burst of
power. If the engine over revs IMMEDIATELY back the power down and reset the prop to more
If the engine fails to reach 6200 then shut the engine down and repitch the prop - with
less pitch. Repeat this until you reach 6200 rpm - or the rpm that is 2 to 300 rpm lower
than that suggested in your manual.
If you have a standard fixed pitch prop and the engine over revs -
IMMEDIATELY and get another propeller DO NOT proceed with an
improper load or your engine WILL seize up!
If you have a standard fixed pitch prop and the engine under revs - STOP
IMMEDIATELY and get another propeller DO NOT proceed with an
improper load or your engine load up and quit in flight!
With the proper prop load achieved you can now start your break in procedure, as per the
Rotax break in schedule
Also check out Bing Carb tuning and troubleshooting!