Glow Engine Tuning Basics
by Matthew Parrish

Understanding Your Engine
The first and foremost consideration when attempting to tune your glow engine is understanding the basic parts and their functions.  By understanding the fundamentals, you can better tune your engine for maximum performance while at the same time, expanding the life of your engine.

The carburetor is the mechanism that mixes fuel and air in very specific proportions and passes it on to the engine through the vacuum intake.  The natural operation of the engines causes of flow of gases to pass through the engine (through the carburetor) and out the exhaust manifold and on to the pipe or muffler.  The exact mechanism for this is unimportant for the scope of this tutorial, however it is important to realize that air and fuel pass into the engine by this vacuum method.  Depending on how you adjust your carburetor, you can either adjust how much of this gas/air mixture reaches the engine and to what proportion of gas to air passes on to the engine.  By reducing the amount of fuel per volume of air, you are making the mixture "lean" and by increasing the amount of fuel, you are making the mixture "rich".

The two types of carburetors are slide and barrel.   The old-style barrel carburetors still dominate the market because of their simplicity in design and because of the tendency for designers to hang on to legacy design.  These have been around since the beginning of glow-fuel planes.  They control gas/air flow by rotating a barrel with a hole cut in either side that allows varying amounts of gas/air mixture to flow through the carburetor as the hole opening enlarges to the venturi (air shaft down the center of the  carb body).

Idle-Speed Adjustment
This is the most basic and easy to understand part of tuning your carburetor.   This spring-tensioned screw limits the closure of the barrel aperture.  Although this doesn't affect the mixture of the fuel it does affect the idle speed.  The more closed the aperture is, the slower the idle, the larger the aperture, the faster.   As you close this aperture up and the idle speed decreases, you will eventually (sooner than later) stall the engine out.  In order for the engine to run, it must have enough inertial energy built up in the engine and flywheel to carry it through the entire ignition cycle.  Generally speaking, you want to adjust this down to the slowest idle, just before it begins to stall.

Low-End Mixture Adjustment
This adjusts the fuel mixture at or near idle.  Some engines lack this low-end mixture valve for reasons of simplicity, however this makes accurate tuning difficult.

For barrel carbs, this mixture valve is generally found where the throttle-arm pivots.  Some are countersunk, others are clearly visible from the outside.  On slide carbs, they are generally found on the opposite side of the carb from the throttle slide shaft (has an accordion billow type rubber boot over it) next to, but below the fuel-inlet and high-end mixture valve.

High-End Mixture Adjustment
Also known as the Main Needle adjustment, this is the primary fuel mixture adjustment.   This is generally found on the top end of the engine, typically next to where the fuel line goes into the engine.  Some are flat-head screws like the low-end mixture, others are hand adjustable valves.

Tuning Basics

It's important to understand that there is a reputation for glow-engines to be difficult to tune. This is a common error in thinking.  With a little bit of know-how, tuning a glow engine can really be a simple, pain-free process.   People that don't properly understand the basics can easily become frustrated by what should be a simple, straightforward process.  Here's how you do it:

Dialing it In
For the purpose of this tutorial we are going to make some basic assumptions.   First, we're going to assume that the rest of your car or truck is properly functioning and that you have everything ready to go.  Second, we're going to assume that you are able to start your engine and that it at least runs for a second or so.  

The first place to start with dialing in your engine is to make sure that you have your idle-speed properly adjusted.  Your engine manual should give you specific instructions on setting the aperture gap to the minimum size.  It's important that we get this resolved before continuing on.  If your engine can't get enough air/gas flow then it won't start/run.  A clockwise rotation opens the aperture and increases the idle RPMs, a counterclockwise slows it down.

Second, you should tune the low-end mixture valve.  This is done before the high-end (main needle) adjustment because an improperly adjusted low-end can affect the high-end performance.  Like most mixture valves, clockwise rotation will "lean" the mixture and a counterclockwise will "richen" the mixture. 

To determine whether the low-end mixture requires tuning, allow the engine to warm up completely, and then allow it to idle, uninterrupted for one full minute.  If the engine continues to run after the minute is up then your low-end mixture is correct and you're ready for the high-end adjustment.  If it dies on you then there are two possibilities; either you are running too rich or too lean.  To determine which is the case you must listen for how the engine dies in its idle test. 

If the engine's RPM's rev up at the last second and then the engine dies than you are running too lean.  To correct this, turn the low-end mixture screw counterclockwise (out) 1/8 of a turn (always make adjustments in 1/8 turn) and retry the idle test.

If, on the other hand, it begins to wind down and you notice a change in how the exhaust sounds in the last few seconds, then your engine is running too rich. To correct this, turn the low-end mixture screw clockwise (in) 1/8 of a turn and then retry the idle test.

Once you have passed the idle test and are able to idle for one full minute (after first warming the engine up, of course) you are ready to continue on.   You may have to repeat the above process a few times until it is properly set.   Remember, only adjust the screw 1/8 of a turn.  It's far too easy to go too far with the adjustment.  Setting changes don't always take effect immediately.   You may have to run your engine for a few minutes for the full effect to take place.

Now that you have dialed in your low end, any carb mixture problems can be isolated to the high-end (main) mixture adjustment.

Acceleration is the tell-tale sign of how to tune your high end.  If you hit the throttle and it takes off suddenly but then suddenly dies or loses power then you have your main mixture set too lean.  Try backing (counterclockwise) the main mixture needle out 1/8 of a turn and retry.  If it bogs immediately when you hit the throttle (sounds like it's choking), then it's most likely running too rich.  Try leaning the mixture out by screwing the main mixture valve in (clockwise) 1/8 of a turn. 

The more accurate way of really dialing in the top-end is to take the engine's temperature.  A properly tuned engine should run between 210 and 220 Fahrenheit. This can only really be ascertained by using and infra-red thermometer such as the type used by automotive mechanics. On-board or direct-transfer types that measure the heat from the head are inaccurate because, assuming the head is properly dissipating heat, it would reflect a lower than accurate temperature as a majority of the heat energy would be dissipated from the exposed surface of the head.  By "looking" at the temperature near the core (actually, area immediately surrounding the glow plug) the temperature can be more accurately read. 

The cheap but easy alternative would be to drop a bead of water down the head on the glow-plug and see whether it boils off.  If it slowly simmers than it probably is running right around 212.  If it boils to quickly then it's probably too lean and needs to be richened.  If it just sits there and doesn't boil at all, then its running too rich and needs to be leaned out.

An engine that is running too lean will run hotter and exceed the 220 degree limit.  This can significantly reduce the life of your engine.   Although it may be tempting to run your engine as lean as possible (does give a short-lived performance boost), this should only be done if you are very wealthy and like swapping engines out every race.  There is no quicker way to kill and engine, honest.   This is simply because as you lean the engine out, it gets less fuel to the engine, and more importantly, less lubricant.  Since glow fuel is the only means of lubrication for your engine, the lack of it means certain death to your powerplant.

A few final do's and don'ts...

  • Give your adjustments time to take affect.  Remember that most adjustments won't be immediately noticeable.  You need to drive your engine through it's full range for at least a minute.  Make sure you make adjustments in 1/8 turn adjustments only!
  • Always run on the rich side.  It's far better to take a slight performance hit than to turn your engine into a paper weight.  Running too lean may give you a temporary thrill, but it's short lived.  Your engine must get the proper amount of lubrication at all times.
  • Changes in temperature affect your tuning!  Whenever the outside temperature changes you will most likely need to re-adjust your engine.   Warmer temperatures require a leaner setting where colder temperatures require a richer setting. 


I hope that this info gets you on the right track.  If all fails, it's always a good idea to get expert advice from the vets down at your local track.  However, be aware of the guy that's too eager to give you advice on how to get that extra performance boost out of your engine.  Unless he or she plans on buying your next engine, I would be weary of any such advice. 

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This page last modified: 07/26/11