Gobler-Hirthmotoren KG 3503 User manual

Operator's Manual
Engine
3503
Göbler Hirthmotoren KG
Max-Eyth-Str. 10
71726 Benningen
Deutschland Germany
Tel.: [phone]; Fax: [phone] Phone:[phone]; Fax:
0049-7144-5415
e-mail: [e-mail] , Internet: www.hirth-engines.de

Operator's Manual
Engine
Model 3503
Read this operator's manual thoroughly before putting the engine into operation for the first time and comply
strictly with the instructions given here.
In the interests of continual development of our engines we must reserve the right to change conditions of
delivery for design, engineering and fixtures. We also request your understanding that no claims can be made
against statements and Figures from this manual.
THIS ENGINE DOES NOT COMPLY WITH FEDERAL SAFETY REGULATIONS FOR
STANDARD AIRCRAFT. THIS ENGINE CAN BE USED IN EXPERIMENTAL AND ULTRA-
LIGHT UNCERTIFIED AIRCRAFT ONLY IN CIRCUMSTANCES WHICH AN ENGINE FAILURE
WILL NOT COMPROMISE SAFETY. BEFORE OPERATING THE ENGINE READ OPERATOR'S
MANUAL INFORMATION AVAILABLE FROM YOUR AUTHORISED HIRTH DISTRIBUTOR.

Table of Contents
Germany.........................................................................................................1
Phone:[phone]; Fax: [phone]..............................................................1
1. Description of the Engine, Mounting Instructions and Technical Data...............5
1.1. Overview.....................................................................................................5
1.2. Description of the carburation system.........................................................5
1.3. Description of the Ignition System...............................................................6
1.4. Description of the cooling system ...............................................................7
2. Model Identification plate................................................................................... 7
3. Installation instructions....................................................................................... 8
3.1. Mounting and securing the engine..............................................................8
3.2. The engine’s air supply...............................................................................8
3.3. The engine’s fuel system and fuel supply...................................................8
3.4. The operation of dual carburettors .............................................................9
4. Monitoring Apparatus.......................................................................................10
4.1. Exhaust gas temperature .........................................................................10
4.2. Fuel Pressure............................................................................................ 11
4.3. Coolant Temperature Gauge.................................................................... 11
5. Instructions for assembling the coolant system ...............................................12
5.1. Operation..................................................................................................12
6. Specifications – 3503 engine...........................................................................14
Operation of the Engine......................................................................................14
6.1. General.....................................................................................................14
6.2. Running in - Recommendation ................................................................. 14
6.3. Initial Inspection........................................................................................15
6.4. Starting Procedure.................................................................................... 15
6.5. Operating condition of the engine............................................................. 15
6.6. Powering down the engine........................................................................ 16
Maintenance .......................................................................................................16
6.7. General.....................................................................................................16
6.8. Tools, special tools and torques ...............................................................16
7. Maintenance rate - Daily inspections...............................................................17
Replacement of Parts .........................................................................................17
7.1. Maintenance instructions - Carburettors...................................................18
7.2. Synchronisation of dual Dellorto carburettors...........................................18
7.3. The basic setting of the mixture regulating screw.....................................18
7.4. Cleaning of the carburettors......................................................................18
7.5. Inspecting the air filters.............................................................................18
7.6. Inspecting the fuel lines ............................................................................18
7.7. Inspecting pulsation lines..........................................................................18
7.8. Spark plugs and sparkplug leads..............................................................19
7.9. Inspecting the condition of the sparkplug leads........................................19
7.10. Installing and removing sparkplugs.........................................................19
7.11. Condition of the sparkplug......................................................................19
8. Coolant Pump..................................................................................................19
8.1. Changing the coolant pump V belt............................................................20
9. Trouble Shooting.............................................................................................. 21
9.1. Engine does not start or is difficult to start................................................ 21
9.2. Engine will not Idle.................................................................................... 21
9.3. Engine Runs Rough, Uneven, and with Insufficient Power.......................21
9.4. Engine does not develop full power.......................................................... 22

9.5. Excessive Liquid Temperature..................................................................22
10. Exhaust System.............................................................................................23
10.1. Exhaust parts and measurements .......................................................... 24
11. Wiring Diagrams ............................................................................................25
11.1. Single PVL Ignition system - 3503.......................................................... 26
11.2. Dual PVL ignition system – 3503............................................................ 27

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1. Description of the Engine, Mounting Instructions and Technical Data
1.1. Overview
The 3503 engine (Figure 1.1.1-1) of Göbler-Hirthmotoren KG is a two cylinder in-line two-stroke liquid
cooled engine.
The cylinders consist of an aluminium alloy with a Nikasil coated running surface that is highly resistant to
wear. The cylinders are mounted onto the crankcase by stud bolts, discs and hexagon nuts. The cylinder
heads consist of a special hypereutectic aluminium alloy that resists high temperatures while maintaining
constant hardness. They are secured onto the cylinder by means of cylinder screws and washer using ‘O’ ring
seals between the head and the cylinder.
The pistons consist of an aluminium alloy and are sealed off from the cylinder running surface by two piston
rings. The piston is connected to the con-rod by the gudgeon pin and a needle bearing.
The crankshaft is made of chrome-molybdenum steel (42 Cr Mo 4) and is mounted in five deep groove ball
bearings. It‘s an assembled crankshaft and the individual parts are pressed together to form a complete unit.
Two deep groove ball bearings are installed on the PTO drive side, two between the con-rods and one on the
ignition side. The drive shaft is tapered and has a centred thread in order to secure to the PTO shaft. The
con-rods are connected to the crankshaft by crank pins and needle bearings.
The crankcase consists of an aluminium alloy and constructed as a case in two halves. It is secured together
by eight fastening studs and nuts which secure the cylinders to the case. Opposite the PTO end of the
crankcase, there is an ignition stator unit. This is fixed to the crankcase and is concealed by the flywheel and
the fan housing..
Figure 1.1.1-1 (Engine 3503)
1.2. Description of the carburation system
The carburation system (Figure 1.1.2-1) of the 3503 engine consists of two Dellorto carburettors. Each
carburettor is connected to the inlet aperture of the crankcase by a rubber flange and is secured to the inlet
aperture by means of a clamp held in a rubber flange. The carburettors have a dry air filters installed.

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Figure 1.1.2-1 (Dual Dellorto Carburation System)
1.3. Description of the Ignition System
The ignition system (Figure 1.3-1, 1.3-2) consists of an armature plate, a magneto, E-Box(s), ignition coils
and the ignition cables with the required number of spark plug leads. The ignition system is completely
electronic and possesses an E-PROM for a freely programmable ignition curve however, this can only be
carried out by the manufacturer. Alternating Current (AC) power for electrical components during running
and charging of the battery is provided. The engine is available with a single or dual ignition system
installed.
An electronic stator plate is secured on the ignition side with the crankcase. A magneto is fixed on the
crankshaft and encloses the armature plate. The fan housing covers the ignition system’s magnetic wheel.
The electronic E-box and ignition coils are mounted on a fixture plate secured to the crankcase as shown in
figure 1.3.3.
Figure 1.3-1 (Single PVL ignition)
Figure 1.1.3-2 (Dual PVL ignition)

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Figure 1.3-3 (View of ignition side with installed ignition system)
1.4. Description of the cooling system
The 3503 engine has a mechanically operated liquid cooling system. (Figure 1.4-1).
The liquid pump is installed above the flywheel housing. It is driven by a V belt pulley that is fixed to the
ignition system’s magnetic flywheel. Coolant is pumped from the radiator system into the cylinder water
jackets then to the cylinder heads. The hot coolant then exits the outlet of the cylinder returning back to the
radiator system. The temperature of the cooled liquid is controlled by a thermostat located between the
radiator and the inlet of the liquid pump.
Figure 1.4-1 (View of the fan housing and the liquid coolant pump)
2. Model Identification plate
The model plate is attached to the intake side on the upper part of the crankcase. This also includes the serial
number of the engine and must be quoted should a warranty claim be required.
Figure 3.5-1 (Position of the Model Plate)

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3. Installation instructions
3.1. Mounting and securing the engine
The six screw threads (M10) located on the underside of the 3503 engine crankcase is for fastening the
engine to a suitable engine mount.
Figure 2.1-1 (Fastening thread points)
The engine should be mounted so that the moment from the engine to the mounts can be absorbed as widely
as possible. The absorbing of the mounts should also be made as hard as possible, since the engine can
become unstable if these mounts are too soft. This also leads to problems within the carburettors fuel bowl as
well as uncontrolled vibration of the fuel pump diaphragm, intimating to an unsafe operation of the engine.
- Suggestion for securing the engine:
The engine is secured to a fastening plate that is about twice the width of the engine (the wider the better).
On the two outer surfaces of the fastening plate six (or more) hard rubber absorber elements (three on each
side) are attached in an axial direction. This complete substructure is secured by the rubber absorber
elements on to a corresponding base plate.
3.2. The engine’s air supply
The engine must be installed so the cooling air stream fed to the engine radiator is sufficient to ensure the
engine is cooled and that air is also supplied to the carburettors.
3.3. The engine’s fuel system and fuel supply
The engine’s fuel supply is provided by a Mikuni diaphragm pump (Figure 2.3-1) that is driven by pressure
pulsation from the crankcase.
This diaphragm pump is positioned in the fuel line between the fuel tank and Dellorto carburettor(s). A fuel
filter should always be installed between the fuel pump and fuel tank.
The Mikuni fuel pump should be secured to a location with the least influence of vibration or heat. It is
important that it is installed upright with the fuel outlet at the top. The assignment of the fuel connection
direction can be identified on the housing by arrows.
The central connection of the Mikuni pump is connected to the vacuum impulse line from the crankcase and
should be as short and straight as possible. When choosing the impulse line and the fuel line it is important to
ensure that rigid lines are used and do not expand under pressure. The impulse line should not exceed a
maximum length of 150mm with a minimum inner diameter of 6 mm.

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Figure 2.3-1 (Fuel-diaphragm pump with impulse connection)
The positioning of the fuel tank over the engine is advantageous as would ensure a reasonable fuel supply
pressure. It is recommended to maintain a consistent fuel pressure under all conditions. Additional
monitoring of fuel pressure by means of a fuel pressure gauge may also be considered.
When installing the fuel tank beneath the engine, the maximum fuel line length between the fuel tank and
diaphragm pump should not exceed 2,000mm. The maximum suction head should not exceed 1,000m with a
minimum inner diameter of 6mm.
Be aware of the possible gradient of the vehicle the engine is installed in during operation i.e. ascent-/descent
with aircraft, uphill-/downhill with ground vehicles.
The maximum length of the fuel outlet pressure line should not exceed 500 mm with a minimum inner
diameter of 6 mm. A maximum pressure head between the centre of the diaphragm pump and centre of the
carburettor float chamber may not be exceeded.
If a Mikuni pump is installed on the engine, it is recommended the diaphragm of the pump stands vertical to
the crankshaft axis (drive or ignition side ). This will ensure that resonance produced by the crankshaft axis
from the engine does not impair the action and function of the fuel pump.
3.4. The operation of dual carburettors
The throttle valves of the Dellorto carburettors are operated via two Bowden type cables. It is advantageous
to run both cables from the throttle into a single cable by means of a Y-shunt or cable splitter.
If at all possible, both throttle cables between the Y-shunt and the carburettors should be the same length.
In idling position (both throttle valves sit on the idle – adjusting screw) there should still be enough play in
the throttle cable to ensure that the throttle valves can reach the idle position stop.
It is essential to ensure that both throttle valves in the Dellorto carburettors are completely synchronised.
The idling position of the throttle valves must be equal in both carburettors. To do this, use the idle adjusting
screw to bring the throttle valves to the same level in idle position.
Measure and match the size of the aperture of both slide valves.
When the slide valves are drawn upwards they should lift simultaneously from the idle-adjusting screws and
open the outlet together.
Securing the spark plug leads in a hanging (inverted) installation
If the engine is installed in a hanging or inverted position, the sparkplug leads should be secured so that any
breaking or interruption of contact by engine vibrations is prevented (support or secure by wires).

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Switching off the Ignition System (dual ignition)
It is essential that both ignition circuit breakers are located on separate grounding switches. It is important
each earthing wire is installed separately to avoid circuit failure. This method also aids the operator to check
each ignition side when the engine is operating.
4. Monitoring Apparatus
It is recommended that the coolant temperatures, the exhaust gas temperature and the fuel pressure be
monitored.
The basic setting of the engine guarantees a problem-free engine function under the general installation
conditions. It is very important for safe running of the engine that the cylinder head, exhaust gas temperature
and the fuel pressure are monitored.
The equipment listed in Table 3-1 can be purchased from Göbler-Hirthmotoren KG.
Table 3-1
Description Part No.
Westach Exhaust gas temperature measuring device (2x, as a set) 029.30
Westach Coolant temperature devise 029.36
Westach Fuel pressure measuring device 029.20
Westach Tachometer 029.14
4.1. Exhaust gas temperature
A gauge that we recommend on all Hirth two stroke engines is the Westach Exhaust Gas Temperature gauge
(EGT) as in figure 3.2.1. This gauge is a reliable measurement tool that measures the exhaust gases as they
exit the engine. A rich fuel/air mixture which can be caused by improper jetting, clogged air filter, improper
prop setting, will generally show up as a cooler temperature. A too lean mixture can be caused by an air leak
or fuel blockage, can result in overheating and engine seizure.
Figure 3.2.1 Westach Dual EGT 029.30
The proper location of the EGT probe on the exhaust manifold is extremely important. Mounting the probe
too close or too far from the piston will result in an incorrect reading.
If the engine is installed with one carburettor, the probe needs to be located in the centre of one of the
exhaust manifold ports 85mm from the exhaust outlet of the engine cylinder exhaust port. If the engine is
installed with two carburettors, each probe needs to be located in the centre of each of the exhaust manifold
port, 85mm from the exhaust outlet of the engine cylinder exhaust port. Install the probe before reinstalling
the manifold and make sure the probe will not touch the other side of the manifold wall.
The 50mm round EGT the gauge will mount from the front of the panel and will require a 52.388mm round
hole, a U bracket holds it in place from the rear.

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The EGT gauges require probes which come with 100mm of lead. To install the probe, drill a 4.763mm hole
in the centre of the exhaust pipe at the recommended distance above. Install the probe into the hole, and
tighten up the clamp. As an option, manifolds are provided to suit the "bayonet probe" type. The terminal
cable can be lengthened to a maximum of 4.5 metres without affecting the accuracy of the instrument.
Route all wires away from the manifold and other heat sources. Support the wires every 300mm intervals,
using tie wire or similar. When going through firewalls etc, use rubber grommets to prevent damaging the
wiring. Do not route these wires with any wires from an AC source, such as a lighting coil, spark wires, tech
wires, etc. The EGT operates on .004 millivolts. Running near an AC source will result in incorrect
readings.
It is important that you verify the EGT readings by careful examination of the spark plugs. A reading on an
EGT of 1100 to 1200 degrees F should give a nice tan coloured spark plug. Lower readings should give
darker colours while higher readings grey/white colours. Once you have verified a spark plug colour and seen
where your gauge reads with the proper colour then it is now more important to watch for a CHANGE in the
gauge reading from this "normal reading."
It is important to set your engine up for proper fuel to air mixture, your EGT gauge and probes can aid in
doing this. A lean mixture will run hotter on the EGT. A rich mixture will run cooler on the EGT but will
make increasingly less power. Ideally the highest EGT temps should come in at settings usually 5200 to 5800
rpm.
Reason for EGT temperature changes include:
•Air intake leak.
•Air fuel mixture too lean
•Air fuel mixture too rich
•Changes in density altitude
•Changes in relative humidity
•Changes in air temperature
•Pre-ignition or detonation
•Fault spark plug or ignition system
•Improper prop load
•Poor quality or grade of fuel
•Improper fuel air ratio, plugged main jet, failed needle jet, or clip
4.2. Fuel Pressure
As mentioned previously, it is recommended to maintain a constant fuel pressure under all conditions.
Additional monitoring of fuel pressure by means of a fuel pressure gauge may also be considered. One such
suitable gauge is manufactured by Westach and is available for pressure of 16psi and 80psi.
Figure 3.3.1 Westach fuel pressure gauge 029.20/029.21
4.3. Coolant Temperature Gauge
The temperature probe is located on top of the cylinder head. Replace the hex screw on the cylinder head that
exits the hot coolant to the radiator system for a correct reading.

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Figure 34.1 Westach Liquid Coolant Temperature gauge 029.36
5. Instructions for assembling the coolant system
1. Compensation tank
2. Filler cap with integrated expansion tank
3. Reduction with throttle
4. Thermostat
5. Ventalation screws
5.1. Operation
The pump forces the coolant into the cylinder jacket the passes into the heads. From which it flows back via
the bypass through the thermostat into the coolant pump. By throttling the flow speed, the reduction resulted
in a faster heating of the engine. As soon as the operating temperature is reached, the thermostat opens the
inlet and closes the bypass opening. Now the cooling liquid is flowing from the engine through the radiator
via the thermostat into the pump.
•The diameter of the hose must be at least 22 mm (internal diameter)
•The material used for the hose must be resistant against cooling withstanding an operation
temperature of 120°C continuously and 140°C shortly with a system pressure of 1.5bar (21.7psi).
•Take care not to exchange the connections of the thermostat.
•Install reducer in the bypass lead.
•Mount the water filler cap on the topmost point of the cooling system.
•Take care to install the cooling system in a way that steam bubbles arising in the engine can escape
into the expansion tank of the filler cap.
•The compensation tank must be placed at a maximum of 200mm below the filler cap.

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•Take care to operate the engine in warmer climates with anti-freezing solution (for aluminium
engines) (protection against corrosion)
•The water filling cap must be filled to the brim.
•In cold conditions the compensation tank must be filled about one third of coolant.
•Take care to vent the cooling system thoroughly before the engine is started,
•Take care to warm up the engine at mid speed (about 3000 rpm) until the coolant temperature is
decreasing (because of retarded thermostat opening the temperature shortly can be more than 90°C)
•When the engine has run for some time it is necessary to check the level of the coolant water and
top up if necessary.
•The coolant temperature should not exceed 118°C.

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6. Specifications – 3503 engine
Table 4 .1
Manufacturer Göbler-Hirthmotoren KG
Model 3503
Operating mode Two stroke
Number of cylinders Two in line
Piston Capacity 625 cm
3
(38.1 cu in)
Stroke Length 69 mm (2.72 in)
Bore 76mm (2.99 in)
Compression ratio 9.5:1
Performance 51.5 kW (70 HP) @ 6,500 rpm.
RPM, max. 6500 rpm
Direction of revs Left, looking towards the drive shaft
Starter Electric Starter and/or Recoil Starter
Ignition System PVL CDI system, single or dual
Generator 250 W, 12 V
Sparkplugs BR 8 ES (NGK) WR 4 CC (Bosch), W 24 ERS (ND)
Ignition Timing 18
0
v. OT (at 2000 rpm)
Carburation 2 DELLORTO PHBE 34 BD 6832, 2 Air filters
Cooling Liquid cooling
Mixing 1:50
Fuel Premium unleaded, 95 Octane
Two Stroke oil Branded two stroke oil for hot (air-cooled) engines
Cylinder head temperature Max
Exhaust gas temperature max 680
0
C / 1256
0
F
Fuel pressure min. 0.3 bar / 5.8 psi
Engine weight with exhaust System 36kg / 79lbs
Operation of the Engine
6.1. General
It is important to observe the instructions provided in this chapter in order to maintain a safe and economical
operation of the engine.
•USE ONLY THE PRESCRIBED TYPE OF TWO STROKE OIL AND THE PROPER GRADE
OF FUEL.
•INSURE THAT FUEL IS MIXED IN THE CORRECT GAS/OIL RATIO. REMOVE THE FUEL
FROM THE CARBURETOR BOWLS AND FUEL TANK WHENEVER THE ENGINE IS NOT
GOING TO BE USED FOR AN EXTENDED PERIOD. AFTER A LENGTHY STANDDOWN,
REMOVE ANY LEFTOVER FUEL IN CARBURETOR BOWLS AND FUEL TANK BEFORE
OPERATING THE ENGINE.
•TAKE CARE THAT THE AIR CIRCULATION IS SUFFICIENT SUPPLY THE
CARBURETORS.
•ENSURE THAT HEATED EXHAUST AIR FROM THE COOLING SYSTEM IS NOT SUCKED
INTO THE CARBURETORS.
•USE ONLY RESISTOR TYPE SPARK PLUGS AND RESISTOR PLUG CAPS.
•ALWAYS ALLOW THE ENGINE A WARMUP AND COOL DOWN PHASE:
6.2. Running in - Recommendation
Göbler-Hirthmotoren KG does not give explicit instructions for running in its engines, since the
nickel-silicium coating of the cylinder and the aluminium pistons do not have to wear to each other. The
engines are ready for full throttle from the time they are built if the maximum temperatures of the cylinder
head and exhaust gases mentioned earlier are not exceeded.

Page 15 of 27
However, it would be wise when the engine is first started, to run the motor at half throttle and load (not over
4000 rpm) for the first hour of operation. Please note, the engine has been initially run at the factory to set the
electronic timing so it is possible when the engine is first started, there may be more smoke than usual. This
is generally due to excess oil possibly still in the engine during assembly and will reduce to normal after the
engine has been running for a couple of minutes.
6.3. Initial Inspection
After the first 10 - 15 hours in operation or after a large service the following service tasks should be carried
out:
- Tighten the engine fitting screws as a safety measure
- Re-torque the cylinder head screws as a safety measure
- Tighten the sparkplugs as a safety measure
- Visual check (leakage, loose nuts/screws etc.)
Note:
It is recommended that the following inspection be carried out for safety reasons before each use of the
engine:
•Starter (check connections, fitting and condition)
•Carburettors (check connections, fixtures and condition)
•Ignition system (check connections, fitting and condition)
•Liquid cooling system (check fitting and condition; check pump fan belts for condition and tension)
•Cylinder head (check connections, fitting and condition)
•Cylinders (check fixtures and condition)
•Exhaust gas manifold (check fitting and condition)
•Exhaust system (check fixtures and condition)
•Sparkplugs and sparkplug leads (check fitting and condition)
•Fuel system (check for leakage and dirty filter)
6.4. Starting Procedure
•Secure the fuel supply (fill the tank, open the fuel cock,...)
•If the engine is cold, engage the choke(s).If the engine is warm or hot do not engage the choke.
•Move the throttle lever into a fast idle position.
•Switch on the ignition system (ON-Position of the ignition switch)
•Start the engine (Activate starter maximum 10 seconds)
•When the engine has started, reduce the choke slowly to the off position and apply a small amount
of throttle.
•Allow the engine to warm up at a ¼ load or 3000 rpm for about 3 minutes.
•The engine should now be ready for operation.
Please note: If the engine has a centrifugal clutch installed within the drive system, then engine should be
run with the clutch firmly engaged to avoid unnecessary wear of the clutch lining.
6.5. Operating condition of the engine
While the engine is running, the liquid coolant temperatures and the exhaust gas temperatures of both
cylinders should be monitored. So long as the maximum temperatures of the coolant and/or exhaust
temperature are not exceeded, the engine can be run without a time limit in the set load range (also full load

Page 16 of 27
at maximum performance). It is further recommended during operation to monitor the fuel pressure. Lack of
fuel pressure will lean the fuel mixture and lead to a loss of performance and cause the engine to fail.
6.6. Powering down the engine
Before the engine is switched off, it should be run for a short period at fast idling revs. This will allow the
cooling system to reduce the temperature of the engine gradually without stressing any components and
overheating.
Maintenance
6.7. General
This chapter contains the necessary directions to allow technically qualified persons to carry out the smaller
repairs and inspections such as:
•Periodic inspections of the engine
•Maintenance of engine parts
•Troubleshooting
6.8. Tools, special tools and torques
The specifications and sizes of tools are given according to the metric system and can be procured
commercially in specialty shops.
Tools:
Description Size Description Size
Adjustable wrench 7 Adjustable wrench 8
Adjustable wrench 10 Adjustable wrench 13
Hexagon socket screw key 3 Hexagon socket screw key 4
Hexagon socket screw key 5 Hexagon socket screw key 6
Sparkplug wrench Sparkplug wrench 20.6/20.8
Torque wrench 0 -100 Nm Screwdriver
Special Tools:
The special tools are available for purchase under the indicated G.-H. numbers from AATI Pty Ltd or a Hirth
distributor.
Description GH-Number
Magnetic wheel - extractor W 138
(Piston ring clip (Piston –aid in assembly)) W 108/15
Torque settings
Description Initial Torque Description Initial Torque
M4 x 2.8 – 3.3 Nm M5 x- 4 –4.4 Nm
M6 x 9 -11 Nm M8 x 23-28Nm
M16 x 70-80 Nm Sparkplug 16 Nm

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7. Maintenance rate - Daily inspections
Before operating the engine, a thorough daily inspection should be carried out. Observe the following points:
1. Air filter - check condition and installation
2. Starter - check condition and installation
3. Ignition system - check condition, fastening, and connections
4. Pump V belt - check condition and belt tension
5. Cowling - check overall condition and the fasteners
6. Crank case and cylinders - check for leaks overall condition of installation
7. Spark plug caps - check condition and the connections
Periodic Inspections
Inspection Rate 25 h 100 h 500 h
Cooling:
Airway: check for free flow and freedom from pollutants X X X
Check V belts for deterioration and damage X
Carburettors:
Clean the air filters X X X
Check the carburettor setting X X X
Clean the carburettors X X X
Fuel system:
Clean the fuel filters X X X
Check fuel lines and –connections X X X
Check the impulse line to the diaphragm pump X X X
Ignition:
Clean the spark plugs X X X
Check the sparkplug air gaps X X X
Exhaust system:
Check for firm fitting and deterioration X X
Check outlet ports for carbonisation X X
Replacement of Parts
The parts listed in Table 3.3-1 should be replaced after the specified number of operating hours.
Consideration should be given to components which may show signs of premature failure and replaced
accordingly.
Table 7-3-1
Number Designation Order number Operating Hours
2 Sparkplug Single 023.29 50
4 Sparkplug Dual 023.28 50
2 Sparkplug cap 024.22 100
4 Sparkplug cap 024.22 100
2 Piston set 1000
1 Crankshaft 271 AC1U 1000
2 Cylinder Head Single ignition 1000
2 Cylinder head Dual ignition 1000
2 Cylinder 1000
2 Carburettors Dellorto 54/38/141 1000
2 Air filter 066.16 100

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7.1. Maintenance instructions - Carburettors
The Dellorto carburettor jets are jetted by the manufacturer so the engine is guaranteed to run thermally
secure. Tampering with the jet systems of the carburettors can lead to overheating and damage to the motor
and should be avoided. Only original replacement parts should be used. Information concerning the
carburettor assembly may be obtained from AATI Pty Ltd or your Hirth distributor. Should any parts be
required, the carburettor specification should be quoted. This is identified on the side of the carburettor
intake (…/…).
7.2. Synchronisation of dual Dellorto carburettors
In order to ensure performance and smooth running of the engine, the two carburettors must be synchronised
precisely. Note that the idle adjustment screw holds the slide in the same position on both carburettors.
Measure and adjust the opening height on both carburettors for this. The two cables which move the slide in
the carburettor usually come together in a Y fitting leading to a single throttle cable. The cables between the
Y fitting and the carburettors need to be adjusted so that there is some play in the cable at the idle position.
This will insure the proper idle position of the slides. When the throttle cable is pulled, the slides of the
carburettors should move upward simultaneously from the idle position. Lubricate cables if difficult to
operate. Defective or frayed cables have to be exchanged for new ones immediately. Bent cable must be
changed immediately.
7.3. The basic setting of the mixture regulating screw
The idle mix regulating screw in the basic setting is opened half a turn in both carburettors. To do this, screw
the idle mix regulating screw lightly in towards the stop, then open half a turn.
7.4. Cleaning of the carburettors
Turn off the fuel valve before cleaning carburettors. Open the spring clip which holds the float chamber
against the carburettor. Move float chamber downward carefully since it is filled with fuel. Remove the fuel
from float chamber along with any debris. Should debris be in float chamber, the fuel filter in the intake line
between tank and fuel pump should be checked and, if necessary, replaced.
Pull plastic strainer from around the mixing tube and, if necessary, clean with fuel. Screw out the main jet
from the mixing tube and check for damage or grime. Replace immediately if damaged; clean with gasoline if
dirty, then blow out with compressed air. Never push a sharp object through the main jet as that will destroy
it.
Reassemble in logical reverse order. When putting the float chamber back on, insure that the bolts are
properly seated. Torn and/or frayed gaskets must be exchanged immediately for original replacement gaskets.
7.5. Inspecting the air filters
To remove the air filters, loosen the clamp on the air filter with a screwdriver, and slide the air filter off from
the carburettor flange.
Wash out the air filter with fuel and then dry. To do this, blow out the filter with compressed air from inside
to out. Check the filter for damage and replace any defective filters and clamps.
To install the air filter, insert over the carburettor flange and fasten the clamp with a screwdriver again.
7.6. Inspecting the fuel lines
Check the fuel lines between fuel – diaphragm pump, carburettors and fuel tank for leakages and/or other
damage. Replace leaking and/or damaged fuel lines.
7.7. Inspecting pulsation lines

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Check the pulsation line between the vacuum nipple on the crankcase and the pulse connection of the fuel –
diaphragm pump for leakage and/or other damage. Replace the leaking and/or damaged pulsation line.
7.8. Spark plugs and sparkplug leads
It is recommended that the spark plugs be checked one after the other to avoid mixing up the sparkplug leads
and their connecting positions.
7.9. Inspecting the condition of the sparkplug leads
Remove the sparkplug leads and check the contact socket inside for corrosion. Check the SAE-contact (the
screwed on contact hood) of the sparkplug for corrosion or "deposits". If there is corrosion or "deposits",
replace the sparkplug lead and SAE- contact of the sparkplug (usually here the sparkplug would be
completely exchanged - see next paragraph).
7.10.Installing and removing sparkplugs
To uninstall the sparkplug, remove the sparkplug lead. Loosen the sparkplug with the sparkplug wrench and
unscrew it.
Before installing the new or still serviceable sparkplug, the air gap (0.8 mm) is to be checked with a feeler
gauge and adjusted as required.
Screw the sparkplug into the cylinder head with a sparkplug wrench and torque to the correct setting.
7.11.Condition of the sparkplug
The spark plug should have a dry bright brown colour tone on the central electrode, the insulator of the
central electrode and the earth side of the case. If the colour tone changes to a bright very dry grey, the
engine is running very hot and the mixture is too lean. Where there is a wet dark brown to black colouring,
the engine is running with too thick a mixture. In both cases, AATI Pty Ltd or your Hirth distributor should
be contacted.
Check the sparkplug central electrode and earth strap for corrosion (burning). If signs of burning are
detected, the sparkplug should be replaced. For more information on how to read sparkplugs, go to the
maintenance pages on the www.hirth-engines.de web site.
8. Coolant Pump
It is necessary that the Pump and V belt remain unobstructed. The belt tension should be about 5 mm of
movement as shown below:
Figure 10.1 Belt tension.

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Adjustment of the belt tension is made by undoing the upper lock nut and adjusting the belt tension by
turning the lower nuts thereby moving the pump up or down. When the satisfactory tension is gained,
retighten the upper locknut.
Figure 10.2 Lock nut locations
8.1. Changing the coolant pump V belt
To change the V belt, loosen lock nuts previously mentioned. Remove the manual recoil starter on the front
of the engine. Replace V belt and reassemble manual recoil starter. Tighten the adjustment nuts to obtain the
correct tension and retighten the lock nuts to secure the pump.