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Warringah Radio Control Society

PO Box 633, St Ives, NSW, 2075

 

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PETROL {GAS IF YOU ARE AMERICAN}ENGINES AND THEIR FOIBLES
BY TOM SPARKES !
These days petrol engines are rapidly becoming more popular, particularly as our models get bigger
and more complex ,and for some very good reasons. They are more reliable than their glo cousins, but
they do have their little idiosyncrasies .The greatest difference lies in the carburettor set up. They have
an inbuilt petrol pump, so you can mount your tank anywhere in the plane, height above the carby
matters not a toss, and with the throttle closed you cannot flood the engine while refueling. Also
because of the fact that you have a spark plug and not a glo plug, the engine can be mounted inverted
with no fears of poor idling or poor running.
The one thing that can be a trap with petrol engines is the little balance port on the pump side of the
carby. If it is in a position that can be influenced by different air pressures, like facing into the airstream
or in a pressure void inside the cowl, erratic mixture settings will result during certain manoeuvres .The
insurance against this is to always when installing a petrol engine, solder a short length of brass fuel
tube over this tiny hole and run a length of fuel tube back into the fuselage,where you will not get
sudden pressure variations. If you are unsure of the location of this hole, ask me or one of the other
petrolheads in the club. Whilst on the subject of fuel tubing, remember that you MUST NOT use
silicon fuel tubing, you must use the TYGON fuel tubing, readily available from hobby stores or from
lawnmower or motor bike shops.Also it is a good idea to put a small cable tie around every junction of
tygon over brass tubing, because the tygon can stretch over time and slip off the end of the brass tube.
Very embarrassing !!Also the stopper in the neck of your tank must be of the gas type and not the glo
fuel type. Most tank brands will supply a gas stopper.
FUEL MIXES. Most makers these days like you to run the engine in on a higher ratio of oil to petrol
mix, usually 25 or 30 to one, using a MINERAL rather than a synthetic oil, so that the surfaces get
bedded in properly. Later when fully run in use a top quality Synthetic oil which you can get from a
Motor bike shop, one of the offroad racing synthetics is the go, and follow the makers advice as to
ratio, usually around 40 or 50 to one. These engines are very frugal as well, you will get about twice to
three times the run time as a glo engine on the same size tank. As you can see they are very cheap to
run.
MIXTURE ADJUSTMENTS. These engines are a bit different to glo in the way you set the high and
low mixture settings. Set the high first, and run it a bit rich for the first hour or so, and remember that
when setting the low speed needle, this usually affects the high setting as well, so if you richen up the
low needle chances are you will have to lean off the high a bit as well. Listen to the transition, if the
motor “dies”on opening the throttle chances are you will have to richen the low speed needle, if it
“hunts “and surges when you open the throttle chances are it is too rich on the low needle. Don’t forget
that as you change mixture settings on the low end your idle speed will vary, so always adjust this after
altering the low needle.
STARTING TECHNIQUE .If your engine is of the Magneto type like a Zenoah,don’t even think about
hand starting, the propeller has to do almost a complete turn before a spark is generated, so just flipping
it over compression is a no brainer. Either use a starter or have the motor fitted with a spring starter,
which spins it over several times quickly. Close the choke FULLY and open the throttle fully, spin the
engine and when enough fuel is pumped into the carby it will “pop” or just fire once. Then open the
choke, put the throttle just above idle, spin it and it will start. If your engine has electronic ignition and
no magneto, then it is possible to hand start because the spark is generated at top dead centre by the
ignition battery without the engine having to do a full turn. However for my money and because I
prefer my fingers to stay attached to my hand, I use a starter even on my electronic ignition engines. In
fact with the bigger engines,say around 80cc up, this becomes a case of prefering my whole arm to stay
attached to my body !!!!
ALWAYS have someone help you and hold the model, those bigger engines can develop a lot of thrust
and power, on full power my 3W80 will almost drag my helper {RON} along the ground with his
heels dug in !!!!
Good luck with your petrol engines !!!! If you need help just ask.

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