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When starting the engine for the first time, set both the 

high speed needle 2 turns open and the slow speed 

needle 2 turns open from the fully closed position.

With this position both high and slow speed mixtures 

will be rich. If the settings are upset, come back to this 

setting and adjust them according to procedure in the 

needle setting section.

Initial needle position

1/4

1/2

3/4

NEEDLE ADJUSTMENTS

The slow speed needle is effective through the high 

speed range, while the high speed needle is not 

effective in the low speed range. Both needles are 

effective in the  mid speed range, while the slow speed 

needle is the most effective. (Refer to the diagram 

below.)

Fuel Flow

Throttle

Opening 

Fully

open

More

Less

1. 

Carry out the needle settings as follows.

Adjustment on the ground

6.

7.

8.

4. 

5. 

3.

Set the throttle stick 2-3 clicks advanced from the 

bottom and measure rpm with a tachometer and 

write it down.
Close the slow speed needle approx. 30 degrees 

and measure rpm with a tachometer and write it 

down. (Be sure to adjust the slow needle only after 

stopping the engine.)
Repeat step 3 to locate the slow speed needle 

position where the maximum rpm is achieved.  

Open the slow speed needle from that position 

reducing 700 – 800 rpm. That is almost the right 

position of the slow speed needle..
Open the throttle valve fully and measure rpm with 

a tachometer and write it down.
Close the high needle approx. 30 degrees and 

measure rpm and write it down. (Be sure to adjust 

the high needle only after stopping the engine.)
Repeat step 6. to locate the high needle position 

where maximum rpm is achieved. Open it approx. 

60 degrees from this position. This is the basic 

high needle position.
Set the throttle trim on the transmitter so that the 

idle rpm is approx. 1,800 (approx. 1,600rpm when 

idle down function is used).

9.

10.

11.

If the engine responds sluggishly to increase rpm, 

the mixture is too rich due to the slow speed needle 

being open too far. Close the slow speed needle 

approx. 15 degrees. Repeat this procedure until the 

engine accelerates smoothly. (Be sure to adjust the 

slow speed needle only after stopping the engine.) 
If the engine hesitates before picking up speed or 

ceases firing, the mixture is too lean due to the 

slow speed needle being open too far. Open the 

needle approx. 15 degrees. Repeat this procedure 

until the engine accelerates smoothly. (Be sure to 

adjust the slow speed needle only after stopping 

the engine.)

Adjustment with flight

If the rpm drops or the exhaust sound weakens 

when vertically climbing from level flight with full 

throttle, the mixture is too lean. Land the model 

and open the high speed needle approx.15 

degrees. Repeat this procedure until the engine 

sound is smooth.

If the above symptom is not detected with 

prolonged torque roll and hovering but sluggish to 

increased rpm when the throttle is fully opened 

abruptly, the mixture is too rich. Land the model 

and close the slow speed needle approx.15 

degrees until sluggish rpm increase disappears.

Note

Generally, a gasoline engine is sensitive to a lean 

mixture compared with a glow  engine, and will 

stop without warning hesitation and stops with 

overheating. It is recommended that the engine be 

run with a slightly richer mixture.

If, on the other hand, exhaust sound is irregular 

when vertically climbing from level flight, the 

mixture is too rich. Land the model and close the 

high speed needle approx. 15 degrees. Repeat this 

procedure until the sound in level flight is steady.

If the power drops gradually or exhaust sound 

weakens (overheating symptom), when performing 

torque roll and hovering, the mixture is too lean. 

Land the model and open the slow speed needle 

approx. 15 degrees. Repeat this procedure until 

power remains steady. 

When the engine is started, make sure the radio 

control system works normally (distance test).
Engine does not run erratic with full throttle.
Idling is stable.
Responds positively to the throttle operation.
Warm-up is finished.

FLIGHT & MAINTENANCE

Checking prior to flight

Warm-up is required as with full size aircraft and car 

engines. Take off the model after warming the engine 

for approx. 10 seconds with full throttle.

Precautions in flight

A slight engine rpm increase and decrease delay is 

normal. Abrupt throttle operation will the cause 

engine to quit. Move throttle smoothly. 
Due to the construction of the carburetor, 80~90% of 

maximum power develops with half throttle.

If you feel uneasy with this, adjust with transmitter 

function (exponential and throttle curve, etc.) or 

differential of throttle linkage.

Maintenance after the day’s flights

Please pay attention to the matter described below 

to ensure that your engine serves you well in 

regard to performance, reliability and long life.

Check the tightening of each screw, especially 

engine installation screws and silencer installation 

screws each time. Also, for the first 4 to 5 flights, 

tighten the screws after each flight.

Cooling is more vitally important to a gasoline engine 

than to a glow engine. If overheating symptoms (loss 

of power at full throttle or exhaust note at mid speed 

changes from cloudy one to clear one) are observed 

during flight, immediately stop flying and carry out 

the following countermeasures.

Enlarge the air intake cutout on the cowling.
Enlarge the air outlet cutout on the cowling. (It is 

vitally important.)
Partly cover the air intake cutout on the cowling 

where air does not hit direct the engine.
Install an air guiding plate on the fuselage and 

cowling so that cooling air may be guided to the 

cylinder portion of the engine and muffler.

When the interval between the flights is short and 

the engine is still hot, it may be possible overheating 

symptoms are observed by circulating the head from 

the former flight through the engine even if the 

overheating symptoms were not observed during 

the former flight. In this case, leave it until the 

engine is fully cooled down (in hot weather, it may 

take more than one hour.) or run the engine for 4 to 

5 minutes at idle.

1) 
2) 

3) 

4) 

Fill the carburetor with fuel at the conclusion of a 

day's flying. (Pay careful attention to fire and ignition 

source when carrying and storing the model.) 

If the engine is stored without filling the carburetor, 

with fuel the inside parts will dry out and not work 

properly at the next running. If the engine quits out 

of fuel, refill the carburetor with fuel.

As previously mentioned, it is vitally important to 

avoid operating the engine in conditions where dust, 

distributed by the propeller, may be deposited on the 

engine and enter its working parts.
Remember to keep your fuel container closed to 

prevent foreign matter from contaminating the fuel.
Install a fuel filter to prevent foreign matter in the fuel 

container from entering the fuel tank. O.S. Super 

Filter (L) is available as an optional extra.
Install an in-line fuel filter between the tank and 

carburetor to prevent foreign matter in the tank from 

entering the carburetor.
Clean these filters periodically.

If these precautions are neglected, restrictions of 

fuel flow may cause the engine to cut out, or the 

fuel/air mixture to become too lean causing the 

engine to overheat.
With a gasoline engine rust hardly occurs. Check 

the exterior to make sure there is nothing wrong and 

wipe off any oil res.

O.S. GENUINE PARTS & ACCESSORIES

LOCK WASHER M6 (10sets)

NON-BUBBLE WEIGHT (S)

SUPER FILTER (L)

2. 

Start the engine. Open the throttle valve slowly 

until fully open and run the engine for 10 seconds 

to warm the engine.

Slow Needle Effective Range

High Needle

Effective Range

After that, tighten the silencer installation screws 

every two to three flights until the gasket has been 

stabilized (total 10 to 15 flights).

After cleaning the engine, dry it well then inject a 

small quantity of oil used to mix fuel and rotate the 

crankshaft several times to distribute the oil well 

inside the engine.  Finally reassemble the engine 

and store it in a dry place after inserting it in a heavy 

vinyl bag.

Starting with a Chicken stick or gloved hand

Turn on the transmitter switch.
Turn on the receiver switch.
Close the choke valve fully (ON).
Pull down the throttle stick on the transmitter fully 

or to the 1/8 opened position ( 2 to 3 clicks).
Turn on the ignition switch.
Flip the propeller counterclockwise until the fuel reaches 

the carburetor and you hear explosion several times.
Open the choke valve fully (OFF).
Tell the helper and onlookers that you will start the 

engine now and have the helper hold the model 

securely.
Tell the helper and onlookers that you will start the 

engine now and have the helper hold the model 

securely.

Flip the propeller counterclockwise with a Chicken 

stick.  (Usually the engine starts with 5 to 6 flips.)

1. 
2. 
3.
4. 

5. 
6.

7.
8.

9.

When the engine is not to be used for a long period 

(more than a year), remove the engine from the 

model, clean the outside then remove the 

carburetor, and plug all tubing. Clean inside the 

engine by rotating the crankshaft with the engine 

immersed in container filled with gasoline.

Also use gasoline to clean the outside of the 

carburetor. 

Clean the outside only because the inside parts are 

sensitive to foreign substances.

SPARK PLUG CM-6 (NGK)

ID. 3mm × OD. 5mm Length 500mm

FLUORORUBBER TUBE

Warm the engine for 10 seconds with the throttle fully 

open, then idle the engine for 5 seconds, then quickly 

open the throttle fully. Make sure the engine does not 

respond sluggishly to increase rpm nor hesitates 

before picking up speed or even ceases firing.

1. 

4. 

3.

2. 

(55500005)

(72403051)
(72403050)

(71531010)

(71669000)

(28382200)

(71669010)

(78300000)

SPARK PLUG CM-6 (Rcexl)
GASOLINE FUEL FILTER (S)

SUPER FILTER (S)

Summary of Contents for GT120T

Page 1: ... installing it in the model or there is a possibility of injury Be sure to use an effective silencer muffler Frequent exposure to an open exhaust may eventually impair your hearing Such noise is also likely to cause annoyance to others over a wide area Mount the engine in your model securely following the manufacturers recommendations For their safety keep all onlookers especially small children w...

Page 2: ...stem Do not use any propeller which has become split cracked or nicked even very slightly or received strong impact even if no apparent damage is visible PROPELLER SPINNER ATTACHMENT Since the GT120T is intended to be started with an electric starter the addition of a spinner assembly for centering the starter sleeve is desirable Special propeller locknut sets are available for use with spinners U...

Page 3: ...nce reliability and long life Check the tightening of each screw especially engine installation screws and silencer installation screws each time Also for the first 4 to 5 flights tighten the screws after each flight Cooling is more vitally important to a gasoline engine than to a glow engine If overheating symptoms loss of power at full throttle or exhaust note at mid speed changes from cloudy on...

Page 4: ...ngine SPECIFICATIONS C Copyright 2017 by O S Engine Mfg Co Ltd All rights reserved Printed in Japan 60093430 011800 1 2 3 4 5 6 7 8 8 1 8 2 9 10 11 11 1 12 13 14 15 16 17 18 19 20 28603100 28614000 28603401 28603200 29706000 29717000 28602100 4AB08000 29708100 29708200 29332000 4AB01000 74002A20 74002321 28631000 4AB02000 26730005 4AB01800 4AB28000 4AB18000 4AB15000 4AB21000 4AB81000 4AB81410 7400...

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