OPERATOR’S MANUAL

91SR (Helicopter Engine)

Fig.1 BREAK-IN 5. The high speed needle refines the fuel mixture for
forward flights without affecting the hovering adjust-
SPECIFICATIONS ment. After the engine is started and warned up, lift
It is not necessary to mount this engine on a set stand
Bore 27mm for break-in. However, the engine should be adjusted off into a hover and check that the engine is running

52mm
Stroke 26mm slightly on the rich setting for the first few flights to insure smooth with a good trail of smoke. If everything is
Displacement 14.89cc proper break-in. Always use a good quality fuel which fine, open the throttle and enter forward flight. Take
contain 15 ~30% nitromethane and an oil content of 18 note of the amount of smoke like we did in a hover
Weight 553g
~23% low viscosity oil. adjustment. It is correct when you see a noticeable
Practical rpm 2,000 ~17,000 steady smoke trail. Adjust the high-speed needle
NEEDLE VALVE ADJUSTMENT valve to obtain slightly rich but consistent setting.
43.5mm 25mm
Idle, Hover and High needle valves all work the same
Hover Needle way. Turning the needles clockwise will lean the fuel mix- STOPPING THE ENGINE
tures and counter clockwise will richer the fuel mixtures.
Fuel Filter 1. Fully close the throttle barrel to stop the engine.
Low Needle 2. As soon as the engine stops running, be sure that a
1. Set needles as described in START-UP.
Tube (B) fuel line clamp is used in Tube-B to prevent fuel from
2. Start the engine and check idle. Adjust as needed. flowing into the engine.
High Needle
3. On the final flight of the day, the fuel line clamp
3. Lift helicopter into a hover and check for the correct should be used to stop the engine in order to prevent
rotor RPM recommended by the kit manufacture. rust and corrosion.
Once this is done take note of the amount of smoke
being produced by the muffler. The mixture is correct
for hover when there is a steady stream of smoke be-
ing produced. FUEL AND GLOW PLUG
Check Valve Tube (A)

Factory Set 4. Land the helicopter for approximately 20~30 seconds. We have found that the fuel and glow plugs listed below
Do Not Adjust Lift the helicopter into a hover again taking note of the will give the best engine performance.
Tube (C) 3 15/16"(10cm)
transition from idle to hover. If the engine exhibits a
large amount of smoke and the throttle response is Fuel
sluggish, you will need to adjust the idle and or the Powermaster 30% Special Heli Blend
FEATURES 3. Tube(C) should be set exactly at 3 15/16” or (10cm) Cool Power 30% Special Heli Blend
hover needle leaner to achieved a smooth transition.
between the engine and check valve. DO NOT use if the engine detonates and the smoke is inconsistent
The 91SR is the latest in helicopter engine technology Glow Plug _ YS #2 Enya #3 OS #8
any other type of check valve. The check valve is or a small amount is produced, the mixture is to lean.
from YS.
● New stroke design for superior operation
designed exclusively for the 91SR.
● New carburetor design for easier adjustments.
IMPORTANT!
4. Adjust your throttle linkage and curve using the align- Silicone rubber is used in many parts of the YS engine.
● Factory preset fuel regulator.
ment indentations on the carburetor body located on Use only glow fuel of methanol for cleaning. Gasoline
● Same physical size as 61ST-2 ,80ST ,91ST making it
the throttle arm side of the carb body. and other volatile solutions will damage silicone if used.
easy to add more power to your helicopter.
PLEASE NOTE that the hover position is at 40% see Fig.2
FIG 2. High speed 100% WARRANTY
CAUTION Strict quality control is implemented by our factory in all
START UP phases, from parts manufacturing to final assembly.
Using gear ratios higher than 9.0 to 1 may result in en- If performance deteriorates or a part fails due to a manu-
gine damage from overspeed. Low speed 0% facturing error, YS engine will repair or replace the en-
1. Always remove Tube(A) at the check valve first
before fueling. The fuel tank is pressurized and you gine at no charge in the period of one year from date of
GEAR RATIO need to relieve the pressure first. Disconnected purchase. Warranty does not cover normal maintenance.
Tube-b at the filter and fill the tank. Reconnect Should the engine be modified, incorrectly assembled or
You should be using an engine to main rotor gear ratio abused, there will be a normal charge for parts and la-
Tube(A) and (B).
of 1-7.5 to 1-8.0. Please check with your helicopter man- bor.
2. From the fully closed (clockwise) position, turn the
ufacturer to obtain the correct gear ratio.
needle valves counter clockwise as follows.
INSTALLATION STARTING
STARTINGSETTING
SETTING // OPERATING
OPERATINGSETTING
SETTINGTIN
1. The fuel lines should be connected to the fuel tank ● Hover 1.75 open 1.75 ~1.50
as shown in the above figure. Be careful to install the ● Low 1.25 open 0.75 ~0.50
check valve in the correct direction. Since the tank is ● Full 1.25 open 1.25 ~0.75 Hover 40%
exposed to high pressures, be sure that all conne-
ctions are tight to prevent pressures leakage.
3. Close the throttle to the idle position and connect the
glow plug driver. The engine is now ready for starting.
2. Since the engine is sensitive to dirt in the fuel, a fuel
filter must be used. (We recommend the 6720 fuel
filter.)
NO. PART# DESCRIPTION QTY
1 S9101 Crankcase 1
2 S9102 Cylinder Head 1 4
3 S9103 Head Gasket 1
4 S5104 Head Screws 6
5 S9105 Cylinder Liner 1 2
6 S9106 Piston 1
7 S9107 Piston Ring 1
7
8 S7108 Wrist Pin 1
3
9 R6108 Wrist Pin Retainer 2
10 S5110 Con Rod 1 22
11 S9111 Crankshaft 1
8 9
12 R6211 Front Bearing 1
9
13 S9113 Rear Bearing 1
32 6
14 R6115 Drive Washer Spacer 1
15 R6217 Propeller Nut 1
21
16 S9116 Back Plate 1 26
17 S9117 Back Plate O ring 1 19
25 10
18 R6120 Back Plate Screw 4 27
19 S9119 Carburetor Body 1 24
20 40
23
20 S5120 Carburetor Gasket 1 27
21 S9121 Throttle Barrel 1 29
34
22 R6124 Throttle Barrel Retainer 1 28
18
23 S4125 Hover Needle Valve 1 30
5 16
31
24 F1546 Hover Needle O Ring 1
25 F1555 Hover Needle Seat 1 17

26 F1556 Hover Needle Seat O Ring 2

11
27 S5128 Needle Detent 2
28 S8130 High Speed Needle 1
29 R6203 High Speed Needle O Ring 1
30 S5131 Low Speed Needle 1
31 S8133 Low Speed Needle O Ring 1 13
41
32 F1260S Throttle Arm 1
33 S7132 Carburetor Screws 2 1
12
34 S5135 Carburetor Subplate 1 14
35 S7134 Regulator Subplate 1 15

36 S8138A Regulator Assy. 1

37 S7136 Diaphragm 1 33

38 S7140 Regulator gasket 1

38
39 S7139 Regulator Screws 2
40 S7143 Nipple 1
35
41 S7144 Check Valve 1
S9143 Gasket Set 37
S9119S Carburetor Assy.
S9146 O ring set
36
S9105S Piston / Ring / Liner Set
S4125S Needle Valve Assy. 39

2007 03