There is definitely no witchcraft involved in this, all that is required is a good bit of patience, plus the knowledge of what to do & why you are doing it. Take your time & you will usually get it right the first time! Over the years I have seen quite a few models hitting the ground - or worse! just because the engine was not set up correctly in the first place & stopped suddenly. At your flying field, you will observe that some flyers just start their engines & fly their models successfully, seldom having to shout "dead stick" or plummetting into the ground, but some other modellers seem to constantly mess about with their engines on a weekly basis. Crashing into trees & stone walls is a slight problem if your engine has stopped & the obstacle is in your flightpath!
Manufacturers often leave the "idle needle" in approximately the right place, but it is dependent on other factors such as engine position in the airframe, fuel type etc. In my experience, usually the high quality end of the engine market come out of the box better setup, but not always. One well known quality British manufacturer actually runs each engine before it is dispatched, & these are ready to go, only a fractional movement, if any, is needed to compensate for the above conditions. Also you will need at least a reasonable quality engine in the first place, preferably with some sort of compression, and without air leaks around the base of the carburretor inlet or needle valves!
Before I start describing the process of "setting up the carb", I need to mention the pros & cons of "exhaust pressure feed". The idea is that a pressure tapping is taken from the silencer via a small nipple & connected by a pipe to the fuel tank. The other vent pipe is blocked with a plug after filling. The exhaust back pressure slightly pressurises the tank, forcing fuel to the carburettor. The problem is that the main needle setting can become more critical, & throttle pickup can become erratic as the pressure from the silencer is much lower during idle. If the pressure pipe should fall off, or the silencer come loose, then the engine becomes too lean & stops running! Some people moan about potential contamination of the fuel with exhaust by-products, but in reality I think that's maybe taking it a bit far. Although you can often see a little carbon & burnt oil in the silencer pressure pipe & in the inside wall of the fuel tank.
I prefer silencer pressure for these reasons:
1) constant head of fuel at the carburettor inlet
2) fuel doesn't run out of the vent pies when the aircraft is inverted (in flight or otherwise), or when carrying it nose down back from the flight line
3) Fuel doesn't run everywhere in the car when you forget to drain the tank when going home after a day's flying
4) Depending on the physical position of the carburettor on some engine types, it is sometimes difficult to get the tank in the right position to ensure a good feed to the carburettor in all flight attitudes without a pressure feed. If you can build from a kit, then it is usually possible to arrange the tank placement to suit the engine, but on the A.R.T.F. type, not all engines have the carburettor on the centre line of the model. If your engine has the carburettor lower than the centre line of the tank, then the fuel siphons out of the tank to the carburettor & drips onto the ground, although at least the engine always has a "head of fuel" when it is the right way up! You then run the risk of a temporary drop in power or complete engine stop, owing to fuel starvation when flying inverted or during a rolling maneuvre. There are specialised pumps available for model engines, but I have never had the need to use them. My models fly well enough for my non-competitive sport flying aspect of this hobby. Non-pressure is usually fine for "vintage" type models which just stooge around slowly, mainly in an upright position, but I have personally found that a well known, very small capacity .30 size four stroke engine from a Chinese manufacturer, seems to run better with pressure, & develops a little more power. I have also seen a very large number of "dead stick" landings from these non-pressure fed "vintage models" with all sorts of engine types & sizes fitted, the engine having stopped suddenly, shortly followed by gliding into walls, trees, fences & other various obstacles - generally culminating in heavy arrivals into the ground with considerable damage to the airframe in the process!
I have always used "inline" fuel filters on all of my engines, as a very small piece of grass or grit can easily block the passage of fuel past the extremely fine main needle valve. If this happens during take off, or climbing out after a slow low pass, then the whole model, radio & engine are at risk. Please note: With these filters, always check that the are screwed together tightly before use, so as not to let any air in, around the "O" ring seal. There are a few different styles of carburettor fitted to model glowplug engines, but the operation of the most common type is described below: First, have a good look at the carburettor's construction & notice that it usually consists of a rotating barrel with a large hole through at 90 degrees to the barrel, with a master needle valve at one side & a second needle at the other side, which is often in the form of a small slotted screw in the end of the barrel. As this barrel is rotated by the throttle servo it does two main things:
1) it allows more or less air into the engine - and at the same time .........
2) it allows more or less fuel into the engine via the position of the barrel relative to the adjustable "bottom end" needle valve.
More fuel & air in the correct ratio equals more RPM. Less fuel & air in the correct ratio equals less RPM. The main needle valve is usually a long, thin pointed needle, allowing very fine adjustments. The idle needle is shorter & blunter, therefore allowing a much quicker change of fuel admission from the spraybar as the throttle barrel is rotated into the open position. Some engines use the "air bleed idle system" but as far as the setup is concerned, it is worth noting that the "air bleed screw" which is usually at the side of the carburettor (not to be confused with the "throttle stop screw") which is usually on the top of the carburettor, must be adjusted in the opposite way to the "internal barrel" type ie: to lean out the idle = unscrew, to richen the idle = screw in The following description assumes the spraybar idle needle type: As the barrel rotates, the spraybar (the thin tube in the centre of the barrel, where the fuel comes out of) moves further away from the idle needle which is positioned centrally to the spraybar, the more it is opened, the further away the spraybar moves away from the idle needle, letting more fuel into the engine's intake. The main needle is in a fixed position always, initially set by the operator, so with the engine on full - (carburettor wide open), the needle is first manually adjusted to let the correct amount of fuel into the spraybar, this is sucked into the engine on each induction stroke in the form of a spray, a bit like a paint spraygun.
So .......... Start the engine with the throttle initially set low, run it up to full power, adjust the top end (master) needle valve until the engine "peaks" out - hold the model nose up to check that you are not running too lean. place the model back on the ground & slowly close the throttle until your engine idles. Set the throttle travel by either adjusting the servo's travel manually or use "ATV" (servo travel end point adjustment) on your radio transmitter, until the engine runs at an acceptable idle speed. The following adjustments should not be carried out until the engine is correctly run in, as a "tight"engine may be reluctant to run slowly at first. Once the engine is idling, open the throttle quickly & observe what it does, if it splutters its way up to full power, then the idle needle is too far out - too rich. If the engine "coughs" & sounds like it is going to cut out, or actually stops, then the idle needle is set too far in - too lean. With a small screwdriver (I have a really long thin screwdriver for this bit, so that I can set the idle needle position while the engine is running, without getting my fingers in the spinning prop arc - ouch! - that brings back memories of my youth, I still have the scars to prove it!) turn the idle needle in or out depending on whether the engine's mixture is lean or rich. Only move about a sixteenth of an inch (or approx 1 millimetre) at a time in or out, then open up the throttle again to see what happens. the ideal situation is when the engine picks up quickly without hesitation. The main needle now needs adjusting slightly, owing to the change of position, inside the end of the spraybar, of the idle needle valve. Some engines are very critical of the mixture settings, others are not. Run the engine on full, re-adjust the main needle, hold the model verically again to verify that you are not going too lean, which you do not want as the aircraft climbs out on take off, then place the model down on the ground again & check for pickup from idle to full power.
Each time that you change the position of the idle needle valve, re-check the position of the main needle valve with the throttle barrel fully open.
As you reach the optimum "bottom end" setting, the engine may be idling too fast, so subsequent re-adjustment of the "ATV" (or servo travel end point adjustment) may need to be carried out once again. Do not, under any circumstances rush this procedure, because if you get it wrong, then you run the risk of a "dead stick" at the wrong time! Once again, don't forget, whenever you adjust the "Bottom End Needle Valve", always re-check the setting of the "Top End Needle Valve" .................................
The engine's mixture settings are very much affected by the fuel type, my "more methanol - less oil mix" needs the settings all to be leaner, as in each induction stroke, the ratio of methanol to oil is a good bit more than normal, so there is more combustable mixture in each cylinder full - if you see what I mean! If you change fuel type, always test your throttle settings before flying the model. Under normal circumstances though, variations in external air temperature, air pressure & moisture density ie: running in Summer & Winter on hot or cold days usually only requires a small adjustment of the main needle valve, if any.
All the above information is not copied from any manuals, it is based on my own experience of running model aero engines over the years with very few fuel/mixture problems. If you disagree with the above brief description on "How to setup a model engine carburettor", Please do not email me, because you could always of course use the cast iron cauldron based method - with "wing of bat, eye of newt"etc etc !