Gas Engine Set-up

RCS 215.jpg (61756 bytes) Tank Placement: Most modern gas motors have carburetors that have pumps and regulators built in. This means that you can mount the fuel tank almost anywhere in the plane. It is recommended that you mount the tank as close as possible to the center of gravity.

Felt Clunks: Header tanks can be used but are generally not needed as Gas isn't as susceptible to foaming as Glow fuel and the carburetors seem fairly unaffected by air in the system. However, the use of a "Felt" clunk is becoming common as they have several properties that make them very attractive. The "Felt" acts as a very effective fuel filter. The "Felt" holds fuel causing a sort of pool that insures a constant fuel supply under all conditions. (For instance, on a down line when the clunk isn't in the fuel) When the tank is not full, the fuel splashes around and small quantities of air enter the system. If you want to use one, you can get them at most "Weed Whacker" shops.

Fuel tubing: Glow fuel tubing is not compatible with gas fuel, the gas will eat the fuel line so use tubing designed for gas, Tygon is one brand.

One thing to remember here is that the line will expand when in contact with gas. This means that you have to use some sort of clamp to hold the line to the fittings, either a spring clip or some other suitable device. This is really important, especially in the tank on the clunk line where the clunk is always pulling on the line.

Carburetor Settings: In most cases, the carburetors will be set from the factory in a fashion that will be close to what is needed to get the motor running and in the air, so, unless you have a problem starting the motor, leave the carburetors needles alone to begin with.

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Limbach 280.jpg (55531 bytes) On the gas carburetors, the low end will effect the high end but the high will not effect the low. This means that you set the low first and the high second. In most cases, the low is set as lean as possible, then set the top for peak and go rich by about the width of a screw driver blade (No RPM drop). You might have to richen the low end to get a good transition or to insure good down line breaking, but remember to keep the bottom end as lean as is practical, this will help insure a good midrange response.

Fuel/Oil mixture: Most 2-cycle oils on the market will work with our Gas motors. Saying that, a high quality synthetic oil will burn cleaner and help the motor last longer. Don't forget though…you must use something like LawnBoy Ashles for at least the first gallon of fuel for break-in. Synthetic fuels would work for break-in but it will take forever for the rings to seat.

  • Amsoil 100-1 mix mixed at 100-1 has a very good reputation for keeping the motor clean and providing very good protection.
  • Klotz R/C lube is another good one mixed at 50-1.

A note on mixture: A Glow motor relies heavily on its fuel for cooling/timing and without a pump and regulator, you have to set them very rich to have a good run. The Gas motor on the other hand really doesn't rely as much on fuel for cooling and has a regulated carburetor. This means that you set the high end for peak and back off to rich about an 1/8 of an inch but not enough to realize a RPM change. This is important because if you set it to rich, you will have a horrible midrange response.

Soft Mounts: Single cylinder gas motors have a reputation for excessive vibration. In some cases (Sachs 5.8) it is deserved, in others (Quadra 100), it isn't. This can be a real pain as far as constantly having to tighten and replace bolts that come loose. In order to combat this issue, several have gone to soft mounting.

There are several soft mounts on the market, some are good, some aren’t. Most consider the Hyde mount the best in function, it's readily available and comes in all sizes. The thing to remember about soft mounts is that the mount has to be suitable to the motor, it has to be tuned to the motor in question's resonance frequency, if it isn’t, it will be little better than dead weight.

Hard Mounts: Many consider hard mounting to be the way to go. It is generally the lighter of the two options and in the case where the motor runs smooth enough, it is the best way.

Vibration: A note on motors and vibration: There are three primary sources for vibration. The motor is one and is pretty much the only one you can’t fix.
  • Propellers are another and one that a good deal of effort should go into getting the balance right. Props like the Mejzlik and Bolly can be had balanced, but you really need to check this, many times these so-called "Balanced" props are not.

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  • The spinner is another source of vibration, this needs to be balanced as well, don't take it for granted that it is balanced from the manufacturer.

  • A balancer that works well on larger sized props is the Dubro HiPoint balancer. The magnetic balancers will not work on the larger props as they cannot support them.

  • Lock-tight every thing is another good piece of advice.

Ignition Types: There are several types available, for lack of better words, the "Magneto" and the "Electronic" The magneto is the simpler of the two and is self contained. The up side to it is that it has no external power supply. The down side is that it has fixed timing and is harder for hand starting. The timing issue is bad because the timing advance needed to produce good top end power is different than what is optimal for a good idle. The result is a lumpy idle and hard starting. Another down side is the speed at which the motor needs to turn to get enough spark to start.

There have been several solutions to the problem of hard starting. One is a spring mounted on the motor. This is good but this will add quite a bit of weight which when added to the weight of the magneto system it self, can make for a heavy motor. The second it a "Jump Start", a device offered by C&H. What it is a plug in battery that boosts the spark for starting and retards the ignition. This make for easy starting with very little weight as the battery is unplugged as soon as the motor is running.

The second type of ignition is the electronic. This is a box that has a battery attached to it and a wire that connects to a crankshaft sensor. Basically, the sensor signals the box every revolution of the crankshaft. With this info, the box can determine the optimal timing of the spark.

Mounting: Mounting the electronic ignition module is very easy. Some ignition systems have little tabs on the box, to these you can attach rubber grommets and bolt the box to the side of the motor box. The grommets are available at most Home Depot type stores. On systems that are supplied with a plane box, the easiest thing to do is mount the box on a pad of foam securing it with two-nylon tie straps orientated in a cross. This requires that you drill into the motor box, four holes big enough for the straps to go through. The nylon tie straps are also available at any Home Depot type store.

Radio Interference: This is an area of Gas motors that can be less than pleasant. Ignition systems work by generation a spark to ignite the fuel/air mixture in the motor. This can lead to potentially dangerous radio interference problems if precautions aren't used.

First: Use separate batteries for the ignition and receiver/servos (this applies to electronic ignitions only)

Second: Make sure the two electrical systems, i.e. receiver/servo's and ignition are at least 8 inches apart, that includes all wiring, switches, batteries and modules.

Third: Mount the throttle servo well back from the motor and do not use a metal rod for a throttle pushrod. Do not use metal cleaves on the throttle and be careful to mount the pushrod so there is adequate space between it and the muffler/s, they get hot.

Forth: If you replace the spark plug on the motor, make sure to get the same type that was there in the first place, at least make sure that it is a "Resistor" type plug.

Fifth: If you are experiencing a loss of range due to ignition interference and the above doesn't solve it, try a Bosch spark plug cap available from various suppliers.


There are several methods of starting gas motors that work very well but here is a method that's good for new motors when you don't know exactly what it will take to get them running.

Step One: Throttle set to ¼, Choke closed, Ignition on, Flip the motor tell it fires (or in the case of a spring start motor, use the spring) and stops.

Step Two: Throttle to 1/8th, Choke open, Ignition on, Flip tell it runs, or in the case of a spring start motor, use the spring. On the larger gas motor's, electric starters are really not necessary.

Running Range Check: This is something you HAVE to do! This step is a must, do not fly the plane with out doing this. With the motor not running and the antenna down on the radio, do a range check, i.e. walk away from the plane tell the surfaces start to jitter, in the case of PCM, keep moving the sticks tell the movement of the surfaces becomes jerky. At this point, walk back to the plane close enough to get a solid signal then walk around the plane keeping the same distance.

Note the minimum distance and if they’re any places as you walked around the plane where the signal faded. Next, start the motor and do the range check again, with a helper holding the plane, walk out and around the plane. Do this several times at different throttle settings including full throttle.

It is likely that you will not have as much range as before, what you need to do is determine how much range you lost. 10% is considered ok, 15% borderline. If you have lost more than that, you should not fly the plane tell you have gone through the system and found the cause of the interference.

Motor Break-In: Breaking in a gas motor is not like running in a glow motor. Due to the difference in design and running requirements, a gas motor should be broken-in ... in the air. In most cases, you should do your running range checks, motor off, motor on, and insure the needle settings are close enough to fly, erring on the rich side, then fly the plane in a gentle fashion for several flights. Be gentle in the beginning and no extended full throttle usage. (i.e. no gyroscopic's or snaps. After three or fore flights of at least 10 minutes, you are free to fly as you please but remember, the nicer you are to the motor when it's new, the better it will run in the long run. The fuel you use can be the same as the fuel you plan to fly with there is no need to use different oil or a different ratio. If you just have to run it in on the ground, you should use a propeller several sizes to small, i.e., if your motor normally turns a 26-10, use a 22-10 on the ground and make sure not to exceed red line by going to full throttle. This will keep the load low and the internal temps down.