Glow Engine Break-In and Tuning

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Written by Andrew Coholic
Tips for breaking in your engine
As seen in the May 2021 issue of Model Aviation.


Bonus Video



an o.s. la .15 engine with an airbleed carburetor
01. An O.S. LA .15 engine with an airbleed carburetor and rear-mounted, high-speed needle. Notice the air-bleed hole and adjustment screw on the carburetor’s body.
an enya tv .15 engine
02. An Enya TV .15 engine, with an airbleed carburetor, swept-back main needle and airbleed hole, and an adjustment screw on the carburetor’s body.
this is an irvine metal bodied twoneedle carburetor
03. This is an Irvine metal-bodied, two-needle carburetor. Notice the high-speed needle and low-speed needle are opposite (on the throttle arm side) with slotted head adjustment.
this supertigre two needle carburetor
04. This SuperTigre two-needle carburetor has the low-speed needle within the throttle arm side of the barrel. The high-speed (main) adjustment needle is opposite it.

Glow engine break-in and tuning is one of those topics that continues to be the specialty of a few "engine gurus" in many clubs. It used to be that we would seek out the more experienced modelers for engine help—but it doesn’t have to be that way. With the proliferation of electric-powered models in the past decade, and the demise of many of the long-established glow engine manufacturers, glow engine-powered models are not the mainstays that they once were.

With what seems to be a resurgence in glow power, many of us are trying it for the first time. This includes buying used engines and older new-in-the-box versions that might not come with instructions. Every modeler should arm himself or herself with the knowledge of the basics on how to set up and operate them for maximum longevity and reliability.

If you break down the "process" of running engines into parts, I find it easier. If you tackle it in segments versus the entire thing, you are less likely to overlook items that might cause issues and the related frustration. The main things to be mindful of are mounting and orientation; glow plug and fuel requirements; propeller choice; carburetor design and adjustment; and any accessories that are either required or suggested for safety or just to make things easier.

Running, and more specifically break-in, varies depending upon the type of engine that you are going to run (a ringed engine such as an ABC/AAC/ABN type, or older lapped-steel and iron construction).

Let’s begin! I’ll assume that you are starting with a new engine. Used engines, specifically those that have been crashed, improperly stored, or are stuck from sitting for too long, can have issues that complicate things.

Most engines from larger manufacturers have a set of instructions that offer a good starting point for where to set the needles, what plugs and propellers to use, fuel selection, and so forth. Some go into great detail; others are quite vague. For companies that provide a good instruction set, I suggest reading and using the manufacturer’s recommendations as a starting point. However, if you find yourself with an engine and no documentation, do not worry. You can still figure things out and achieve success.


Engines should initially be run (or "broken in") on a test stand, or in a model with easy access to both the engine and the fuel system. You do not want to place a new engine in a tightly cowled situation with a hidden fuel system. That makes it difficult to diagnose any issues that don’t involve the engine itself, such as leaking fuel lines, an inadequately tightened carburetor or muffler, poor cooling, etc.

I have learned that any engine will benefit from at least a few tanks of fuel being run through it on a test stand or in an airframe such as an Ugly Stik, where you have easy access to everything and can ensure that the engine receives adequate cooling.

You can also mount the engine to either a fiberglass-reinforced plastic or metal mount then to a plank, or use a piece of plywood cut out for the engine to sit in. Whatever means you use, make sure that you have the mounting lugs of the engine sitting flat and parallel.

Bolting or screwing an engine down to an uneven surface can introduce distortion and stress, which can cause friction and uneven wear or even cause stress cracks that make it impossible for the engine to run reliably. If you use an airframe to break in the engine and set it up, the same rules apply. You should also make sure that whatever means is used to mount the engine (drilled and tapped machine screws, coarsethread self-tapping screws, machine bolts and locking nuts, etc.) is suitable for the size and power of the engine. If you soft-mount an engine, make sure it cannot vibrate and contact anything around it. Follow the same rule later when it is in the airframe.

Propeller choice is based on what the manufacturer recommends, or rules of thumb gathered by experience. For example, .15 cu. in. glow engines usually run well with a 7 × 4 to 8 × 4 propeller. A good place to start with .25 cu. in. engines is an 8 × 4 to a 9 × 6 propeller. The .40- or .46-class engines run well with 10 × 6 to 11 × 6 propellers. A large 1.50 cu. in. or 1.80 cu. in. glow engine would run best with an 17 × 8 or 18 × 8 propeller.

Many propeller and engine manufacturers offer charts that give a good starting place to guide users. It is always a good idea for new propellers to be checked for balance and corrected if needed. Molded propellers can have extremely sharp edges; taking a bit of 220-grit sandpaper to these edges can save you from cuts. Sand a small amount off the face of the heavier blade to ensure that it balances and runs smoothly.

use a good quality plug wrench
04. Use a good-quality plug wrench (do not cross-thread and don’t over torque), and a name-brand glow plug. An O.S. #8 glow plug is ideal for many engines.

Glow plugs are often suggested in the instructions. Various engines will prefer (depending upon the fuel and propeller choice) a hotter or cooler plug. A cold plug will ignite the fuel/air mixture in the cylinder slightly later in the cycle, while a hot plug will do the opposite. Whether you run an engine in the summer or winter can also make plug selection critical. Use a hotter plug when the weather is colder and the opposite in the heat of summer.

Running smaller propellers at a higher rpm or with a higher percentage of nitromethane in the fuel might warrant the use of a plug with a cooler heat rating. My go-to plug in most sport two-stroke engines that run on 5% to 15% glow fuel is the O.S. #8.


Most engines (with some exceptions) will run well on any commercially available fuel with 5% to 15% nitromethane. Oil content is generally from 16% to 20% with either a synthetic oil, castor oil, or a combination of the two used for lubrication. Most older engines will definitely run better and last longer using a higher-percentage of castor-based fuel.

Newer ringed or ABC-type engines run well with less oil and either all-synthetic oil or fuel with a small percentage of castor and the remainder being synthetic. The primary thing is to keep the fuel clean, both when filling the tanks and feeding it to the engine. Fuel filters are inexpensive and can keep dirt and grime from fouling the carburetor passageways.

The Break-In Process

The break-in process is twofold. It allows operators to familiarize themselves with a particular engine, making setup and tuning easier. It also allows a new engine a chance to run with adequate lubrication at optimum temperatures. So, let’s get to it!

We are try to start most engines while the main fuel adjustment (high-speed or primary needle) is open to allow plenty of fuel to reach the engine. Starting an engine lean is a quick way to wear things out prematurely. We want to let the excess fuel (and oil) run through the bearing and wear surfaces on the engine, and take away the excess heat.

If you do not know the initial recommended needle settings, I have found that fully opening the carburetor barrel works well. Place a clean piece of fuel tubing on the inlet and blow through it while backing off the main needle valve. After you hit the point of least restriction, the needle is fully open.

You can try to start the engine knowing that it will not be lean. With the glow ignitor on the plug, make sure that the coils are glowing a healthy orange, and with the carburetor wide open or partially closed (but able to be moved once started), start the engine. Using an electric starter is the simplest and safest method for most engines. If you want to try to hand-start your engine, I suggest using either a chicken stick or a gloved hand. Under no circumstances do I recommend using your bare fingers.

After the engine starts, leave the glow ignitor connected to ensure that the engine remains running and you can (safely) start to slowly close down the needle valve to get to the point where the engine will remain running—albeit very rich. If you are running a ringed or lapped iron/steel engine, you can let the engine run for 30 seconds or so then close the carburetor (or pinch off the fuel supply) to stop it.

Let the engine cool down and continue this process while leaning the engine’s high-speed needle a small amount (1/8 to 1/4 turn) every few runs. Letting the engine run, heat up, and then cool off will let the metal-to-metal contact parts wear in nicely.

If you are running a lapped engine with an aluminum piston (ABC type), you do not want to initially run it quite as rich. After the engine is started and has warmed up for 20 seconds, lean out the high-speed needle so that the engine is running well on its own (without the glow ignitor) but is still running on the rich side. Some oil and unburned fuel exiting the muffler is good, but preserve the tight piston-to-cylinder fit at top dead center, you must not run these engines too rich. They need to run at an elevated temperature to allow the sleeve to expand to the proper running fit.

After a tank of fuel has run through the engine, I suggest setting the high-speed needle (or rpm with the carburetor barrel set wide open) 200 or 300 rpm slower than what can be achieved. Do not set the engine to remain running at maximum rpm and never lean it past that point. Although an experienced engine operator can adjust this by ear, I suggest using an optical tachometer to ensure that you are running the engine at the required slower-than-peak rpm.

balance your propellers on a proper balancer
06. Balance your propellers on a proper balancer (such as the Du-Bro unit that is shown) by sanding the face of the heavier blade until the propeller is level.

To set the low speed of the engine, you need to know whether you have a carburetor with an air-bleed screw or a true dual-needle-metered carburetor. On an air-bleed-type carburetor at an idle setting, the air-bleed screw covers or uncovers a small hole that allows air into the engine. Closing off this hole (moving the screw inward) reduces the air-to-fuel ratio, thereby richening the setting at idle. Unscrewing the air bleed will allow more air in and thereby causes a leaner setting at idle. This adjustment doesn’t affect the engine’s speed or behavior at any other setting. Set the air bleed for the most reliable engine running at idle and use the idle stop to prevent trying to get the rpm too low.

With a two-needle-type carburetor, there is usually a secondary needle to adjust on the opposite side of the main or high-speed needle. This can be adjusted with a slot screwdriver or Allen key. An engine will often come from the factory with the low-speed needle set well enough to run.

I do a similar procedure for the low-speed needle as I do for the main needle adjustment. Place a clean piece of fuel tubing on the inlet. This time, close the carburetor barrel so that it is only slightly open and adjust the screw so that you can hear air coming through the jet. Set it on the open side, which will ensure a rich setting.

Engine acceleration and idle are controlled by the screw. As with the main needle, the more the needle is screwed in, the leaner the mixture becomes. Opening the needle will richen the mixture. You want to achieve a reliable idle with a decent acceleration (no sputtering or varying rpm while accelerating).

If the engine suddenly dies, you have the low speed set too lean. After you have the idle set to your liking, it is a good idea to recheck the high-speed needle and adjust it accordingly, if required. Most dual-needle carburetors have a slight amount of overlap, where both needles can affect the tuning, going through midrange and up.

After the idle setting is adequately adjusted, test the transition from idle to maximum rpm. With the engine warmed up, you should be able to cleanly transition from an idle setting to mid-throttle or higher, with minimal sputtering or hesitation.

By the time you have run the engine enough to tune it up, it can generally be placed into a model and flown to finish the break-in procedure.

Some engines need more run time to achieve a reliable idle and transition. Ringed engines need run time to seat properly. Iron/steel metallurgy needs time and heat cycling to achieve a proper fit. If your engine doesn’t run well at full throttle (overheats and sags), continue the break-in at a reduced rpm by richening the high-speed needle.

After the engine runs indefinitely at full throttle, idles for prolonged periods, and accelerates cleanly, you will be rewarded with a well-running, reliable powerplant.

For longevity, I suggest applying an after-run oil (generally apply some in the carburetor opening after running the engine dry). The methanol, which is the main constituent of glow fuels, is hygroscopic, meaning that it attracts moisture. If you leave fuel in the engine, it can lead to rusted crankshafts, ruined bearings, etc. I usually pinch the fuel line while running the engine to burn off the fuel in the crankcase, empty the tank, and apply the after-run oil. Turn the engine over several times to disperse the oil.


After you get a few engines running and familiarize yourself with the procedures to confidently set up, tune, and run your glow engines, you will be rewarded with reliable power at the field. It’s always nice to see the modelers who take their airplanes out to the flight-line, start up, and fly without issues.

You can be that modeler as well! It’s not difficult if you take your time and learn the basics. Good luck and enjoy the sounds, smells, and excitement of running your own glow engines.

You can see me perform these outlined procedures on a new SuperTigre G51 glow engine (a ringed-type piston engine with a two-needle style carburetor) that I am breaking in and setting up to go into a classic-style sport Pattern airframe in the digital edition’s bonus content.

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