INSTRUCTIONS FOR O.S. TYPE 3H AUTOMATIC CARBURETTOR

This new carburettor incorporates an automatic mixture control device which ensures that the engine receives acorrectly balanced mixture of fuel and air at all throttle settings. The device progressively reduces the effective sizeof the fuel jet orifice as the throttle is closed, thereby preventing the engine from running too rich at low speeds. Thisalso means that an airbleed is no longer required and, with its elimination, maximum suction is maintained at the fueljet at all times. This is a most important factor where manoeuvres have to be executed at low engine speeds andthrough wide variations of fuel level within the fuel tank.

Three adjustable controls are provided on this carburettor• The Needle-Valve: For adjusting the mixture strength

when the thrott le is ful ly open.• The Mixture Control Screw: For manually adjusting the

automatic mixture control valve in order to obtain astable idle and a smooth transition through the mediumspeed range.

• The Throttle Stop Screw: For setting the position wherethe carburettor rotor is closed.

Note: The Mixture Control Screw has been factory set forthe approximate best result. However, different fuelsand/or climatic conditions may necessitate minorcompensatory adjustment. Therefore, first run theengine as received and re-set the Mixture ControlScrew only if necessary.

PROVISIONAL SETTINGSBefore starting the engine, check that the controls are set asfollows:1) In the case of a fixed-wing aircraft, use a muffler pres-

surized fuel feed, and open the Needle-Valve 1 1/2 to2 turns from the fully closed position. For helicopteruse, open the Needle-Valve 2 1/2 turns (or 2 turns if amuffler-pressurized fuel feed is used) from the fullyclosed position.

2) Make sure that the Throttle Stop Screw is set, in conjunc-tion with the throttle lever linkage, so that (a) the throttlerotor is fully open when the transmitter throttle stick isfully advanced and (b) that the throttle rotor is fullyclosed when both the throttle'stick and throttle trim leverare fully pulled back.

ADJUSTMENT (HELICOPTERS)In helicopter f l ight, thrott le response at medium revolutions(i.e. hovering speeds) is most important, since this is a speedrange widely used in helicopter f l ight. The optimum mixtureat mid range will be obtained with a balanced adjustment ofboth the Needle-Valve and the Mixture Control Screw.A — Low Speed Performance — Mixture Control Screw1) Start the engine with the transmitter throttle stick in the

closed position and the throttle trim lever set at the midposition.

2) Adjust the throttle trim so that the highest possible idlingr.p.m. are obtained without actually engaging the clutch.

3) Now increase engine r.p.m. by means of the throttle stickuntil the helicopter just 'floats' above the ground, allowthe engine to warm up for about 10 seconds, then gradual-ly reduce r.p.m. and allow the engine to idle for about 5seconds.

4) Repeat, several times, this procedure of increasing r.p.m.so that the model 'floats' after idling for 5 seconds on theground. While doing this, carefully observe the exhaustsmoke and the behaviour of the engine. If, as the throttleis opened, the engine tends to hesitate and to run with alow-pitched exhaust note and an excess of exhaust smoke,the mixture is too rich.

Throttle Rotor Set-Screw

Needle Valve

Mixture ControlScrew

If, on the other hand, when the throttle is opened, thereis a marked lack of exhaust smoke and the engine quicklyspeeds up but runs 'hard', with a higher pitched exhaustnote and a tendency to overheat, the mixture is too lean.

5) If the mixture is too rich. turn the Mixture ControlScrew clockwise 45 degrees. If, on the other hand, themixture is too lean, turn the Mixture Control Screwcounter-clockwise about 90 degrees, then turn it clock-wise 45 degrees.

Turn the Mixture Control Screw 30 to 45 degrees at a time inthe early stages. When the response improves, turn the Mix-ture Control Screw 10 to 15 degrees at a time. Carry outadjustments progressively and patiently, remembering thesymptoms of rich and lean running, quoted above, until theengine responds quickly and positively to the throttle.

B — High Speed Performance — Needle-ValveThe optimum Needle-Valve setting is usually 1/2 turn (or 1/4turn if a muffler pressurized fuel feed is used) closed fromthe starting position [i.e. 2 1/2 turns open (or 2 turns open ifa muffler-pressurized fuel feed is used) from the fully closedposition]. The precise setting depends on (a) the individualcharacteristics of the helicopter and its engine/muffler in-stallation, (b) the fuel used and (c) atmospheric conditions.The Needle-Valve should be gradually closed until the highestperformance is obtained in flight at fu l l power.However, take care not to close the needle-valve to too lean asetting and risk overheating the engine, in the pursuit ofmaximum performance.

C — Mid-Range Performance — Throttle ResponseAs previously stated, in helicopter fl ight, throttle response atmedium (hovering) speeds — i.e. engine speeds widely used inhelicopter flight — is most important. If the Mixture ControlScrew (low speed) and Needle-Valve (high speed) are proper-ly balanced, the mixture will be correct, also, through themedium speed range. If , however, minor readjustments proveto be necessary for the best possible throttle response, proceedas indicated by the following symptoms and required correc-tive action:1) If the mixture at mid-range is lean, gradually increased

engine speed is accompanied by diminished exhaustsmoke and a high-pitched exhaust note if hovering isprolonged. In extreme cases, the engine will overheatand lose speed, so that hovering cannot be maintained.In this event, open the Needle-Valve 15 to 30 degrees andturn the Mixture Control Screw 30 to 45 degrees counter-clockwise.

2) If the mixture at mid-range is too rich, positive and quickthrottle response will not be obtained, the exhaust smokewill be dense and the exhaust note will be lower, probablyaccompanied by fuselage vibration.In this event, close the Needle-Valve 30 to 60 degrees andturn the Mixture Control Screw 45 to 90 degrees clock-wise.

Note: It is most important to make the Needle-Valve adjust-ment in small increments of not more than 1 5 — 3 0degrees and the Mixture Control Screw adjustment notmore than 30 — 45 degrees as advised, when seekingoptimum mid-range throttle response. Excessive re-adjustment will upset either (A) Low-Speed Perfor-mance or (B) High-Speed Performance. Carry out re-adjustments patiently until the best balance of idling,hovering and high-speed performance is achieved andthe engine responds quickly and positively to thethrottle control.Throttle response at hovering speeds is also affectedby the relationship of the main rotor pitch angle tothrottle opening. If the optimum mid-range throttleresponse cannot be obtained by the carburettor adjust-ments described above, try adjusting the helicopter'spitch control characteristics.

ADJUSTMENT (FIXED-WING AIRCRAFT)1. Start the engine and, with the throttle fully open, gradual-

ly close the Needle-Valve until it is running at its maxi-mum speed.

Caution: Do not close the Needle-Valve to too "lean" asetting as this will cause the engine to overheatand slow up. Set the Needle-Valve very slightlyto the "rich" side of the peak r.p.m. setting.Make sure that the engine is fully "broken-in"(about 1 hour of total running time in short runs)before operating it continuously at full throttle.

2. Having set the Needle-Valve as detailed above, close thethrottle. The engine should idle continuously and steadilywithout further adjustment.(a) If, however, the engine begins to idle unevenly, open

the throttle. If the engine then hesitates before pick-ing up to full speed, it is probable that the idlingmixture is too rich. Check this by closing the throttleagain and letting the engine idle for a little longerbefore again opening up. If the engine now puffs outa good deal of smoke and hesitates or even stops, itwill be necessary to close the Mixture Control Screw.Do this by turning it clockwise: about 10~20 degreesshould be sufficient.

(b) If instead of being set too rich, the Mixture ControlScrew is set too lean, the engine will stop when thethrottle is closed, or will lose speed while idling andthen cut-out abruptly (without smoking) when thethrottle is opened again. In this case, turn the MixtureControl Screw counter-clockwise about 90 degrees,then turn it clockwise slowly to find the best setting.Mixture Control Screw adjustment is not critical andby remembering the symptoms of rich and lean run-ning quoted above, it is a very simple matter toestablish the best setting.

Note 1 If the engine stops when the transmitter throttlestick is fully pulled back, advance the throttle trimlever so that the idling speed is slightly raised. Donot change the Needle-Valve setting.

Note 2 As previously stated, the Mixture Control Screw ispre-set before the engine leaves the factory. If theoriginal setting is lost, it may be reestablished asfollows: 1. Close the throttle completely. 2. Screwin the Mixture Control Screw carefully until itstops. 3. Unscrew the Mixture Control Screwexactly 2 1/2 turns.

CARBURETTOR CLEANLINESSThe correct functioning of the carburettor depends on itssmall fuel orifices remaining clear. The minute particles offoreign matter that are present in any fuel, can easily partial-ly obstruct these orifices and upset mixture strength so thatengine performance becomes erratic and unreliable.O.S. 'Super-Filters' (large and small) are available, as optionalextras, to deal with this problem. One of these filters, fittedto the outlet tube inside your refueling container, will pre-vent the entry of foreign material into the fuel tank. It is alsorecommended that a good in-line filter be installed betweenthe tank and carburettor.Do not forget to clean the filters regularly to remove dirt andlint that accumulate on the filter screens. Also, clean thecarburettor itself occasionally.If, despite the use of f i l ters, foreign matter should reach thecarburettor, causing it to malfunction, it should be carefullydismantled and cleaned as follows:

1. Remove complete carburettor from engine. Clean off anyexternal dirt.

2. Remove throttle stop screw.3. Withdraw rotor from carburettor body, taking care not to

lose the rotor spring placed behind rotor.

4. Remove needle from needle-valve assembly, followed byhexagonal retaining nut and ratchet spring. Now pushnozzle unit through body from outside.

5. Remove fuel inlet nipple.6. Immerse parts in solvent (e.g. gasoline, alcohol or carbon-

tetrachloride) and clean thoroughly.7. Re-assemble in reverse order. Note that periphery of

nozzle flange is specially shaped to key into carburettorbody.

Note:This carburettor is fitted with a short tubular seal (2.5 mmbore, 5.0 mm outer dia., 7 mm length) between the needleand body to prevent air leakage into, or fuel leakage from,the needle-valve. When the seal becomes worn or damagedafter a long period of use, replace with the spare seal pro-vided, or cut a short piece of silicone fuel tubing as a replace-ment.

PARTS LIST

Description

Throttle lever assemblyThrottle lever assemblyThrottle lever fixing screwThrottle leverThrottle leverMixture control screw"0" ringThrottle rotorRotor springCarburettor bodyThrottle stop screw (w/nut)Nipple (w/washer)Carburettor rubber gasketCarburettor fixing screwsNeedle valve assemblyNeedleSet-screwNozzle assemblyRatchet spring"0" ring

c°,8=]-hCode No.

2298140022781401227814202298141022781411229816002278180022981200224815062298110022681306226819532261500023081706229819004518197326381501229819602261130224881824

The specifications are subject to alteration for improvement without notice

6-15 3-chome Imagawa H i g a s h i s u m i y o s h i - k uOsaka 546, Japan T E L (06) 702-0225

FAX (06)704-2722

© Copyright 1992 by O.S. Engines Mfg. Co., Ltd. All rights reserved. Printed in Japan.109301