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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.
Carburetor
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".
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
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.
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