Understanding
Your Engine first?
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".
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. If
you can't get your engine to even start,
then go back to our Engine Troubleshooting
section before continuing on.
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.
Good
luck!
