Why is it some modeller's engines just purr along and seem so effortless about their task?

PRELUDE: This article is written primarily to help the advanced beginner through the difficult transition to competition flying. The information is a collection of observations and remedies from years of experience about with stunt engines in New Zealand conditions. It is also intended to highlight many of the simple solutions to problems before getting seriously side tracked into engine modifications.
There is a lot written about getting engines to run well, but still every time at stunt contests I seem to end up helping sort out models and engines. It seems that old basic problems keep showing their faces.
There are no real secrets but my belief is that you have to KEEP THE SET-UP SIMPLE, EASY TO MAINTAIN, and most of all BE THOROUGH ABOUT IT. A simple set-up can take a lot of time initially but it will require the minimum of maintenance to keep operational.
Before you get anywhere near the flying field, there are a lot of things to check and possibly remedy, some of which would be best done in the process of building the model. I will break it down into sections and will try to keep it as brief as possible. Be methodical about it and check off each item when it has been given the "Stuntman's seal of Approval'.

ENGINE MOUNTING: Mount the engine on rigid mounts securely, preferably using additional
locknuts to avoid the bolts vibrating loose.
TANKS: Design the tank mounting to be AS CLOSE AS POSSIBLE behind the engine. Make
provision for it to be adjustable above or below the centreline of the needle valve. Profile
models generally require the tank to be higher than the centreline of the prop shaft. Tank
adjustment is extremely important for "trimming" the engine run to be equal upright and
inverted. It also helps to skew the tank sideways in the fuselage to help make the engine cut
cleanly at the end of the run. (Back outboard corner - outboard.)
Mount the tank securely but on a layer of foam plastic to absorb some of the vibration.
Tank construction: Some sort of symmetrical metal wedge tank. The joints MUST be
overlapped and Clean solder joints with no leaks.
Tank Plumbing: USE uniflow venting.
Feed Pipe: This should end approximately 1.5mm from the back outside corner of the wedge.
Uniflow Vent: This needs to be between 15 & 16mm directly forward of the feed on the
Horizontal centreline of the tank. The vent pipe needs to exit the fuselage above the top level
of the tank and pointing forward into the breeze. (The tank can be connected to the exit tube
with a length of silicon fuel tubing).
The uniflow vent also serves as the filler.
Overflow Pipe: This is usually drilled through the bottom of the tank and goes to the top
inside corner of the tank. The overflow must be BLOCKED OFF in flight with a cap that is air
tight. Plug a piece of fuel tube with a solid rivet, bind with cotton and cyano.
Note: 1/8" or 3mm thick wall copper tube is better than the thin walled brass tube because it
is easier to bend and it helps control fuel surging. Be sure to solder each pipe in two places in
the tank, firstly where the pipe enters the tank and secondly where the pipe ends inside the
tank.
After completion of soldering, flush the tank thoroughly with lots of water to neutralise the
flux and then flush with fuel.
MUFFLER PRESSURE: DO NOT USE MUFFLER PRESSURE! I have seen people struggling for
years using muffler pressure and it is more trouble than it is worth for stunt. Set up your engine
to run on suction feed with a suitable sized venturi to achieve the run you require. For .40 to .50
size engines a 7mm inside diameter venturi using a 4mm diameter needle valve spray bar is a good
starter.
FUEL LINES: The feed line from the tank to engine must be on the OUTBOARD SIDE of the
fuselage and the most direct route possible. i.e. On the horizontal centre line of the needle
valve for inverted engines. Use an in-line fuel filter.
Fuel lines that can flap around are a NO GOOD. Use sensible sized fuel line, not the 'garden
hose' variety used on large radio models. Adjust the length of the fuel line so'-that bends are
smooth with no kinks. If the needle valve fuel tube junction or any others are not a tight
reliable fit, wire them on with 15 Amp fuse wire or try smaller tube.

NEEDLE VALVE ASSEMBLIES: Personally I prefer the Super Tigre style N.V.A. that has a
collet to lock the needle in position.
These should be tightened so you can just turn the needle without excessive force. The OS.
style N.V.A. do not give a reliable setting because the needle can vibrate like a tuning fork,
and sometimes you need a half a click!
The spray bar jet hole should be just past horizontal pointing downwards. You should not be
able to see the hole. Periodically the needle and fuel lines need to be removed and the spray
bar back flushed. NOT back into the tank!
FUEL CLEANLINESS: This is vitally important, filter your fuel into your bottle, have a filter
on the outlet of your bottle and another in the model. MAKE SURE THE FILTERS ARE
EFFECTIVE! I replace the gauze in commercial filters with much finer gauze and ensure the
gauze is a tight fit inside the filter case so crap can't sneak around the side.
ENGINE CLEANLINESS: Remove the muffler and examine the piston through the exhaust
port while turning the engine over. If there is a brown lacquer build up on the piston the
engine is due for a clean out. It would be wise at this stage to look at the cooling system
because chances are the engine is running too hot. For cowled engines the outlet area must be
considerably larger than the inlet area. As a rule of thumb the outlet should be twice the size
of the inlet.
If you are nervous about dismantling the engine, try and find someone to help you who knows
the "Art" of engine dismantling and cleaning. For a reliable engine run it is important that
there is no brown lacquer build up, particularly on moving parts.
When the engine is dismantled, look carefully at the following areas:
1. Piston and the inside bore of the liner.
2. Outside of the liner and inside of the cylinder jacket.
3. Piston ring and groove.
4. Inside of the gudgeon pin hole in piston.
5. Top, little end of con-rod (bearing)
6. Edge of cylinder head / combustion chamber that slips into the liner.

I like to put all the parts in a sealed golden syrup tin or screw top jar to soak for several days in lacquer thinners.
To remove the lacquer from holes, wrap a small piece of 1,200 grit wet & dry paper (2,000 grit is better) around a suitable sized drill shank so it just slides into the hole. VERY CAREFULLY sand trying not to remove any metal.
After spot sanding the worst areas, the internal bore of the liner can be done in a similar way with 400 - 600 wet & dry paper wrapped around a piece of smoothly machined bar for support. Sand in a spiral pattern in both directions. Check that the piston ring is free in its groove If gummed up it should be very carefully removed and the ring plus the groove cleaned.
The external parts of the engine that are castor oil baked can be cleaned by boiling them in a pot with a teaspoon of Cold Water Surf or dishwasher powder for 10 to 15 minutes then scrubbing well with an old toothbrush. This also works well for the piston and liner but avoid this treatment on ball races. After this, scrub everything thoroughly in a tin of white spirits or kerosene and flush parts individually with a squeeze bottle, placing them to drain on a clean newspaper.
Reassemble the engine, then before fitting the cylinder head and back plate, flush the engine one last time and give it a squirt of CRC 5.56, WD4O or sewing machine oil. Ensure the cylinder head and back plate gaskets seal properly and the screws are tightened evenly.
The engine will take about 6 runs for the moving parts to seat in again and you will probably need to lean out each run. Once run in, the oil from the exhaust should be clean. If it is not then something in the engine is wearing or the muffler is loose.
FUEL: For the purposes of the tests on the flying field (coming eventually...) it is essential
that you "KEEP THE FUEL BREW CONSISTENT"
For most engines's. a mix containing 5% Nitro, 20% Castor Oil and 75% Methanol is a good
guide. Plain bearing engines should have a little extra oil, 22.5%. It is not usually necessafy in
the New Zealand climate to run higher oil contents that you read about in American articles
because we don't have to contend with the very hot conditions.

Older ringed piston engines and those with cast iron piston/steel bore should have fuel containing Castor Oil or at least 10% Castor Oil if Synthetic oil is used. Like everything, Castor Oil has disadvantages but it seems to give the engine run a sweetness that synthetic oil doesn't.
You might have noticed some of the Tro's" putting a squirt of "Armor ALL" (Silicone spray) in their fuel can. This acts as an anti foaming agent and is a good idea to help eliminate foaming fuel caused by engine vibration. The tiniest drop in a litre of fuel is sufficient but note that it needs to be done before you go flying because it does not seem to last long periods.
GLOW PLUGS: Forget the "OS No.8" cliche or at least turn it into and 05 No.7! There are no hard and fast rules, in fact if you really know your stuff you can set up an engine to run satisfactory on almost any glow plug heat. However on the fuel suggested containing 5% Nitro, a MIEDIIJM HEAT plug is where to start. A lot of people use plugs that are too hot and their engines behave too aggressively.

In the flight testing I will describe glow plugs in more detail but for now I thoroughly recommend you try an 05 No.7 Idle bar plug. This is the plug that I have success with and it seems to have helped sort out numerous problem engines over the years I have flown stunt.
Idle bar plugs offer advantages for engine runs where the engine is breaking from 4 stroke (rich) to 2 stroke (lean) to control the speed of the model through manoeuvres. On smaller engines and those set up to run at a constant relatively high RPM, standard plugs are OK. The 05 No.3 is very similar to the No.7 but is a standard non-idle bar plug.
The Firepower range is also worth a try. I have a complete set for testing engines and there is an excellent selection of heat ranges. e.g. Cold, Cool, Medium, Warm, Hot. I use the set to determine where to start with adjusting fuel and compression ratio.
PROPELLERS: Props must be balanced. My preference is for wooden propellers because they
are very light and do not act like a flywheel. The object is to have the engine relatively lightly
loaded so that it will gently switch from rich to lean and vice versa when required. Heavy
plastic props load the engine up delaying this action that will mean the power will turn on or
switch off too late!
For most engine / model combinations, pitches between 5" and 6" will generally work best. The
manufacture's instructions will guide you with the approximate diameters to work with. Some
engines will tolerate being really loaded down with big props but it pays to avoid this if you can
because if the engine is working hard it introduces additional problems, e.g. overheating,
lacquering of the internal engine parts which in turn leads to poor inconsistent runs.
FLIGHT TESTING: If you are still with us we are now probably ready to start flight testing
to tune the engine correctly for the model, and the flier!
Begin trials by fitting a medium glow plug and set the engine to a comfortable speed so that
the model will not 'fall out of the sky" when doing basic manoeuvres like loops. Prop the engine
so it is not too heavily loaded. On a 35 to 60 powered model at 60 ft to 70 ft line length, this
equates to about 5 to 5.5 second laps, 50-60 mph. It is a rare model that can be trimmed to
fly safely with speeds lower than this, i.e. higher lap times.
Lap times: With the help of a timekeeper, time the lap times both upright and inverted. The
object is to have them identical. If the engine run inverted is SLOWER (richer) than upright
then the tank needs to be adjusted or shimmed higher in the fuselage. If it is FASTER
inverted then the tank needs to be lowered. It is important to get this even before going any
further.
It is also useful to time the lap times early in the run and late in the run preferably on a flight
Where a schedule is flown so the engine is working normally. If the engine is speeding up
towards the end of the run (getting leaner) then it could mean the engine is running too hot. If
a uniflow tank system is used, with the overflow blocked off airtight, the problem is most
likely with the engine.
Look carefully at the cooling system and improve it if you can. Next you can experiment with
Plug heats, trying a hotter plug first.
If the engine is slower towards the end of the run (getting richer) then the engine is
overcooling. If the model is still safely flyable, leave this for the time being but at a later
stage if it still persists then the oil content can be lowered slightly. I have had to go as low as
17% oil content to cure this problem with some engines.

4/2 Stroke switching: The next steps involve a lot of flight trials to tune the engine to suit
your flying style. Try flying loops or horizontal eights not worrying too much about quality of
shape and keeping a little higher than normal so that you can "focus your ears" on the engine
run. Listen to the note of the engine as the model climbs into a loop and goes over the top and
down again. If you are running the engine slightly rich so that it is 4 stroking in level flight
then the aim is for the engine to gently switch lean as the model climbs into a loop. The engine
should then softly switch back to rich as the model starts to descend in the second half of
the loop to govern the speed of the model.
If the engine stays lean right through the loop or switches off too late so the loops gradually
get faster and faster and it's a grim matter of hanging on by the third loop then something
definitely needs to be changed!
At this stage I usually do some simple glow plug trials to determine what to do with the
compression ratio and/or Nitro content in the fuel. To play safe, try a warmer plug first but I
suspect in a lot of cases a cooler plug might tame the engine to something nearer comfortable.
When I first flight tested my Moki 51 it required a cold plug for it to perform somewhere
near normal! If the change of plug heat makes a big difference it tells us what we can do next.
I now like to tune the engine to run on my favourite glow plug, the OS No.7, medium heat, idle
bar.
If you find a cooler plug is necessary to remove the aggressiveness from the engine run so it
"purrs" then it tells us the compression ratio is too high. Make or purchase some head shims
and keep adding them until the switching from rich to lean and back to rich in loops are just at
the right points on the loop to achieve an even comfortable model speed. You can also lower
the Nitro content in the fuel but it is wise to have at least 2-3% to ease starting, because in
stunt, starting IS a manoeuvre!
If a warmer glow plug is necessary then you could if you want to, run a higher Nitro content or
higher compression but these are not really necessary and it would be easiest to stay with a
warm or hot plug heat.
Once you arrive at the combination that suits you it is vitally important you stick with the
same fuel brew. Even a change of oil content or change from castor to synthetic oil is enough
to throw everything out of balance. For example if you change from using a fuel with castor oil
to a fuel with a blend of castor-synthetic, the engine run will change dramatically and you will
probably need to lower the compression ratio further.
Most engines for stunt seem to tolerate quite a lot of decompression to remove
aggressiveness from the run. My Moki 51 required an extra 0.5mm (0.020") of head shims to
make me reasonably happy. It was obviously factory set for straight FAI 80/20 fuel.
GENERAL: In reality changing plug heat, Nitro content in fuel and compression ratio can all
achieve the same thing. In the end it is usually a combination of all three that is most
practical.
Propellers will make a big difference to the engine run. When you start to get results with the
engine set up, try lowering the pitch of the prop to gear the engine down a little so that the
engine will have more freedom to switch. If you can run 5" pitch you will most probably be
surprised by how much more lively the model feels.

In most cases the changes I have described will improve the majority of engines being used.
You can experiment further with venturi sizes and if really serious, change the exhaust and
transfer timing, but I have not intended this article to delve that deeply, trying to
concentrate on the simple solutions.
Lastly, it is important to use an after run oil following a day's flying. For quite a number of
years I have gotten into the habit of sucking the last few drops of unused fuel from the tank
then squirting CRC 5.56 into the venturi, flicking the prop to disperse it through the engine. I
then plug the venturi and exhaust with a wad of tissue paper. At the same time give your
control lines a squirt and put them in a sealed plastic bag. However you need to use a more
permanent oil for long storage of engines, but CRC or WD4O is fine for several months. Apart
from keeping the engine free, it penetrates the needle valve and stops oil setting in there.
If you forget to do this treatment you will find the first run(s) of the day will be lean as the
fuel flow cleanses the needle valve assembly. By using the after run treatment you should not
have to make needle adjustments as often.
If you can persevere and sort out the engine department you will remove a lot of worry so
that you can concentrate on the FLYING!!!