YS 91SR S User manual

3. Tube(C) should be set exactly at 3 15/16” or (10cm)
between the engine and check valve. DO NOT use
any other type of check valve. The check valve is
designed exclusively for the 91SRS.
4. Adjust your throttle linkage and curve using the align-
ment indentations on the carburetor body located on
the throttle arm side of the carb body.
PLEASE NOTE that the hover position is at 40% see
FIG 2.
START UP
1. AlwaysremoveTube(A)atthecheckvalverstbefore
fueling. The fuel tank is pressurized and you need to
relieve the pressure first. Disconnected Tube(B) at
 thelterandllthetank.ReconnectTube(A)and(B).
2. From the fully closed (clockwise) position, turn the
needle valves counter clockwise as follows.
STARTING SETTING / OPERATING SETTINGSTARTIN
●Hover 1.75 open 1.75 ~1.50
●Low 1.25 open 0.75 ~0.50
●Full 1.25 open 1.25 ~0.75
3. Close the throttle to the idle position and connect the
glow plug driver. The engine is now ready for starting.
SPECIFICATIONS
Bore 27mm
Stroke 26mm
Displacement 14.89cc
Weight 553g
Practical rpm 2,000 ~17,000
OPERATOR’S MANUAL
91SR S (Helicopter Engine/ Part No.YS0074)
FEATURES
The 91SRS is the latest in helicopter engine technology
from YS.
●New stroke design for superior operation
●New carburetor design for easier adjustments.
●Factory preset fuel regulator.
CAUTION
Using gear ratios higher than 9.0 to 1 may result in en-
gine damage from overspeed.
GEAR RATIO
You should be using an engine to main rotor gear ratio
of 1-7.5 to 1-8.0. Please check with your helicopter man-
ufacturer to obtain the correct gear ratio.
INSTALLATION
1. The fuel lines should be connected to the fuel tank as
shown in the above figure. Be careful to install the
check valve in the correct direction. Since the tank is
exposed to high pressures, be sure that all connections
are tight to prevent pressures leakage.
2. Since the engine is sensitive to dirt in the fuel, a fuel
 ltermustbeused.(WerecommendtheYS1195fuel
 lter.)
BREAK-IN
It is not necessary to mount this engine on a set stand
for break-in. However, the engine should be adjusted
slightlyontherichsettingfortherstfewightstoinsure
proper break-in. Always use a good quality fuel which
contain 15 ~30% nitromethane and an oil content of 18
~23% low viscosity oil.
NEEDLE VALVE ADJUSTMENT
Idle, Hover and High needle valves all work the same
way. Turning the needles clockwise will lean the fuel mix-
tures and counter clockwise will richer the fuel mixtures.
1. Set needles as described in START-UP.
2. Start the engine and check idle. Adjust as needed.
3. Lift helicopter into a hover and check for the correct
rotor RPM recommended by the kit manufacture.
Once this is done take note of the amount of smoke
 beingproducedbythemufer.Themixtureiscorrect
for hover when there is a steady stream of smoke
being produced.
4. Land the helicopter for approximately 20~30 seconds.
Lift the helicopter into a hover again taking note of the
transition from idle to hover. If the engine exhibits a
large amount of smoke and the throttle response is
sluggish, you will need to adjust the idle and or the
hover needle leaner to achieved a smooth transition.
If the engine detonates and the smoke is inconsistent
or a small amount is produced, the mixture is too lean.
5. Thehighspeedneedlerenesthefuelmixturefor
forward flights without affecting the hovering adjust-
ment. After the engine is started and warmed up, lift
off into a hover and check that the engine is running
smoothwithagoodtrailofsmoke.Ifeverythingisne,
open the throttle and enter forward flight. Take note
of the amount of smoke like we did in a hover adjust-
ment. It is correct when you see a noticeable steady
smoke trail. Adjust the high-speed needle valve to ob-
tain slightly rich but consistent setting.
STOPPING THE ENGINE
1. Fully close the throttle barrel to stop the engine.
2. As soon as the engine stops running, be sure that a
fuel line clamp is used in Tube(B) to prevent fuel from
 owingintotheengine.
3. Onthenalightoftheday,thefuellineclampshould
be used to stop the engine in order to prevent rust
and corrosion.
FUEL AND GLOW PLUG
We have found that the fuel and glow plugs listed below
will give the best engine performance.
Fuel
Powermaster 30% Special Heli Blend
Cool Power 30% Special Heli Blend
Glow Plug _ YS #2 Enya #3 OS #8
IMPORTANT!
Silicone rubber is used in many parts of the YS engine.
Use only glow fuel of methanol for cleaning. Gasoline
and other volatile solutions will damage silicone if used.
WARRANTY
Strict quality control is implemented by our factory in all
phases,frompartsmanufacturingtonalassembly.
If performance deteriorates or a part fails due to a manu-
facturing error, YS Parts and Service will repair or re-
place the engine at no charge in the period of one year
from date of purchase. Warranty does not cover normal
maintenance.Shouldtheenginebemodied,incorrectly
assembled or abused, there will be a normal charge for
parts and labor.
Fig.1
Fig.2
High speed 100%
Low speed 0%
Hover 40%
52mm
43.5mm 25mm
Hover Needle
Fuel Filter
Tube (B)
Tube (A)
Check Valve
Tube (C) 3 15/16"(10cm)
Low Needle
High Needle
Factory Set
Do Not Adjust
STARTING SETTING / OPERATING SETTING

3
9
6
7
8
9
10
5
1
26
25
27
23
2
24
28
31
30
29
27
4
34
19
20
21
32
22
11
17
16
18
40
41
13
33
38
35
37
36
39
12
14
15
YS Parts and Service
1370 PORTER DRIVE MINDEN NEVADA 89423
Phone: [phone] Fax: [phone]
2010 03
NO. PART# DESCRIPTION QTY
1 YS6505 Crankcase 1
2 YS6510 Cylinder Head 1
3 YS5830 Head Gasket 1
4 YS4785 Head Screws 6
5 YS6520 Cylinder Liner 1
6 YS6525 Piston 1
7 YS5850 Piston Ring 1
8 YS2385 Wrist Pin 1
9 YS4803 Wrist Pin Retainer 2
10 YS4805 Con Rod 1
11 YS6530 Crankshaft 1
12 YS1050 Front Bearing 1
13 YS5860 Rear Bearing 1
14 YS5243 Drive Washer Spacer 1
15 YS0490 Propeller Nut 1
16 YS5865 Back Plate 1
17 YS5870 Back Plate O ring 1
18 YS1070 Back Plate Screw 4
19 YS5880 Carburetor Body 1
20 YS4830 Carburetor Gasket 1
21 YS5885 Throttle Barrel 1
22 YS1090 Throttle Barrel Retainer 1
23 YS4485 Hover Needle Valve 1
24 YS2695 Hover Needle O Ring 1
25 YS2700 Hover Needle Seat 1
26 YS2705 Hover Needle Seat O Ring 2
27 YS2710 Needle Detent 1
28 YS4140 High Speed Needle 1
29 YS2695 High Speed Needle O Ring 1
30 YS4845 Low Speed Needle 1
31 YS4150 Low Speed Needle O Ring 1
32 YS0200 Throttle Arm 1
33 YS2435 Carburetor Screws 2
34 YS4850 Carburetor Subplate 1
35 YS2445 Regulator Subplate 1
36 YS4155 Regulator Assy. 1
37 YS2460 Diaphragm 1
38 YS2480 Regulator gasket 1
39 YS2475 Regulator Screws 2
40 YS3036 Nipple 1
41 YS2510 Check Valve 1
YS5890 Gasket Set
YS5875 Carburetor Assy.
YS5895 O ring set
YS6515 Piston / Ring / Liner Set
YS4480 Needle Valve Assy.