February 5

Setting Up And Tuning RC Nitro Engines

For many RC flyers there is a certain magic to the sound andEngine Checks smell of Glow/Nitro Engines powering their models around the skies. If you are one of these then the ease of operation and reliability of your engine is an important aspect of your hobby.

In this post I aim to try and help you make life as easy as possible when it comes to operating your rc nitro engines.

Until fairly recently all of my planes were powered by glow/nitro engines and I managed to get the starting and running down to a fine art. There was rarely a time when I needed to spend ages trying to get my engines to perform correctly at the field. Sad to say , this was not so for many of my fellow modellers who often spent a great deal of time trying to get their engines to start and run reliably.

Preparation and Priming

The key to a safe and reliable start up procedure is just this; preparation and priming. These two aspects ensure  confidence in our installation and the knowledge that our engine will start reliably. Let’s take these two subjects in turn.


a) Tank Installation and Plumbing:  Although the fuel tank arrangement is not too critical once the engine is running, there are some basic precautions that need to be considered.

Glow Engine and Tank Installation

The fuel tank needs to be installed as close as possible to the engine. Ideally its centre line on or very slightly below the level of the carburettor spray bar assembly as shown here. This arrangement helps avoid flooding of the carburettor when the tank us full. It also gives the muffler pressure a good chance of maintaining fuel flow to the spray bar throughout the flight from tank full to tank empty.

An additional consideration when installing the engine and tank is to ensure that the fuel line from tank to spray bar does not rise above the full level at the top of the tank. With the fuel line as shown in the diagram positive pressure, due to gravity, will ensure that fuel flows to the spray bar for initial priming and is retained throughout the starting procedure.

The diagram above only shows two lines, one for fuel to3-line-tank plumbing for easy de-fueling spray bar and the other for muffler pressure. I personally prefer a three line system as shown here. This avoids having to remove the line from the spray bar in order to refuel/defuel the tank. Note, however, that you need to cap off the extra fuel/defuel line before starting the engine.

Another aspect of this setup is the use of two clunk lines running to the bottom of the tank. Many installations have the fuel/defuel pipe alongside the vent/pressure pipe inside the tank. The setup shown here enables defueling without having to invert the model. It does ,however, require an extra clunk weight to keep the line at the bottom of the tank.

There is also another slight modification that can be made to this arrangement and that is to put a length of rigid fuel pipe from the engine feed line clunk, approximately half the length of the tank, to prevent the tube doubling back on itself ( a “hang up”) in the event of a heavy landing.

As a precautionary part of the engine and tank installation process be sure to remove any burrs from the ends of rigid tubes and around access holes in bulkheads, etc. Check that there are no pin holes in any of the flexible silicone tubes both inside and outside the tank.

Another good precaution is to fit a fuel filter in the fuel fill line. Some people also fit fuel filters in the tank to spray bar line but I feel that so long as the fuel is filtered on its way into the tank, it shouldn’t be necessary to filter it again.

Check that there are no kinks or tight corners in any of the feed, pressure and fill lines that might restrict fuel flow.

b) Engine Integrity

Before you actually fit the engine into the plane check it over for any potential air or gas leaks. To do this carefully inspect the fit and tightness of the front  shaft housing to the crankcase, the back plate to the crankcase and the cylinder head to the crankcase. All of the screws, nuts and bolts, etc. should be checked for tightness.

Ensure that the carburettor is correctly seated in its spigot hard against the rubber “O” seal ring. Make sure there is no dust or other debris in the spray bar assembly and jet.

Some fuel adjustment needles can be a fairly slack fit in theirAir Bleed carb 2 threads and can move in or out under engine vibration. To prevent this, a small section of silicone fuel tubing can be slid over the needle thread before refitting it. this will create a resistance to free movement of the needle.

The next thing to do is to check the glow plug fitted to your engine. Do this by removing it using a suitable glow plug spanner, connecting it to your glow plug driver and observing that it glows a bright red colour.

Make sure it is the correct type of glow plug. Consult the engine manufacturers data sheet for guidance here. If you are a newcomer to rc flying I suggest a glow plug with an “idle-bar” would be appropriate. This type helps retain heat in the plug element when running at idle and tick-over speeds.

c) Throttle Servo Linkage SetupThrottle Arm Positions

To ensure full range of control the throttle to servo linkage should be arranged so that there is full movement between “tick-over” and “full throttle” with final cut off being achieved via the throttle trim control or throttle cut switch at the transmitter.

If you look vertically down on the air intake of your engine’s throttle barrel then the three barrel air openings that correspond to the arm positions shown here will be those Throttle Barrel Settingson the right. It is important to ensure that the servo output arm positions the throttle control arm at the correct positions for you engine to run correctly and give you the full range of control required.

The opening at “idle” setting should be between 1 & 2mm and should give  tick-over of between 2,500 & 3,000 rpm. The final adjustment will be dependent on the size of your propeller and the fuel used.

Getting Ready To StartProp 2 o clock

Your propeller should be fitted so that the blades are at a 8 o’clock/2 o’clock position when pushed against compression as shown here on the right. This ensures a smart positive flick can be made.

Close the main needle valve so that flooding of the carburettor does not occur while you fill the fuel tank. Once the tank is full you can open the needle again. It needs to be set 1.5 to 2.5 turns from the fully closed position.

If your tank is installed as suggested above the fuel delivery tube from tank to spray bar should fill under gravitational pressure. It may not have been possible for you to achieve a straight line between the tank and spray bar and any unavoidable hump in the tube may prevent the desired flow of fuel.

This can be rectified by placing your finger over the air intake of the carburettor (finger choking) and turn the propeller over until the fuel is drawn through. If the fuel starts to flood into the carburettor by siphon action, raise the nose of the model a little.

Alternatively, if the fuel refuses to flow into the feed tube, raise the tail of the model and repeat the choking procedure or cover the exhaust outlet with your finger and smartly flick the propeller over a few times. This creates a pumping action via the exhaust pressure system.

Priming The Engine & Testing For Fuel

A properly primed engine will usually start immediately so follow this procedure carefully.

With a full fuel line up to the spray bar and the throttle fully open, choke the carburettor using your finger as explained above. Turn the propeller over four or five times to draw some fuel into the engine crankcase.

Take your finger off the carburettor air intake and flick the propeller over repeatedly until the engine feels loose and free. Do not skimp on the flicks, it may take ten or more to ensure the fuel reaches all parts of the inside of the engine.

You will tell this has happened when the propeller turns freely over compression with a wet ‘plopping’ sound. If this does not happen, choke the engine again another two turns and repeat the priming procedure.

The best way to tell if this priming has worked successfully is to connect the glow plug to a fully charged glow driver. Firmly grasp the propeller and slowly turn it through compression. You should detect a kick as it goes over top dead centre (the compression point).

If this kick does not occur the engine is not ready to start and will require further priming or the glow plug is not functioning correctly or it is flooded. This should not happen under the routine above but excessive priming could cause the problem.

Starting and Setting The Main Needle

You are now ready to start your engine for the first time. This is done with the throttle set to ‘fully open’. The reason for this is that if there is any problem with the slow running jet setting, it will not affect the start-up unless someone has screwed the slow running needle fully in. Note that future starts are made with the throttle set to about 1.3rd.

Assuming you have correctly primed the engine it should start on the first or second flick of the propeller or with a mere touch of a starter. Keep in mind that your throttle is set to fully open so ensure the model is securely tethered or restrained and be prepared for a burst of full power!

If your engine bursts into full song but cuts soon after, it is too lean and you will need to open the main needle another half turn. Re-prime your engine and try again.

The other extreme is that the engine starts but runs rough and eventually stops. This means that the needle is set too rich and requires screwing in half a turn at a time until the engine continues to run.

In the happy event that the engine continues to run from the start you are ready to make adjustments to the main needle to obtain maximum revolutions.

You will know when the correct setting is achieved if by raising the nose of the plane upward toward the vertical the engine neither gains or looses revs.

If a drop in revs is detected with nose up, place the model back on the ground and open the main needle slightly and check again.

An alternative approach is to keep the plane on the ground and squeeze the exhaust pressure tube flat. This will eliminate the pressurisation from the tank. If the engine looses revs then a slight opening adjustment of the main needle is required.

Slow running & Throttle Response

All new engines should be set up for reliable slow running before they leave the manufacturer and should rarely need adjustment to the slow running jet. In all of my years of experience with new engines I have only once needed to make an adjustment to a slow running jet.

My advice would be to check the throttle response of your new engine by steadily closing and opening the throttle and unless it does not behave as you would expect, LEAVE WELL ALONE!

If for any reason the pick up from idle is not consistent then a SMALL adjustment may be required. I DO emphasise here the word ‘SMALL’. The difference between reliable and unreliable ‘pick-up’ is usually no more the 1/8th of a turn of the adjustment screw.

Depending on the make of engine you have purchased there will be one of two types of carburettor.

The ‘air-bleed’ type. This has a small screw with a spring on the sideAir bleed Carb of the barrel assembly that partially closes off a small air bleed hole in the front of the body.

If, when the throttle is smoothly opened the motor speeds up but then slows and cuts, then the air bleed screw is set too lean (too open).

The solution here is to turn the set screw IN no more than 1/8th of a turn. Re-start the engine and check the response again. If it still fails to keep running, adjust the screw another 1/8th of a turn although this should rarely be necessary.

The other type of carburettor is the ‘dual or twin needle’ type. As you can Twin needle carbsee, this type has a secondary needle to control the slow running which is situated in the centre of the rotating throttle barrel.

In this instance, rather than screwing the needle in to create a richer mix of air and fuel, the screw is turned OUT. The difference being that you are increasing the amount of fuel in the mix rather than, in the case above, reducing the amount of air in the mix.

An over rich mixture at the idle position will cause the engine to be reluctant to speed up smoothly and you may detect fuel being sprayed out of the carburettor. Stop the engine and make the appropriate adjustment to rectify this.

In the case of the ‘air bleed’ type turn the screw OUT 1/8th of a turn or, in the case of the ‘twin needle’ type, turn the needle IN 1/8th of a turn.

Repeat these adjustments until the engine speeds up to full power without hesitating when the throttle is opened fully in about one second.

I repeat what I said at the beginning of this section: New engines should NOT require adjustment to the slow running jet.

General Problems

If your engine gives you trouble starting then it is likely that one or other of the following possibilities has arisen:

  1. The battery is not properly charged or the glow clip / leads have a bad connection or continuity.
  2. The glow plug has failed, is the wrong type, the filament has become distorted or the engine requires a longer ‘run-in’ period.
  3. The carburettor is worn or is poorly machined.
  4. There is a gas leak: at the plug washer; around the plug’s central electrode seal; on the carburettor or engine.
  5. The fuel filter is blocked or the main carburettor jet is dirty ( often bits of silicone rubber from the feed tubes can cause this).
  6. The tank clunk has become ‘hung-up’ (doubled back on itself) or the muffler pressure nipple is blocked.
  7. The throttle servo linkage is ‘wandering’ in operation due to excessive vibration and wear.

Glow plugs are disposable items and do need to be replaced from time to time. Repeated replacement due to ‘blowing’ them is often a result of running an engine too lean at high load. Excessively high compression ratio can also cause this problem as can the presence of metal swarf and bits of grit either in a new engine or due to slow disintegration of an old engine.

Final Thoughts

There is no valid reason why modern RC Nitro Engines should present their operators with problems providing they understand the workings of said engines and know how to get the best from them. This post is aimed at helping you do exactly that and, providing you follow the guidelines closely, you should gain great enjoyment from their operation.

Don’t forget that these miniature power plants can bite you! Treat them with the greatest of respect and don’t take chances. Whirling propellers and human flesh are not the greatest of friends. They also get very hot so beware!

Please don’t hesitate to contact me via the comments facility here at the end of this post if you need my help. If you have found the post helpful, be sure to visit my website www.rookiercflyer.com especially if you are new to the hobby. Everything you need to know about getting started is there.

Looking forward to hearing from you.


















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Posted February 5, 2016 by Colin Bedson in category "Glow Powered Flight

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