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Reproduced by Brian Porman with the kind permission of Brian Winch from RCSA Jan. 1997. BP's comments are in italics.

CARBURETTORS - HOW TO SET THEM

The carburettors used on model aircraft are of three different types. The first is a diaphragm or butterfly carburettor and this is the type most commonly found on the large engines, particularly the convert type engines. Examples of these are Walbro and Dellorto. (This section, which may only have a small readership, could be reproduced in a later article if there is general interest and members with or thinking of using this carbie would like the BW low down. Please let me know)

AIR BLEED CARBURETTOR.

The next carby is the air bleed type as is found on Enya and some OS as an example. The sketch shows the hole in the body of the carburettor and in the Enya, a spring loaded screw at the top, and in the side for the OS. The hole and the screw serve one purpose only and that is to adjust the amount of air available to the engine at idle rpm.
As the spring loaded screw is wound IN so that it partially blocks the hole it, obviously, impedes the flow of air. Winding IN gives a RICH mixture and OUT gives a LEAN mixture. The richness and leanness of a fuel mixture relates to the amount of liquid fuel supplied to the engine.
A lean mix is a mixture with too little fuel/too much air and a rich mix is too much fuel/too little air. The fuel comes from the main jet but the rotor of the carby is closed in the idle position so the air has to be supplied by the hole.
When tuning, the control, run the engine until well warm and correctly adjust the main mixture needle. Close (pull back) the throttle until the engine is idling close to the desired speed, generally around 2,800 to 3,000 rpm. Very carefully adjust the air bleed screw in small amounts and listen to the engine. If the rpm increases and the engine stops then the mix is too lean - wind the screw in a little.
If the engine slowly loses rpm and stops then the mixture is too rich - wind the screw out. Adjust the (main mixture) needle until you have the highest rpm the engine can retain without stopping, then wind (the screw) in about 45 degrees [1/8th of a turn] to give a very slightly rich mixture.
Except in the most extreme circumstances, this is a set and forget area as it is very tolerant of prop loads, plug changes and extremes of weather.
(Air bleed) is a very simple and reliable mixture control so why isn’t it used on more carbies?
It has two drawbacks! To choke the engine for starting the throttle must be open otherwise air is drawn through the hole and it drastically reduces the suction on the fuel supply.
The other problem is that it controls the fuel mix for a very small amount of throttle movement, so that the engine is slobbery rich in the mid range.
The method of overcoming this to some extent is to fit a carby with a small venturi and this is the case with the Enya and OS and this has a side benefit in that the suction from the tank is greater so tank position is not so critical with most of the engines fitted with these carbies.
Before we move too far away, on both these carbies you can see another spring loaded screw, in both cases, on top of the carby. This is the rotor retention and idle speed screw.
Serving a double purpose it retains the rotor in the carby body and can also be adjusted to set the speed of the idle. In most carbies, if this screw comes out, the rotor follows quite rapidly so keep an eye on it occasionally.

THE MAIN NEEDLE.

The main mix for these carbies is the same as most others and that is a tapered needle in a stepped hole tube commonly called the needle valve. The end of this tube is exposed to the air flowing through the venturi section - the main hole through the carby into the engine, - and there it does the job of a fuel jet or spray bar.
The needle valve is drilled to a diameter that is a neat fit on the needle and, close to the jet end, the diameter of the hole is reduced.
As the needle is wound in, the pointed end enters the stepped down hole and gauges the flow of the fuel. Further in, less fuel and vice-versa.
Adjusting this mixture is common practice gained by experience and the experienced modeller knows well to tune a little on the rich side as the engine unloads in the air and a fully tuned engine on the ground is a lean engine in the air.

THROTTLE ARM ADJUSTMENT.

The other adjustment on the carby is the throttle arm and this is the most neglected adjustment of all in most cases. I am certain you know well the methods of setting differential aileron travel, for example, by using a disc on the servo and connecting the aileron rods behind the centre of the disc.
Also, when installing a control horn on a surface such as elevator or rudder, the importance of having the connection pivot in line with the hinge line to obtain equal travel.
The same problems crop up with throttle connections where we need a Set your engine servo also in the halfway position and then make the connections between the two. Now you will find there is no "ganging up" of the throttle movement and the changes will be reasonably even as you move the stick on the Tx.
The big benefit is that you will have very nice control at low throttle which, as any scale flier knows, is extremely important when coming in for a landing talking of which, it is about time for me to come to the final leg.

FUEL METERING CARBURETTORS

This type of carby is so called due to the high speed and idle adjustments being controls over the flow of liquid fuel. The air is the absolute domain of the aperture set by the opening of the rotor. Typically the main mixture is adjusted by a needle with a fine point or acutely angled end and its rotary movement is controlled by a spring detent (ratchet), compression spring or friction provided by a tight fitting ‘O’ ring.
Between the main needle and the venturi will be a fuel nipple leading to a fuel chamber. A fuel chamber is a small reservoir that maintains enough fuel to supply the demands as the rotor is opened, in order that the increase in engine speed is instant.
This is as close to a float chamber as you get on a simple carby. It surrounds, or is adjacent to the needle seat through which the metered fuel flows. [I’ll wager not everyone was aware of that?]. The seat is a small hole, large enough for most of the tapered section of the needle to enter. The further the needle enters the seat, the less fuel flows. This is leaning the mixture. The basic principle is the same for all types and that is the supply and metering of the fuel.
It goes without saying that any interruption in this area means engine failure in some form. Either the engine will stop due to lack of fuel, overheat due to a lean mix or run erratically due to an uneven fuel supply. Luckily this area is generally trouble free mechanically but the human element is hard to beat. The main cause of any problems are foreign materials. The final aperture(s) for the metered fuel is extremely fine, it takes only a minute particle to cause problems. The first problem is dirt - a word encompassing dust, grass seed, lint, animal hair, rust flakes, paint flakes wood dust and fibres, particles of glue, glass fibres or just unrecognisable grot.
There is only one way of preventing these ‘nasties’ from getting into the fuel stream and that is effective filtering. If you use only one filter it has to be as close to the fuel nipple of the carby as possible.
Why take chances? Filter the fuel as it is mixed (if you mix your own). Filter it as you fill the tank and filter it between the tank and the engine. Would you run your car without a fuel filter? [a filter on the pressure line from the muffler is also practised by some]
The other type of blockage in the fuel chamber is the ‘mouse’s eyelash’. This is a tiny crescent of silicone about the size of a ‘mouse’s eyelash, and it can be a real ‘bitch’. You might have no problems for several runs then an engine failure. The engine will often start again and run for several tanks then fail again - very frustrating.
Inside the chamber the eyelash floats around and every so often, one will enter the fuel system and disrupt the flow. When the engine stops, the eyelash floats back into the chamber until it is in position again to sometime cause another disruption.
This ‘eyelash’ is a tiny sliver of silicone fuel tubing caused by a sharp edge on a metal fuel tube cutting it off the inside of the silicone tubing as it is slipped on the metal tube. Smooth all metal tube ends and nipples with super fine wet and dry paper and polish with metal polish to be certain there are no sharp edges. To clear the ‘eyelash’, backflush through the jet tube with fuel, having removed the fuel nipple if possible.

JET PROPELLED

Follow the flow of fuel through the needle seat, along the jet tube and out of the aperture. This aperture is our carby jet and can be in the form of a crescent shape slit, a straight and narrow slit, a rectangular slot across the diameter or just straight out the end of the tube. The fuel is sucked out of the aperture by the action of the air changing pressure and speed through the venturi. Air is sucked into the carby and down the venturi which narrows at the middle causing a change of air pressure and subsequent speeding as it passes the jet.
In so doing, it sucks the measured fuel supply with it to supply the demands of the engine. Too much and the mixture is too rich. Too little fuel and the mixture is lean. The engine will run on a disproportionate air/fuel ratio until the mix is so far out the engine stops. Stopping rich is no great problem but stopping lean can kill an engine in a remarkably short time.

JUST IDLING ALONG

On the opposite side of the jet tube to the main needle is the idle mixture control and this can be in the form of a tapered needle, parallel needle or encompassing tube all of which impede the flow of fuel from the jet. A fuel metering carby of the common type has a helical slot in the rotor so that rotary movement induces side movement. This causes the idle needle or tube to slide into or over the main jet and ‘meter’ the amount of fuel in proportion to the now reduced venturi opening.
This adjustment needs considerable care and checking as it also controls the change from idle to mid-range operation. In all types of idle metering devices the amount of movement for a considerable result is very small - 1/10 of a turn at a time. Fortunately, once set they are not prone to changes in weather like the main needle so it is rare that you would need to readjust once it is set provide you do not make radical fuel changes.

OTHER KNOBS AND BOBS

Generally, once the high and low mix is set, the engine is brought to mid-rpm and adjustment made to mid-range to obtain smooth running. Other bits will be a spring loaded bolt or bolt with a collet nut such as in OS carbies. These are idle speed adjustments and are a simple mechanical adjustment to stop the travel of the rotor. Unless it is a large engine, you should set your idle speed between 2,500 and 3,500 rpm. Under this and the engine might stop as it cools down on the landing approach, higher than this might provide too much speed for landing. Some carbies do not have this adjustment, so set it with your servo.
The usual method for reliable idle for back stick - high trim on the transmitter and low trim to kill the engine. These carbies will have a slotted or hexagon head bolt to retain the rotor which must remain firmly tight at all times. If the carby has both adjustments and retaining bolt you might find that removing the adjustment bolt will provide more control from the T/x.
It would be wise to replace the bolt with a shorter one screwed snug to prevent air leak.

SETTING THE JETS

Connect a length of fuel tubing to the nipple and close both needles gently. Open the rotor fully and blow into the tubing as you slowly open the main needle. As soon as you can feel the air flowing, stop adjusting.
Do the same thing with the idle (needle) after closing the rotor so that just a fine crescent area is open. Now open the main needle two more turns so we don’t start lean. Start the engine, take it up to full rpm and let it heat up for a minute or so.
Two clicks at a time close the needle valve (assuming the engine is running rich, if the engine is a Saito then open the needle four full turns) and listen to the rpm. Wait about 10 seconds after each adjustment for the engine to settle, then close down another two clicks and wait. Continue doing this until you do not hear a change in the engine speed.
At this point the engine is fully adjusted for static running. When the model is in the air, the static load on the engine is reduced and the rpm will increase. For this reason we are going to tune slightly rich, so wind the needle open four clicks.
The absolute final adjustment will be achieved in the test flight. By now the engine is nicely hot so we can set the idle. Take the speed down to about 2,800 rpm and listen to the engine. If it stops or loses rpm it is too rich, so wind the needle in two clicks. If it speeds up and stops it is too lean, so wind out two clicks. Keep adjusting until you obtain the maximum steady rpm at which the engine will keep running.
You might have to readjust the idle stop screw during this operation. Very slowly take the engine up to full rpm and let it run there for a minute or so. This will clear any residual fuel in the case, then let the engine return to the optimum operating temperature as this generally lowers during idle. Now back down to idle and try the transition from idle to high speed, moving the throttle lever at a speed similar to which the servo will move it. If the engine falters after idle, make a small adjustment to the idle mix as it is too lean. If the engine chokes and coughs it’s too rich.
From now on, using the same fuel, prop and plug you will need only a very small adjustment of the main needle for extremes of weather. Do not wind the needle back and forth each flight, as only minor adjustments will be needed. If you have a bit more than adequate power for the model, set richer at the start and never touch the needles. If the engine runs richer some days it won’t matter as you are not after every last drop of power.

REMOTELY SPEAKING

You can fit a remote mixture control for inflightmixture changes. Before you consider this, make sure you aren’t having yourself on that you are a beaut engine tuner. You need a very good ear to understand what the engine is telling you in the air, so don’t fit a remote tuner just for the sake of having a complex gadget or one-up-man-ship. If you must, I recommend you consider the Varsane Inflight Mixture Control due to the precision manufacturing and reliability. These units work very well and will last. The instruction sheet is comprehensive for the simple method of operation.
The other remote to consider is the needle valve. Any engine will run from a remote needle valve and it is worth considering for the sake of easy operation with awkward cowl situations, also for finger safety. Again, I can recommend the Varsane unit for the same reasons. Just keep them mounted and all connections airtight. The new OS Fx series engines come complete with a remote needle that is part of the backplate.

CLEAN AIR

Sucking dust and even grass seeds into the engine via the air intake rarely bothers the fuel supply or causes running problems. It does cause accelerated wear (in the case of four strokes, power loss if it upsets the seat of the intake valve). Another consideration is the supply of air to the engine and the problems of hot air, tight cowls and air suction pulling the fuel out of the carby.

Should you wish to ask Brian a question on these topics please send a self addressed envelope and print your name, to Brian Winch, 33 Hillview Parade, - LURNEA NSW 2170