Setup Carburettors by Steve Dorling (RC Model World)

The typical model engine carburettor is extremely simple relying on a venturi and variable metered fuel supply with none of the complexities of the true carburettor such as float chambers and accelerator pumps and the like. Because the model engine carburettor is so very basic and simple one would be forgiven for thinking that operation and maintenance of this component should be almost foolproof but unfortunately the model engine carburettor can be a source of frustration and misunderstanding. The two most commonplace types of carburettor are the twin needle and the air bleed device, so we will concentrate on those in the main, and then take a look at some of the variations on the theme later on in the chapter.

The very simplest way of metering the correct ratio of fuel and air is a straight venturi and needle valve set up, such as those seen on countless
control line and free flight engines over the past goodness knows how many years. The engine sucks in air and the depression in the intake draws fuel and air into the venturi. A small hole or pair of holes in the spraybar assembly allow fuel to be drawn into the intake and in order to precisely meter the quantity of fuel that mixes with the air, a variable needle valve allows the ratio to be set to optimum. The drawback with this splendidly simple arrangement is a total lack of throttle control. Lack of a throttle was hardly an issue before the advent of radio control but today we expect our engines to have a throttled carburettor that will give us a range of engine speeds from a slow idle to flat out. So how do we effect engine speed variation then ?

To reduce the power output of an engine all we need to do is reduce the quantity of air and fuel that it is able to ingest. If we reduce the choke
size of the venturi that feeds the inlet port of the engine then that will obviously have an effect on the amount of air that the engine can draw, but
just closing off the air intake won't work because the fuel and air ratio will be all wrong. What the carburettor needs to be able to do is reduce
the air and the fuel in a precise and measured way so that as the airflow is restricted, perhaps by a butterfly valve or slide assembly, or more commonly in the model engine, a rotating drum valve, the fuel flow reduces automatically pro rata. We need to maintain about a 7 to 1 ratio of air to fuel in a methanol burning glow engine and about twice that for a petrol device and once the needle valve is set up for flat out maximum power, you may assume that is what you have achieved. Less straightforward a situation to effectively manage is to maintain that constant and precise ratio from wide open throttle to idle setting and invariably as the throttle closes beyond a certain point, some fine adjustment will be required to maintain that essential 7:1 ( or 14:1) ratio. Its no earthly use moving the main needle valve for this adjustment because if you do that then the wide open setting that you just set will be wrong when you throttle up again. Take a look at a model engine carburettor and you will see that there is a screw somewhere in the body of the device that allows adjustment for low speed throttle settings. This screw may be either a secondary needle valve or an air bleed screw. Let's explore the air bleed screw type of carburettor first then.

The air bleed carburettor is the most simple and basic of the throttling carburettors used on model engines and as the name would suggest, the air bleed screw affects the passage of air that is allowed to "bleed" through a hole in the carburettor body when the throttle is closed. If you examine an air bleed carburettor closely then you will see that there is a minute drilling that allows a very small quantity of air to be drawn into the intake even when the throttle is closed. This small quantity of air is enough for the engine to idle on and because the amount of air and fuel
needed by an engine at idle is so little, the air bleed screw is provided to allow precise and small adjustments to be made to provide us with a nice reliable tick over and low throttle setting. All the air bleed screw does is partially block that tiny air passage so that the critical fuel air ratio
can be maintained at idle speeds. You can't get much less fancy than an air bleed carburettor, yet they work just fine for our needs. The main thing to take on board with an air bleed adjustment is that the more you unscrew it
the weaker the idle mixture will be, which of course is totally opposite to the main mixture adjustment where unscrewing the needle valve gives the engine a richer fuel air mixture. It is all quite logical when you evaluate the situation because the air bleed screw affects the air supply whereas the needle valve alters the fuel supply. That's all there is to the air bleed carburettor, it really is that simple.

The next and today, the most popular type of carburettor seen fitted to your everyday glow engine is the ubiquitous twin needle device. The twin needle carburettor has a needle valve at both ends of its spraybar and the main needle behaves identically to that found in its air bleed counterpart. There is no air bleed screw on this carburettor, instead we find a secondary and smaller needle valve on the opposite end of the spraybar and this one effects the low throttle setting. With any of the twin needle carburettors, the idle adjustment is out, or anticlockwise for rich, in other words, movement is in the same direction as the main needle valve and opposite to that of the aforementioned air bleed screw.

Setting up a carburettor is a five minute job at most, whatever the type, make or nationality of the device in question, yet visit any flying site and
you will almost certainly be greeted by the pitiful sight of a gaggle of helpers arguing about which way you turn what and generally getting into a
mess with an engine that is misbehaving. Invariably the problem may be attributed to a less than methodical approach allied to a lack of
understanding of the basic principles involved. The very first thing that one needs to be absolutely sure of before even
thinking of trying to set up a carburettor, is that the tank and plumbing are in first class tip top order. If the fuel tank, its placement and
mounting (crucial)and the associated plumbing give no cause for concern following inspection, the glowplug should be checked and verified as
suitable for the job because it is no earthly use trying to set up an engine on the wrong or a worn out plug. Lastly is the fuel ok and the right
mixture for the engine, because again, like the plug and the tank, correct fuel is essential and to this end the manufacturers information and advice should be adhered to rigidly. Let's assume then that the tank and pipe work is fine, which for my money means a TWO pipe system, no filters, T pieces, fancy filler connections or other complications that simply offer the beginner a greater chance of intorducing a problem, that the plug is new and of the correct heat range and your fuel is just fine and dandy too. Oh and one more thing, do make sure that your glow battery is well charged and that the clip is a nice fit and deviod of opportunities to short circuit and here I strongly advocate a seperate glow and starter battery because as soon as you put a starter to an engine the load on the battery causes a voltage drop, which causes the glow
to sag. Flight panels that are sophistacated transistorised devices allied to large car type batteries are fine, but anything less hefty and
sophisticated is courting failure. The first thing you need to ascertain before you attempt to start the engine is that the carburettor is roughly
set up to give you a suitable starting point. Set air bleed screws and secondary needles to their midway settings and don't assume that because
your mates "acme whizzo .46" started straight from the box, that yours will do likewise. Those midway settings will put you in the ball park and you won't be too far off the mark, but that will not do for the main needle. My method for initially checking an engine is to disconnect the fuel feed pipe and with a wide open throttle, blow into it whilst listening for the hiss of air passing through the needle and spraybar assembly. Close the needle until no air is able to pass through it and then open it a couple of turns or until you can hear and feel air escaping through the spraybar and you should be able to start the engine and adjust the main high speed settings. When the engine fires open the throttle wide and adjust the main needle until the engine is obviously rich, which on a glow means a barking four stroke exhaust note. Turn in the main needle a quarter of a turn at a time, pausing to let each increment take effect and when the optimum point of max power and rpm is reached, richen the setting slightly and that is that. If the engine is fitted to a model then this should be done on a half full tank of fuel and with the nose raised to about sixty degrees as this replicates an average condition of mid term flight. Once the main needle has been adjusted you can turn your attention to adjusting the idle and low throttle response - not the other way round as many seem to think.

A lot of carburettor problems attributed to either brand or price of an engine tend to be self induced and this is predominantly because we
modellers can be a lazy bunch. New year, new season, time to dust off the remains of that favourite model that you chucked up over the eaves in the garage at the end of last year and check out the furry ball of corrosion that was once a beautiful engine ! Sound familiar ?

Most carb problems are associated with gumming or dirt, both of which clog up the minute little galleries in the spraybar and carburettor body itself and O-ring seals that no longer perform that function. After a prolonged period of non-usage fuel residues will congeal and block the fuel and airways. Castor oil is the worst culprit for this malady but any oil will attract dirt and muck and those critical O-rings go hard and cracked.
Fortunately most model carburettors are pretty similar and of the two aforementioned distinct types, either the air bleed or the twin needle type.
Stripping and cleaning either type is simple and setting the serviced carburettor up afterwards is pretty straightforward as long as you are
methodical.

Disassembling any part of a model engine should be done with cleanliness and care to the forefront. Never ever use force and make sure the tools that you intend to use are the correct ones. I hate to see rounded screws and pliers and vice marks on a piece of precision engineering. Your carb will be affixed to the inlet tract of the engine casing with a pinch bolt or grub screws and they just need slackening off a little for the carb to be liberated. Under the carburettor body you will find a rubber O-ring and this should be removed and inspected for hardening or damage. This little component is responsible for an awful lot of dead stick arrivals and lousy throttle responses, so if in doubt replace it. Lay a nice clean sheet of kitchen roll on your work surface and lay out the parts in a methodical way as you remove them to help you remember what went where and how. Start by taking the needle valve completely out, be aware of springs or O rings behind the needle. Take out the secondary needle or the air bleed screw, (same applies-re springs etc) and then take out the screw that holds the barrel in place. This bolt usually doubles up as a fixed idle adjustment. I always back this off completely and set up my idle on the servo. Once removed the barrel should slide out. Be careful, sometimes there is a spring behind it.

You can take the spray bar out of the throttle barrel on some carbs, but as you can do all you need to do with it still in place, just clean and clear the galleries leaving the actual spraybar in place seems wise to me. Blowing though airways using a piece of fuel tubing usually clears any
blockages, but stubborn gunge may see one resort to a cocktail stick or a very soft piece of copper wire, but please don't resort to anything metallic that can scratch or distort! Once you are happy, reassemble in reverse order, replacing damaged O-rings or gaskets and just before you pop the needle(s) back into their respective housings, roll them on a flat surface, or between your fingertips to make sure that they haven't been bent during an arrival!

To initially set up ANY carburettor, gently turn the main needle fully home, then undo it about three whole turns. Air bleed screws and secondary needles should be set at their mid points to start with (common sense approach) and that tip for determining a preliminary main needle setting mentioned earlier, to blow into the spray bar through a length of fuel tubing whilst slowly opening the needle works well. About a turn out from the first hiss of escaping air will at least let the engine run. Start the engine and open the throttle wide, this is the ONLY setting that will allow you to set up the main needle valve. Once you are happy with it at full power, turn your attention to the idle screw following the instructions outlined earlier. All this takes rather longer to read than do and the reliability you will achieve makes it a worthwhile exercise, so let's run through the two types and give each one a bit of a service!

The air bleed carburettor whilst less common than the twin needle device, can be found on some current engines. The air bleed carburettor has only one needle valve, which is for full power adjustment only. To slow the engine, a rotating barrel (usually), gradually reduces the air and fuel
quantities as the throttle is closed. At idle, the minute adjustment needed to achieve a steady tick over and an "air bleed" screw affects a clean pick up. Many new modellers and a few not so new.fiddle and diddle with the main needle valve in a vain effort to improve the engine pick up, which is a waste of time because the air bleed screw, a small bolt, positioned in that tiny airway that you will see in the carburettor body somewhere is the one to adjust. It only affects the idle mixture and it does so by reducing or increasing, very minutely, how much air is allowed into the intake on a closed throttle. Open the throttle wide and adjust the main needle for optimum running and once you are happy with that, close the throttle and adjust the air bleed screw until you achieve a nice idle and a clean pick up. The air bleed screw is turned clockwise to richen the idle mix, (opposite to a secondary needle). If the engine lags and dies on throttle up, then the idle setting is lean or weak (too much air), so screwing the bleed screw in (clockwise) will reduce the air to fuel ratio, effectively richening the low end setting. If the engine splutters its way up the RPM range, coughing great plumes of fuel spray into the air, then go the opposite way because this denotes richness. Make one half turn adjustment at a time and WAIT...for it to take effect before going further. The twin needle carburettor dispenses with the air bleed arrangement and uses instead, a secondary needle, which you will find opposite the main needle valve. There is usually a small screw slot in this one to facilitate adjustment. This time, because you are varying the fuel rather than the air at low idle, it's in for lean and out for rich (exactly like the main needle) but that apart, the two carburettors behave similarly.

Model engine carburettors are very simple and very straight forward to adjust. If you persistently have to adjust and readjust yours then look for
a solution to a different problem because by and large, once set up the carburettor should stay that way. Related snags might be anything from poor or contaminated fuel to a less than good or incorrect heat range glowplug. Air leaks from needle valves, fuel pipes and exhaust systems are commonplace and the source of much misery and frustration. The O-ring at the base of the carburettor where it abuts the crankcase venturi should be perfectly sealed because an air leak here will play havoc with low throttle settings. At high engine speeds air leaks can be compensated for by a tweak on the main needle and therefore tend not to show up so readily, but if your engine throttle response is less than dependable look for leaky plumbing or air leaks in the aforementioned areas and please don't automatically slate an engine or its throttling performance because of the name on the box. The engines that cross my bench tend towards at least merchantable and reliable quality
almost without exception today and I often state that there are few if any bad engines to be found these days because simple economics mean that in a global market the bad ones do not survive for very long. If you doubt this for one minute then take a look at the world and national championship results in competition circles to see the diverse range of marques and types employed by the winners, it can be enlightening.

Steve Dorling

 

Article by Steve Dorling from RC Model World
26st May 2001