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Glow Engine Tuning & Setup

This is the first installment of a series of articles on general engine care and maintenance of both glow and gasoline engines. I will cover most of the basics, including setting up your engines, basic cleaning, maintenance and troubleshooting. This month I will discuss the basic setup and tuning of two-stroke glow engines (without a fuel pump), still by far the most commonly used engines in our models.

Super Tigre G-34 two-needle carburetor. A is the high speed adjustment needle. The low-speed needle has a slotted head and is not visible in the end of the throttle arm tube at B.


Before you can properly tune your engine, you must make sure it is set up properly. Having your engine mounted properly is often overlooked, yet is so very important. It should be securely mounted on a mount with straight and parallel beams. Machined metal mounts are ideal. Injection molded mounts are also good, but you must make sure that there isnt any distortion or heavy mold flashing that prevents your engine from sitting properly on the beams. If the engines lugs are not sitting perfectly flat on the mount, stresses will be introduced through the crankcase that can cause friction and premature wear on critical components in the engine. If your engine is soft-mounted, make sure that no part of the engine, especially the carburetor, can contact the cowl or firewall with extreme flexing of the mount. Running an engine that is not adequately mounted (with loose bolts, for instance) can be very dangerous both to the model and the modeler.

This side view of a Super Tigre two-needle carburetor gives a clear view of the low-speed adjustment needle inside the throttle barrel, and opposite the high-speed needle.

If you are inexperienced with running engines, I suggest you stay with a side or upright mount for your engine. You can tune inverted engines to run just as well as upright engines; however there are a few additional issues that can complicate operation. Getting too much fuel into the combustion chamber can drown the glow plug or cause a hydraulic lock for the lesser experienced. Stick with a simpler installation if it is your first or second engine.


Mounting the fuel tank in its ideal location is the next important consideration. In most cases, having the tank centerline on the same plane as the carburetor or slightly below is a good starting point. If the tank is too high, the fuel may siphon down, flooding the engine and making a mess. If the tank is too low, the engine can have a hard time pulling fuel up to the carburetor and it may run lean or quit altogether, especially in maneuvers that place the fuel under load (hard pull-ups and inside loops) or toward the end of a flight.

Some manufacturers are going to larger and larger air inlet throats on their carburetors to produce more power. The increased airflow will pull more fuel and make more power, up to a point. It will also make the tank placement more critical, so it is always best to get as close to the ideal tank position as possible.


Other items on my list may not have to do with the engine itself, but are just as important. They are your fuel, glow plug and propeller. Please use a good fresh jug of fuel! Many companies, such as Wildcat, produce a great factory fuel that is guaranteed fresh. Choose a nitromethane percentage that suits your particular engine, along with an appropriate percentage and type of oil. Most engines today will run fine with anything from 5% to 15% nitro and 18% oil, but check with the manufacturers operating instructions for recommendations. Using a fuel filter will prevent dirt from contaminating the small passageways in the needle valve. It is vitally important to keep your fuel as clean as possible, especially in small engines. Mainly methyl alcohol, our fuel happily absorbs moisture from the air every time the container is open. Use a cap that lets you attach a fuel pump without removing the cap each time you need to refuel. Once water contaminates the fuel, it can cause erratic running and rough idling. With a little care, you can elimi-nate this potential problem before you even get to the engine tuning.

O.S. air bleed style carburetor. Again, the A is the high-speed adjustment needle. The idle air inlet hole is centered just above the O.S. logo. The air bleed adjustment screw is just in front of the throttle arm barrel on the left hand side in this photo. The screw to the top left is a low throttle lock screw.

The glow plug will greatly contribute to the tuning success of your engine. Some engines perform much better with a particular brand of plug. Some brand-new engines come with less than ideal plugs and can also find much improved and easier tuning with an aftermarket plug. Certain brands will turn a hard-to-tune engine into a real gem. I personally have had much success with the O.S. #8 in most engines. Glow plugs do not last forever, and even if they are still intact, they can indeed be worn out. Changing to a new plug can often cure an engine tuning problem.

Finally, the choice of propeller can also affect the engines ability to be set for good running, idle and throttle response. Choose a good quality prop that is free of nicks and any visible damage, and balance it. An unbalanced propeller can greatly impair the engines running ability by causing a lot of vibrationthis causes the fuel to form air bubbles, which make tuning impossible. If a propeller is too big it places the engine under more load, increasing stress and heat that impair tuning. If the prop is too small, you may not be able to tune the engine for a decent idle.


Since it is rare to acquire a new or used engine with the carburetor set properly, I go through the following procedure for initial settings on a two-needle carburetor. First place a piece of clean fuel tubing on the fuel nipple and open the throttle barrel fully. Now completely close the high-speed needle, then blow into the fuel tubing while slowly opening the needle valve. You will feel less resistance as the needle valve opens. Once the needle valve seems to be fully open, leave it. Many engines have the high-speed needle setting wide open at somewhere between three and five turns.

Two more examples, a Fox air bleed carburetor on the left, and a Webra two-needle model on the right.

For the low-speed needle, close the carburetor barrel fully and try the same blowing through the fuel tubing method. This time, adjust the low (or idle) speed needle. The engine is now ready to start up. It will almost surely be way too rich at these wideopen settings, but that is never a bad way to start up an engine. You will be assured that the engine is getting adequate fuel and lubrication, and if there is any problem, it is not lack of fuel.

With an air-bleed carburetor, the highspeed needle is set up the same way. I adjust the low speed by tightening the airbleed screw until half of the air-bleed hole is blocked off. That is a safe place to start.


We are ready to get your engine running, but first lets look at a few accessories that will help you start and tune the engine. These are the glow plug igniter, electric starter and a tachometer. Glow igniters can be either cordless with a unitized battery and glow plug clip, or a clip with wires leading to a power panel. Some sort of igniter is required to provide the initial heat for the glow plug. Some modelers prefer the safety of a cordless model, others prefer the additional kick a corded igniter can provide. The glow plug should glow bright orange when connected to a strong igniter.

Electric starters are not essential, but very nice to have. I find for smaller glow engines under a cubic inch displacement, an electric starter is a quicker and easier way to get an engine off and running for the initial runs. A chicken stick or a gloved hand is fine tooit may just take a little longer. Again, starters come in cordless and corded versions.

I also strongly recommend an inexpensive digital tachometer. Experienced modelers can do pretty well setting up an engine by ear, but very few can match the precision of having a tach to give real numbers.

Lastly, if you are using an engine test stand, make sure that it has some sort of throttle pushrod linkage with a friction lock. You do not want to be operating the carburetor throttle arm by hand.


With the engine properly secured on a test stand or in the model, install the correct glow plug and propeller and fill the fuel tank. Set the carburetor with the throttle barrel approximately 1/4 to 1/3 open and try to start the engine. It should fire and come to life easily. Dont be concerned with removing the glow igniter. Since we are almost guaranteed to be running too rich, the engine may not run without it keeping the plug lit. Slowly increase the carburetor opening to full while simultaneously leaning out the high-speed needle valve (turn inward, clockwise), until the engine is clearly running in at least a dirty two-stroke modegoing back and forth between a clean two stroke and four stroking (missing). Open the throttle barrel fully and now lean out the engine further while watching the rpm with your tach. When it peaks and will hold the peaked setting back it off a few hundred rpm. This whole scenario shouldnt take very long, less than a minute or two, and even if you are running an ABC/ABN-type lapped engine, it will not be running at a too-rich setting for long enough to cause any harm. The high speed is now roughly set and we can turn our attention to the low-speed setting.


Once the high end is set up, you can adjust the low end. Lets look at the two-needle style carburetor first. Start up the engine at 1/4 to 1/3 throttle as previously done. Since cold engines usually do not idle well, open it to run at full throttle for 30 seconds or so to warm up the engine. Now slowly close the throttle to the idle setting. If the engine stays running, open the throttle smoothly and slowly back up to full. If the engine sputters and raw fuel exits the muffler while slowly coming up to speed, it is too rich. Shut it off, and screw in the low-speed needle 1/8 to 1/4 (maximum) turns clockwise and repeat the whole procedure. Once the engine will idle for 30 seconds to a minute and then throttle up cleanly and without hesitation, the low speed is set. If you find the engine quits abruptly when you try to increase the throttle, it may be too lean. Stop the engine and open the low-speed needle slightly and repeat the test. The high and low-speed needles in some carburetors can have quite an effect on each other. If the low end is too lean it may cause the high end to run lean as well, even possibly with the high-speed needle open fully. Fortunately the needle interaction is not this pronounced with most carburetors.

If you are running an engine with an air bleed carburetor, the adjustment is as follows. If the engine sputters rich on throttle up, the air bleed hole needs to be opened to allow more air into the carburetor. This will lean out the fuel/air mixture. You need to unscrew (counterclockwise) the air bleed screw. Likewise, you will need to screw in the air bleed screw if it is lean, closing off a greater portion of the air bleed hole and richening the mixture. The air bleed adjustment has absolutely no affect on the high speed.

Once the low end is set properly you can go back and readjust the engines running at full throttle again. The engine should be set now and ready to get airborne!


Once you have a glow engine running well and the needles set up properly, you really shouldnt have to fiddle too much with the needles each time you go flying. I notice that in cooler weather I may have to richen up the mixture, and in hot weather I slightly lean out the engine. Cold air is denser and contains more oxygen for the same volume, so the engine may need a little more or less fuel. These are generally small adjustments, seldom more than 1/2 a turn, so take it easy.

If you change either fuel brands or nitromethane content you may need to make a small adjustment. A major change in propeller size can also mean retuning your engine. Fortunately you will already be close so the adjustments should be slight.

One mistake I often see fellow flyers make is unnecessarily adjusting their carburetors. When I go flying I do a check on my high speed by leaning the engine to peak and then backing off a few hundred rpm, and thats it. Idle seldom needs readjustment unless a there is major weather change or you have a new prop or fuel.


Once you become more in tune with your engines, and better familiarize yourself with the tuning procedures, it becomes a fast and easy thing to do. Just remember, take things slowly and always follow the same steps. It is vitally important for enjoying glow flying to become comfortable setting up your engines to run reliably. If there is anything that you would like to know about glow engines, or if you have a specific engine problem, please email me at andrewc@flyrc.com.


FOX Manufacturing

www.foxmanufacturing.com , (479) 646-1656

O.S. Engines, distributed exclusively by Great Planes Model Distributors

www.osengines.com , (800) 682-8948

Super Tigre, distributed by Great Planes Model Distributors

(800) 682-8948

Webra Engines, distributed by Horizon Hobby Distributors

www.horizonhobby.com , (877) 504-0233