Glow Plug Selection & Engine Tuning.

Safety Note The propellor of a model aircraft engine is essentially a circular saw revolving at 20,000 rpm and can cause horrendous injuries. Ideally you should wear goggles, mesh gloves and full body armour but extreme care and common sense are usually sufficient. Never try to adjust an idle screw with the engine running, it usually needs a small screwdriver and if this flicks into the rotating propellor the results do not bear thinking about. Always stand BEHIND the rotating propellor disc and read all your club rules regarding engine operation and safety. Fully read the manufactures instructions on engine safety, running in and operation. Most clubs do not like lone flyers for reasons of safety. See also the BMFA guidelines on this subject. You treat engine safety lightly at your peril. Always use a proper model restraint and ensure the model is always pulled forward into the restraint. For safety reasons many model engine designers are now fitting the carburetor at the engines rear which makes adjustments a little safer. Bear this in mind when purchasing a new engine. As a final warning Methanol can KILL or cause BLINDNESS if swallowed. If you have decided to stick with the hobby, read on:-

1) Glow Plug Selection

If you are like me, you tend to use any old glow plug and then wonder why your engine will not start or does not perform as well as it should. A quick look at any model aircraft engine catalogue will show you there is quite a range of glow plugs to choose from, so the engine manufactures must feel the need to supply different grades and types. As a first step go to the engine manufactures web site and see what is recommended for your engine. For best results this is a good starting point. Make a note of your fuel type and the methanol content, as this has a major influence on plug selection. With a new engine care should be taken to follow the running in guide and let the engine settle down before any serious attempt is made to tune for peak revs. You will need to run at least 4 full tanks of fuel through a new engine with frequent stops to prevent overheating, low revs and the mixture set slightly on the rich side giving plenty of oil for lubrication. Before the engine starts to loosen up or gets run in the golden rule is not to overheat or let the engine "labour". Some engines are very tight at top dead center (TDC) when new but this is all part of the design, as the bore/piston is very slightly tapered. Careful running in will ensure a perfect fit. A bit of castor oil (2%) will aid running in, as it is a far better lubricant than any synthetic oil and will not fail at high temperatures. Links are given below to the major engine manufactures web sites where you will find plug grades for your motor. Failing this, the following guide will help, or your local model shop will usually supply the information required.

Note:- Glow plugs are very often unmarked, once removed from the package the identity is lost (OS Plugs are an exception) Leave in the bubble pack until required as this also prevents damage to the filament.. Keep used plugs wrapped in tissue in marked tins. A plug in good condition should give a bright orange glow. If it is dull on 1.5 volts there is little chance of a good glow with the engine running and the battery removed. Do not use plated plugs, the platinum flakes off in time and performance falls. It is false economy. When you buy cheap, you buy twice. Replace plugs every 12 months if you fly regularly. If the revs fall when you remove the glow battery the plug is too cold or more nitro is needed. If it backfires when hand cranking the plug is too hot or less nitro is needed. Hot plugs need 2 volts, cold plugs need 1.2 - 1.5 volts. Ducted Fan Engines with high nitro and high revs need cold plugs. Four Strokes generally run with less oil (10%) and no nitro and therefore need hot plugs. A high methanol percentage usually means a low nitro percentage as the oil content remains more or less constant. For example an 80% methanol fuel will have no nitro as the rest will be oil. (20%) The basic 80/20 mix. Most club flyers use between 5% and 10% methanol so a No. 4, F5 or #8 Plug is probably most suitable. See Fuel Mixing Section for more detail.

Idle Bars. These are designed to prevent un-vapourized fuel hitting the glow plug coil and cooling it. Useful for motors that tend to run wet or old motors

2) Tuning the Glow Motor

The carburetors on most model glow engines are, by automobile standards very crude, but the do exactly the same job in converting the liquid fuel/oil mix into a very fine mist or vapour. For the engine to run well they need to do this over a wide RPM range with instant pickup. An engine that cuts out as you open the throttle quickly is not much use. You will soon get fed up shouting "Dead Stick". However very few engines react well to the throttle being slammed fully open in a split second, allow at least a second from closed to fully open. As these are simple carburetors they only have two adjustments, a needle valve for top end mixture and a simple screw for the low idle range. With the correct plug and some half decent fuel close the needle valve fully then open two full turns. Connect the glow plug battery and with the throttle about 2/3rds open try to start the engine. Slowly open the needle valve and the engine should start to run after a fashion. Once the engine is running allow 30 - 40 seconds running to warm up.

Peak Revs or Top End adjustment.

1) Slowly close the needle valve a couple of clicks at a time until the engine revs peak and then start to fall off. This is usually done with the engine running so take extreme care.

2) Back off for peak revs.

3) Squeeze the fuel line gently between each adjustment and note if the revs rise. Let go of the fuel line as soon as the revs alter. If the revs rise, continue to turn needle slowly clockwise.

4) When the engine is properly adjusted the RPM will increase a little if you gently squeeze the fuel line. If the engine cuts out when the fuel line is squeezed, the engine is too lean.

5) Finally, hold aircraft with the nose pointing up 45 degrees. If the engine cuts out then it is too lean. If it starts to misfire or splutter it is too rich.

6) The mixture on some models tends to alter in flight, I always set my mixture a couple of clicks rich, but only experience can help here.

Idle or Bottom End adjustment.

See Safety note on adjusting this screw.

1) Close the slow running screw and then open by approximately 2 – 3 full turns.

2) Start engine and run at full throttle for about 20 seconds. Bring engine to idle (2,000 rpm) for about 20 seconds.

3) Squeeze the fuel line and note any change in revs. It the revs increase and then the motor stops, the bottom end is too rich. Turn the needle 1/4 turn clockwise. If the revs decrease and the motor stops, the bottom end is too lean. (turn the needle 1/4 turn anti-clockwise)

4) Repeat this process until the engine does not gain or lose revs before stopping when the fuel line is squeezed. When you are getting close to perfection it will be necessary to make very small adjustments, perhaps only 1/8th of turn at a time.

5) Run engine at full throttle and ensure that all is well, making any small adjustments that may be necessary. Ensure that the top end is properly adjusted before attempting to make any bottom end adjustments.

An engine correctly set up should idle reliably and then pick up cleanly as the throttle is opened reasonably quickly. Very rapid opening "gags" the motor. If it is a bit rich it will hesitate for a second before the revs rise. If it is a bit lean it is liable to cut out. When opening up from a long idle a lot of smoke indicates a rich bottom end setting.

Always ensure that the fuel pickup in the tank is 1/2" above or below the carburetors centerline and the tank is insulated to isolate it from vibration. This can cause the fuel to foam. Bubbles in the fuel line always indicate a leak or possible foaming. For reliable running this MUST be cured. Some fuels have an anti-foaming addative.

OS Engines Go to Product Manuals

Model Technics Fuel Web Site inc. Maxflash & Firepower Glow Plug information

Enya Engines & Glow Plugs. Full details of Enya Glow Plugs

Steve Webb Models for Enya, Saito, Thunder Tigre etc.

Ripmax Not a lot of technical help on this site

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Text © Colin Usher 2008 Illustrations © Colin Usher 2008

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