By Adam Hunt
On
November 15th 2003 Ruth and I attended the Rotax Two–Stroke
Engine Level 1 Course held at Aero Propulsion Technologies in St Lazare, Quebec.
Daniel Sasseville and Julien Mauroy taught the course. It was well worthwhile
attending!
Here is what we learned,
or at least what we wrote down for course notes:
“The weakness of the two-stroke engine is air temperature and air density”
Rotax has identified 16 reasons for engine failures on two-stroke engines. Every one has been the subject of a Service Bulletin from Rotax to correct these or to educate the operators. These are:
1. The
Inertia Moment of the Propeller exceeding the capability
of the engine. Addressed by a Service Instruction on how to select a propeller
SI 11 UL 91. Ensure your prop does not exceed 3000 kg-cm2 or 1025
lb-in2.
2. Contaminated
Fuel – Usually due to water, bugs or dust. All of these
normally get into the fuel system via air transmission.
3. Storage
And Aging Of Fuel – No fuel should be used if it is more than
three weeks old. Mixing with newer fuel, however, can extend the life of older
fuel. A maximum content of 5% alcohol is acceptable. Alcohol causes the oil to
separate and not do its job as effectively. Alcohol also absorbs water, which
can cause corrosion.
4. Fuel
Pressure And Carb Venting – The fuel supply line must be <11 feet
in length. The fuel pump pulse line must be <11 inches in length and as
short as possible to increase effectiveness. The pulse line must be very rigid
and not capable of being compressed to collapse by the fingers. The fuel pump
diaphragms should be checked annually for deformation. The deformation must be
less than 200% of the diaphragm thickness. The diaphragms are made of Mylar and
are subject to deformation from both use and aging. The carb vent lines should
be a 1800 line with four drilled holes. The holes must total twice
the diameter of the vent tube.
5. Calibration of the Carburetors and the
Jets – To adjust the air idle screw properly screw it in to the bottom of
travel (be careful not to turn it beyond the point of just touching the bottom
to ensure it doesn’t become damaged by over-tightening). Back it out ½ turn for
rough adjustment. This should give 850 – 9500F idle EGT. Backing the
screw out makes the idle leaner. Permissible adjustment of the Rotax 503 is +
¼ turn. Movement of 1/8 of a turn should give a change of 500F.
6. The
Type of Air Filter and the Maintenance Done to It –
Ensure the right type of K&N filter is installed. If the filter has <50%
light visible then it is still serviceable. The filter must be serviced with
the K&N filter service kit. The oil applied not only makes the filter work
correctly but also helps prevent carb icing, by keeping moisture out.
7. The
Brand of Oil Used – The oil is critical to preventing wear in
the engine. Use no-ash oil. Do not use fully synthetic oil, use a mineral-based
oil instead. Recommended is the Sinto Racing Performa Two-Stroke engine
oil. Gearbox oil recommended is the ANSoil 75W140 Synthetic Gear Lube.
8. Engine
Mount and Cowling Blocking Air – The installation must
allow free flow of air through the engine.
9. Any
Inappropriate Cooling System Installed – There have been many
problems with adapting automotive or other radiators for the Rotax liquid
cooled engines. These are not designed for the same purpose.
10.
Insufficient Time For Engine Warm-Up –
To avoid cold seizures on take-off, air-cooled engines require 8 minutes of
warm up and a CHT of not less than 2500F
11.
Engine Mount Design –
Ensure that the engine mount does effectively reduce vibration.
12.
Improper Monitoring of Engine Systems –
Using the wrong engine instruments or not using engine instruments at all to
monitor the health of the engine.
13.
Exhaust System –
Modification or leaking system. The exhaust system is a tuned system and it
must be installed right and work right to enable to engine to produce any power
at all.
14.
Corrosion prevention –
Follow the Rotax recommendations regarding engine corrosion prevention and
preservation. Use the choke to stop the engine on the last flight of each day
to ensure a good coating of oil is deposited in the crankcase. Block the intake
and exhaust anytime the aircraft is not flying.
15.
Maintenance Schedule –
You need a comprehensive maintenance schedule to ensure that all the correct
items are taken care of when they need to be done. This cannot be done by
memory.
16.
Preflight Inspection –
Before every flight do a preflight inspection to look for problems. The
inspection at the beginning of the day should be more extensive.
Compression Test – if conducting a
compression test there should be a maximum of 10% difference between cylinders.
Cylinders normally average 120 lbs. 90 lbs is the minimum.
Setting the Idle With Dual Carbs –
The large screw on the carb is the idle speed adjustment. Verify the height of
the slider with a drill bit – must be the same height. Idle should be set to
2000-2200 rpm. Both sliders must reach the top of the opening at the same time.
Use the cable screw adjustment to achieve this. The cable should have 1/16”
slack at idle. To ensure both sliders are closed.
The idle
adjustment needs doing often due to wear and cable stretch. Start by adjusting
the idle – move the screw in until first slider movement, then increase one
full turn. Then check the full throttle adjustment.
Next adjust
while running, using EGT as a guide.
At full
power the EGT should indicate 1050 – 11000F, but not lower than 10000F
and not over 12000F.
The engine
runs with an EGT hotspot between 4100-4600 rpm by design. The ideal value
(temperature) for this range is 11000F - 12000F, but not
above 12000F, of course! Adjust using the jet needle. Each notch on
the needle is 500F warmer (moving up the needle) or cooler (moving
down).
On the
Rotax 503 the difference between cylinders should be <500F. Check
this with a vacuum gauge or by EGT once the idle is set. Then adjust to eliminate
the EGT differential at cruise with adjustment to the cable screw.
The Rotax intake silencer
will reduce horsepower by about 2-3%, while the after muffler will not cost any
power. The total noise reduction possible is about 20% from both of these. The
intake silencer makes a bigger difference in total noise while the after
muffler clips the peaks on the exhaust pulses in the noise output. The intake
silencer and the after muffler can be installed independently.
The EGT
probe fittings in the Rotax exhaust manifold are actually at the hottest place
in the exhaust system. This is due to the fact that the hot exhaust keeps
burning after it leaves the cylinder.
EGT probes
should be set to the same height to ensure that they give the same readings.
The carbs
are designed to angle slightly down when viewed from the side. If they don’t do
this then the intake manifold needs removing and reversing. The down-angle is
to prevent flooding of the carbs. The rubber carb mounting sockets are actually
vibration isolation for the carbs to help prevent foaming in the float
chambers.
The correct
spark plugs for the upright Rotax 503 are the BR8ES plugs. Use the ones with
the solid core, not the spring core, as these ones do not retain the spark plug
lead caps!
Do not clean spark plus –
replace them.
The spark
advance is set to 40 advance at idle and 260 at full
power. If the spark timing is out by more than 0.004” then the crankshaft is bent
and requires replacing.
Spark plugs
should be installed with Permatex 22064 Dielectric Tune-Up Grease to
prevent seizing.
Do not turn
over the engine without all sparkplugs grounded, to prevent damage to the
diodes in the dual electronic ignition units.
The small
regulator/rectifier requires a minimum 1A load to operate. This can be achieved
by the use of a light.
The fan belt needs checking regularly as per the maintenance schedule. The central retaining nut should be safetied with Locktite 221/222 (Purple).
The fan can
only be safely removed with the special tool available from Rotax. The fan
shims are stored behind the pulley, on the fan shaft.
Exhaust system springs should not touch the exhaust pipes, as that will prevent them from doing their job. The spring brackets can be bent to raise them to ensure that the springs clear the pipes.
The springs
should be lockwired to prevent loss of the springs and ensuing prop damage in pusher
installations. The lockwire should be loose enough to allow the springs to
move. Use a screwdriver under the lockwire when installing it to ensure that
the wire is loose. Use a bead of hi-temp silicone on the springs to absorb
vibrations and also to help retain any parts should the springs break.
There are
two spring lengths available from Rotax. Springs can be stretched if required
to fit. A proper spring fit means that the springs should only be stretched
with a pull of 1/8 – ¼” on installation.
The exhaust
manifolds should be mounted with just two screws even though there are four
holes. This is to allow flexing in the system due to thermo-expansion.
Apply anti-seize
compound to the exhaust ball joints every ten hours on a new installation for
the first 40 hours to ensure a good application.
The CHT
probes should be routed under the cowling and then in between the fins to
provide the most secure routing.
The “B”
Gearbox requires a check of the spring washers every 100 hours. This check
requires a hydraulic press to accomplish and so is best done by an engine
maintenance shop.
Our Many Thanks to Daniel and Julien – it was
a great course – we are looking forward to doing the Level 2 course!