FIG.1
PROPELLER INSTALLATION
Due to the high torque of the FZ91 engine, we have equipped it
with double locknuts for safety.
1. Mount the propeller and tighten the rear nut. Next, tighten the
front nut as shown in fig.2.
2. Select a good quality propeller that will turn in the 8,000 to
11,000rpm range. We recommend sizes 13~12-14, 13.5~10-14,
14~10-14.
FIG.2
START-UP
1. Remove tube(B)from the filter;remove tube(A)from the check
valve, then fill the tank.
Caution:If tank is filled or under pressure remove tube(A) first;
then tube(B). Fuel will eject if tube(B)is removed first while the
tank is pressurized.
2. Open the needle valve 1 1/2 from the fully closed position.
3. Open the throttle fully and slowly turn the propeller ten turn. This
primes the system by pressurizing the tank and sending fuel to
the carburetor.
4. Pour several drops of fuel into the carburetor.
5. Close the throttle to the idle position and connect the glow plug
cord. The engine is now ready for starting.
Do not attempt to start at full throttle, as this is very dangerous.
BREAK-IN
To maximize engine performance and increase durability, please
follow this break-in procedure:
1. Use the same size(or slightly smaller) propeller than you intend
to use in flying.
2. Use a good quality fuel which contains 15-20% nitromethane
and an oil content of 15-20%. Synthetic or castor oil can be
used, or a combination of synthetic and caster. Do not use four
cycle fuel due to low oil content.
3. The needle valve should be set so that the engine is running at
a rich setting. Run the engine approximately 20 minutes with this
setting.
4. Mount the engine to the model and fly ten times with this setting.
This concludes the break-in procedure. It is advisable to always
use a slightly rich setting to keep the moving parts lubricated,
even after the break-in period.
HIGH SPEED ADJUSTMENT
1. Adjustment of high speed is done by the carburetor needle
valve. When the needle valve is turned clockwise, the mixture is
leaner. When it is turned counterclockwise, the mixture is richer.
A good starting position for the high speed needle valve is 1 1/2
turns open from fully close position.
2. When the engine is started, open the throttle gradually. Next,
find the peak position(highest RPM) by adjusting the needle
valve. Then the needle valve should be opened approximately
1/8 of a turn from full RPM to achieve best performance. The
engine may stop if the throttle is opened to full immediately after
starting. Wait unit the engine temperature rises and then open
the throttle slowly.
3. For flying, it is advisable to use a slightly richer mixture setting.
By using a richer mixture, the engine temperature is maintained
and RPM stability improves.
LOW SPEED ADJUSTMENT
This engine is equipped with a new low speed needle valve to
adjust the mixture from low to mid throttle. This needle valve is
located on the side of the throttle barrel opposite the throttle
arm(fig.1).
1. Open the low speed needle to 1 1/2 turns from fully closed
position.
2. The low speed needle valve should be set after the high speed
needle valve has been adjusted. Close the throttle gradually to a
idle(approximately 2500rpm). Let it idle for 20 to 30 seconds and
then slowly advance the throttle. The adjustment is satisfactory
at low speed if transition is smooth at this time.
3. If the engine is running rough on idle, the low speed mixture is
rich. If the engine starts to speed up and dies on idle or starts to
detonate, when advancing the throttle, the mixture is lean. Turn
the low speed needle valve clockwise to richen and
counterclockwise for a leaner mixture(note that the direction of
the low speed needle valve is opposite the high speed needle
valve). Adjustments to the low speed needle valve should be 1/8
to 1/4 of a turn increment at a time to achieve smooth throttle
response.
I
MPORTANT! The regulator adjusting screw on this engine is
factory set. No further adjustments are necessary. If for some
reason you have to disassemble the regulator assembly, the
regulator adjusting screw should be set flush with the regulator
body.
SPECIFICATIONS
Bore
27.7mm
Stroke
24.8mm
Displacement
14.95cc
Weight
750g
Practical rpm
2,000-13,000rpm
FEATURES
The FZ91 is the most powerful 91 four cycle engine available.
This engine offers many exclusive features that have been proven
on other YS engines.
Supercharged system with simplified structure to keep weight to
a minimum while still retaining maximum efficiency.
Air chamber that uses crankcase pressure coupled with a double
throttle valve system which allows a bigger charge of fuel and air
mixture to enter the intake valve for more power.
Same proven piston design as the 140Limited which creates
more crankcase pressure for a more consistent fuel flow to the
regulator.
Fuel injection system for superior throttle response. This system
is unaffected by tank position or by the attitude of the model.
GROW PLUG
Select the most appropriate glow plug from those designed
specifically for 4 cycle engines. GLOW plug selection greatly
affects the maximum engine output and low idle. If RPM's decrease
or stop when the booster cord is removed, replace the plug.
We recommend YS #4 (P0040).
INSTALLATION
1
. Connect the engine to the tank as shown in fig.1. Since high
pressure is applied to the tank, tighten all connections carefully.
Care must be taken to prevent pressure leakage due to
undertightening of the check valve or by kinking the fuel lines.
2. Always uses a fuel filter. We recommend the YS filter.
3. Match the direction of the check valve arrow to fig.1, with the
arrow facing towards the tank.
FIG.3
TAPPETCLEARANCE
ADJUSTMENT
1. Tappet clearance is factory preset. No adjustment is necessary
unit after 1 hour of operation(includiing break-in period).
2. Clearance adjustment should be done when the engine is cool.
When the engine temperature is high, clearance is higher due to
thermal expansion.
3. The proper clearance setting should be at 0.04-0.1mm. The
adjustment is achieved by loosing the locknut(fig.3) and turning
the adjusting screw.Tighten the locknut after the adjustment is
achieved. After the initial 1 hour adjustment, this procedure
should be performed after every 2 hours of use.
DIAPHRAGM AND CHECK VALVE
DISASSEMBLY
Diaphragm:
1. Remove the adjustment screw of the valve, and then remove the
inside valve and spring.
2. Clean the inside with alcohol or appropriate cleaner. Reassemble.
3. Screw in the regulator screw until flush with the diaghragm body.
Check valve:
1. Open the valve by rotating the body counterclockwise.
2. Reassemble the check valve carefully.
IMPORTANT! Silicone rubber is used in many parts of the YS
engine. Use only glow fuel or methanol for cleaning. Gasoline and
other volatile solutions will damage the silicone if used.
WARRANTY
Strict quality control is implemented by our factory in all phases,
from parts manufacturing to final assembly. If performance
deteriorates or a part fails due to a manufacturing error, YS will
repair or replace the engine at no charge.
Should the engine be modified, incorrectly assembled or abused,
there will be a nominal charge for parts and labor. The use of four
cycle fuel due to the low oil content will also void warranty.
(Diagram) Rear Nut
(Diagram) Front Nut
0.04-0.1mm
YS FZ91
OPERATOR'S MANUA
L
Tube
（
A
）
High Speed Needle
Low Speed Needle
Check Valve
Fuel Filter( Not included)
Fuel Filter(Not included)
Tube
（
B
）
52
57
25
120.7
102.5
21.5
43
