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Gas Engines

by blackfingernail » 20 Apr 2009, 14:58

I have been playing with glow and diesel motors for 50 years, my first glow was a Macoy 19 red head my first diesel was an ED BEE 1cc. but my first Gas engine arrived about 18 months ago a 3W 85cc. and it is a totally new ball game.
So over the past 18 months i have been collecting information on how to handle and operate Gas engines, most of my information comes from the Gas engine site on RCU.
Most of this comes from a post of Bob Pastorello and other posters it is very good information on the how and why of Gas engines, don,t pass over any of these points without thinking twice it could turn out to be expensive, his writing style is somewhat to the point but very valid


1. All tank and plumbing accessories must be "gas" rated. Dubro brown color stoppers are the "standard". Yellow Tygon is widely used. Clear tubing is crap. All fittings need to be "secured" over their attachment barbs. All smooth brass tubing needs some kind of bump, solder blob, or barb to keep Tygon on, as it swells in use. BEFORE it gets rigid and brittle with age. NORMAL.

1a. Klunks and filters.....gotta have 'em someplace. Klunks in the tanks that are like the little Walbro "felt clunks" in your weedeater work REALLY well. So do more exotic ceramic particle filters. Gotta have a klunk. Gas should be filtered in the jug, too, before it gets INTO your airplane. And yes, tanks only need two lines.... vent to atmosphere and line to carb. You can use a tee with a fuel dot to fill and drain the tank. It will work forever. You also can use three lines, but that gets a little more complicated. Use filters.

1b. Pulse Lines - MANY gassers have a piece of tygon fuel line running from a fitting on the crankcase or intake adapter to an inlet pipe on one of the "sides" of the carb. This line is NOT for gas. It must not be kinked. It cannot have holes. If not hooked to the pulse line on the carb, the carb will not pump. Ever. When in doubt, call the manufacturer. Most of them ship their engines with the pulse lines already properly connected. Sometimes they even have manuals with pictures in them that MAY show this line and it's fittings.

2. Every engine with a Walbro or "clone" carb MUST HAVE it's Low and HIGH speed needles re-adjusted from factory settings to get GOOD performance. EVERY ENGINE!!!

2a. Walbro carbs (and other similar) have PUMPS. Rocking the prop back and forth or flipping it with fuel will PUMP the head full. Spark doesn't work under fluid. The tank can be a long way from the carb, and it's okay. As you rock the prop, watch the air bubble/fuel move up the line. If the bubble isn't moving....the pump isn't pumping. Get the pump wet inside somehow. Fact.

3. Breaking an engine in on the ground is generally not a real good idea. Use the manufacturer's recommended oil ratio for starters, get a couple tanks (or less) through the engine to make sure everything is okay, then go fly the damm thing so it will have SOME form of cooling air over it.

4. Spark plug cap fittings usually fit very tightly if they are the "push on" type, and you will have to push HARD. It's normal. If the plug cap isn't on correctly, it will not run right. EVER. And you'll get unreliable starting, running, and radio interference (all not-good things).

5. Prop drilling - if you have a jig, use it. If you don't, use the prop washer as a guide and try to get straight holes. ALWAYS balance a prop before using it.

6. Most carbs will require you to modify the throttle arm to be "user friendly" for our pushrod/ball link setups. Get over it. It's the way it is. Use a short servo arm to attach to the crappy little plate they supply on the throttle butterfly shaft. Use ball links and very rigid pushrod setups with GOOD servos.

7. Remove and toss the "stop screw" on the throttle arm plate, then unhook the little spring that pulls the butterfly closed, unless you want your throttle servo under constant load/tension. Do NOT REMOVE THE SPRING.

8. Electronic ignitions work wonderfully, if they are getting power. Use a reliable battery with the voltage rating and milliamp hour rating of the manufacturer. If they say nothing, go with a 4 cell 1100mah NiCad, or NimH, or a 5 cell system on a GOOD regulator. Not recommended for newbies. Get some experience with this area. Always use a switch, and setup throttle so you can kill it.

8a. Ignitions all have three chunks of wire. ONE, the spark plug lead, also contains the "ground", and must make contact with the outside of the plug hex (where your socket wrench grabs the plug). It can't have holes, or cracks, and should always be prevented from rubbing on ANYTHING. TWO - "Power", usually a "female" battery-type connector with a red and black or red and white wire/connector. This is where the "OUT" of your switch provides power to the ignition. NEVER get the red and other color mixed up ever!!! THREE - the "sensor pickup" - usually a black/white/red (or similar) wire that connects to the thing that is mounted up near the prop hub (on most engines). This is the "hall switch", and picks up the rotation of the prop hub by sensing a magnet when it rotates past the face of this switch. ALL three wires have to connect, and the polarity of these three wires can never be wrong, either. The igniton senses the magnet rotating past the switch on
the hub/crankcase to figure out when to send the spark to the plug. It only "knows" when the magnet has passed it. ALL of these work the same way. Rotating the hub magnet past the hall sensor/switch means that the IGNITION **WILL FIRE THE PLUG** if it has power. Many people with injured fingers thought an igniton was OFF when it wasn't and found out by casual flicking of a "hot" prop. The ED department visit usually costs more than an engine. AVOID this, always. Treat a gasser with an ignition attached as if it is HOT, 100% of the time.

9. In general, a new carb will not pump if it has been dry for awhile. Period. Get some gas into the thing by normal "priming", close the choke, flip through a few times, ignition on, throttle at high idle. Engine pops starts, dies, open choke, flip till starts. RESTRAIN you AIRPLANE while doing this. ALWAYS.

10. These gassers need GAS. Not some other kind of make-believe fuel. If yours has ethanol, it will be ok.. 95% of our engines will do just fine on regular unleaded 87 octane pump stuff. They also have to have OIL. Nearly all of these need about 30 or 32:1 ratios for their first starts. That's 4 oz to a gallon. Easy to remember. Won't hurt the engine. Use a good oil for 2 strokes. If in doubt, call the ENGINE seller about ratios and types.

11. No Metal to metal from the servo to carb. Use nylon rod and sleeve.

12. Place the Rx as far as possible away from the CDI

13. black Friday - dont fly on this day.

14. If it isn't running right on the ground, it will NOT improve in the air. ADJUST IT!!! And do that ONLY after it is at it's "normal" operating temp.

15. More prop does not mean more power. It means more load, and generally that isn't a good thing. EVERY prop change on EVERY engine will need re-adjustment. PERIOD.

16. If someone helps you and things get worse with your engine - get a different helper. ALWAYS.

17. Mufflers - nearly all of them are NOT; they are exhaust cans with diverter outlet tubes. They are usually louder than you think. If you want quiet, you need a canister, period. Not necessarily for newbies.

18. Install muffers with no gasket, use a thin film of Permatex High Temp Permagasket on the face of the muffler flange. Put RED "permanent" or "high temp" Loctite on the screws, and tighten them. Tightly.

19. If you have to use a Pitts muffler, brace the canister part to the engine/firewall with a bracket or strap of some kind. Tighten the muffler to the engine exhaust THEN install the strap/canister attachment. Don't do it the other way.

20. Smoke is not for a newbie. If you must smoke, get help with it, as you've instantly doubled your plumbing complexity, tanks, added weight, electronics, etc. If you have to have it, post here on "How to Setup Smoke". It works great and looks like a million $$$ but most folks don't mess with the mess.

21. Spark plugs. These are not glow plugs. They need ignition sources. They have to be properly gapped. They should not be oily-looking, fouled, or have shiny metal flakes on the porcelain. ANY of these things are symptoms that something else is wrong.

22. If your engine doesn't "take throttle" (hesitates), the low end is probably lean. If it "takes throttle", but bogs down, coughs, sputters, etc. it is low end RICH. NO doubt about these. Properly-adjusted gasser carbs operate very well. Poorly adjusted ones will make you insane.

23. If you have been flipping until your arm is ready to fall off.....go back to the first post on this thread. SOMETHING ain't right. All of these engines need air, gas, compression, and ignition. Given all of them in the proper portion and time, these engines ALWAYS start. If they do not....SOMETHING IS NOT RIGHT!!!!!! do.....figure out what's missing from the thing. If it's all there, THEN post for help. We will be happy to help....those who try.
24. Gassers vibrate. Period. Firewalls must be STRONG, and pinning around their circumference with wood dowels or epoxy/nails is a good practice. Pulling the engine box off an ARF is easier to do than you think.

25. Your engine will probably require "standoffs". Measure your ARF from intended prop spinner backplate to firewall. Deduct the overall "mounting length" of your engine, plus 1/8". Whatever is left is the "standoff". You can buy metal ones, or make your own out of hardwood dowels, whatever. BUT - they have to mount real solid-like. Find sources of long #10-32 hex head bolts, split washers, flat washers, fender washers and nylon-insert Locking nuts. USE them. It is a BAD plan to have an engine come loose.

26. Allow "exit area". You cannot avoid it. You either make holes to let LOTS of air out of your cowling/nose, or you cook your engine. Period. If you are able to do it, direct air by dams or ducting directly over the fins of the engine, THEN to the "big exit area".
If you hover, or do strenuous 3D....skip this step and you'll cook the motor, probably seizing it up at a low-altitude hover by overheating, thereby crashing your airplane ALSO.

27. Props - the most important accessory for your engine. Bad props give sucky performance, break easily, or disappoint you in their performance. Any good prop of the manufacturers rated size will make you a happy gasser.

27a. Prop Balancing - probably an oft-ignored and vitally-important "thing" for gasser guys. A good balancer is essential. A bad balancer is a waste of money. I prefer the Dubro Spin balancer....very sensitive. Blades are balanced by lightly spraying the back of the blade with clear spray lacquer- lightly...until blades balance. If blades are balanced, but moves anyway, check the hub. Put the prop on the balancer with blade vertical. Release blade. Watch top blade....if it falls LEFT (as you're looking at it) then the RIGHT hub needs weight, and vice versa. Weight on the hub is the curly velcro, ca'd in place, a few drops of thick ca, then kicker. Recheck balance. Keep at it until the prop remains motionless wherever you position it on the balancer. THAT is perfect balance. Takes about a half hour on a 22", more on bigger. DO IT.

28. RPM readings. Nearly meaningless for one user to compare to another user. Even if using the same prop. Carbs are very sensitive and being off the sweet spot by 1/16th turn on the H needle can make a big difference in tach readings. Plus, most tachs are semi accurate, part of the time, and light/angle sensitive all of the time. Unless you know me, and how I tune, my altitude, air density, oil mix ratio, type of oil, and heat temperature....my peak rpm is of no consequence to you except for "ball park", and it's a helluva big park. Most 50cc gassers and above should be somewhere around 7000 with the "stated" prop from the manufacturer. Larger displacement, lower peak rpm. That's how it goes.

29. Freqently check your ignition battery voltage. Under load, if possible. An ignition battery that is losing it's guts will create bizarre unreliable and misfiring engines.

30. If your engine was fine yesterday, and today it's NFG, then *something* changed. Find what changed, and fix it. Don't jump to conclusions and speculate. Work through the systems. Start at the tank, work through the plumbing. Start at the battery, work through to the plug, start at the servo, work through to the butterfly. Only a VERY few, and "special" engines are actually sold with Gremlins built into them. Usually they have already been owned by someone who couldn't find and kill the gremlin, so you got it. Think of a finicky motor as a PC virus....there is a solution....you just have to find it.

31. When you have a new engine....buy a new (correct) spark plug. And as soon as you can afford it a Spare Ignition, and a Hall Sensor/switch. Spare carb if you aren't a "carb tweaker". Carb rebuild kit whether you are or are not. Buy replacement prop bolts, and be sure to use split and flat washers so the prop washer doesn't get all boogered up. Many manufacturers don't know about this yet, so they don't include screws long enough for thick hubs and spinner backplates, much less the split and flat washer. It is worth investing in.

32. Buy extra metric screws and ball drivers. NEVER trick yourself into believing that a 10-32 is the same as a 5mm. They ain't.

33. ALWAYS provide for some form of emergency engine/ignition cutoff. At a minimum, install an external switch that cuts off power to the ignition. Better yet, add to that an internal ignition cutoff, that is connected to a spare channel on your radio. Optical remote cutoff modules from SmartFly or other providers, work really well. Make sure the assigned switch is out of the way, so you do not hit that switch accidentally while flying.

34. Tygon tubing, of the type mostly used for gas engine in model airplanes, degrades over time. Check the tubing on any airplane you have acquired, especially at the ends. The piece of tubing inside the tank that is attached to the clunk and dipped in fuel, will likely get hard in a year, loose its flexibility and could be the source of fuel draw issues if not checked.

35. OIL - It is vital that you protect a gasser with the RIGHT kind of internal oil film for operation. Too little oil and it will die a quick and horrible death. Too much oil, and you plugs and airplane covering will look "ugly". It is never a good idea to use an oil that intentionally introduces abrasives to "wear in" an engine. Let a proper amount of good oil do that during FLYING !!!

Everyone has their preferences; some are based on experience, some on knowledge, and even fewer on BOTH. Ask the person with BOTH what they recommend.

ALWAYS pay attention to what the company that is providing WARRANTY on your engine says about oil. Do it "their way", or be prepared to fix the engine yourself for ANYTHING even remotely-oil-related.

36. KILL SYSTEMS - Gassers, because of their power (and usually size of airplane) **SHOULD** have some sort of a "kill device" either mechanical, or electrical. In general, most folks agree that a transmitter-operated kill switch is a good safety device. If airborne, with a typical large tank, your throttle servo fails or disconnects, you either land hot and break something, or fly around risking killing batteries. If you have a switch to flip that chokes the engine, or removes power from the ignition, YOU can decide when to land - safely.

a. Choke Servo - this involves hooking up a choke servo to an extra channel, and assigning to a switch or lever on the TX that closes the choke when you actuate it. Overly-rich, with air cutoff, the engine will die quickly. Pros and cons to this system.

b. Electronic Kill Device - this involves an electronic switch that is operated by a spare RX channel, activated by the TX that cuts off power to the ignition module. These are nearly always installed "after" the on-board separate ignition switch, and "before" the module in the power lead. Usually optically-coupled electronic switches, they don't induce any harmful interference when properly installed. Lots of these available. If going this way, ask around your area, or on RCU to help make your decision.

Whatever you do, please ALWAYS consider the safety benefit of being able to shut off your gasser when YOU want to. It is a vital element of overall RC safety,
from David Foster

A couple of weekends ago I was in the shed listening-in to a few guys discussing the percentage of nitromethane they use in their fuels. The conversation went like this:

“I use 5%”.

“Is that all - I use 15%”.

“That’s too much - I reckon 10% is about right”

It seems to me there’s a lot of misinformation out there about nitro. Let me see if I can clear some of it up.

Yes, NITRO = POWER - but it doesn’t add power because it is a “hot” chemical. In fact the methanol in fuel (methyl alcohol) is by far the most flammable ingredient - nearly twice as flammable as nitromethane. If you put a flaming match into nitro the match would go out.

Well, how does nitro add power? Every internal combustion engine burns a mixture of air and fuel of some sort - in our case a liquid glow fuel. The purpose of the carburettor is to meter these two ingredients in just the right proportions, and every engine requires a specific proportion of liquid fuel and air to perform at its optimum. If we try to push more liquid in without enough air, it won’t run.

However there is a way of running more fuel through our engines without increasing the air supply - by adding nitromethane to the methanol/oil mix. An engine can burn almost 3 times as much nitro to a given volume of air than it can methanol. Voila! More power! That’s all there is to it.

However there are a few practical aspects to consider. Not the least of these is fuel cost .

Nitro costs around $20.00 a litre and ready-mixed 10% nitro fuel costs about 30% more than “straight” fuel.

I know some flyers who don’t use any nitro at all. Pylon racing engines designed for international FAI competition run on no nitro at all, due to their rules. Yet they go much harder then engines running on high nitro fuels. This is because these engines have compression ratios, intake and exhaust timings etc designed especially for FAI fuel (4:1 methanol and oil). Even then they won’t idle at all and can be a serious bitch to tune and run - just ask Ranjit!

A popular misconception is that nitro gives you an immediate power jump. In the 5% - 25% nitro range you will probably see an rpm increase of about 100 rpm static (sitting on the ground or in a test stand) for each 5% nitro increase. However in the air the engine will unload and achieve a greater increase, and it will idle better too.

At the other end of the scale it’s possible to use too much. When I was running powerful racing engines in Old Timer competition, where optimum power is important to get maximum climb from a short motor run, I found virtually no incremental improvement in performance with nitro contents above 30%.

Most of our popular 2-stroke sport engines are designed to run on 5% to 10% nitro, 4-strokes 10% to 15%. Most European engines will run successfully on less, because they are built to do so. In the UK, nitro costs between $200 and $300 a gallon! Reason enough?

Conversely engines made in Asia, as most of those we run are, are designed to run on nitro-containing fuels. The vast majority of model engines manufactured in Asia end up in USA, and nitro is very cheap there. This is because the only manufacturer of nitromethane in the Western Hemisphere happens to be in the USA. ‘Nuff said.

Going back to the beginning, how much nitro do you really need? From a practical standpoint, virtually all our everyday sport flying can be done on fuels containing from 5% to 20%. If you’re flying something like a trainer or a Cub with a 2-stroke engine, there’s no reason why 5% won’t work perfectly well.

Need a little extra power? Move up to 10% or 15%. Four strokes need a little more - 10% to 20%. OS 4-strokes are adjusted at the factory for fuel containing 10% nitro.

I wouldn't recommend going higher than those percentages. It won’t do you much good and it’s a waste of money.

I have found 5% in 2-stroke fuels and 10% in 4-stroke fuels is about right for me, giving easy starting, good top-end performance, reliable idle and instant pick-up. If I could afford it I’d up these percentages by 5%. However I use about 50 litres of fuel a year, so adding more nitro would add substantially to my fuel costs.

Reproduced by Brian Porman with the kind permission of Brian Winch from RCSA Jan. 1997. BP's comments are in italics.



The carburettors used on model aircraft are of three different types. The first is a diaphragm or butterfly carburettor and this is the type most commonly found on the large engines, particularly the convert type engines. Examples of these are Walbro and Dellorto. (This section, which may only have a small readership, could be reproduced in a later article if there is general interest and members with or thinking of using this carbie would like the BW low down. Please let me know)




The next carby is the air bleed type as is found on Enya and some OS as an example. The sketch shows the hole in the body of the carburettor and in the Enya, a spring loaded screw at the top, and in the side for the OS. The hole and the screw serve one purpose only and that is to adjust the amount of air available to the engine at idle rpm.
As the spring loaded screw is wound IN so that it partially blocks the hole it, obviously, impedes the flow of air. Winding IN gives a RICH mixture and OUT gives a LEAN mixture. The richness and leanness of a fuel mixture relates to the amount of liquid fuel supplied to the engine.
A lean mix is a mixture with too little fuel/too much air and a rich mix is too much fuel/too little air. The fuel comes from the main jet but the rotor of the carby is closed in the idle position so the air has to be supplied by the hole.
When tuning, the control, run the engine until well warm and correctly adjust the main mixture needle. Close (pull back) the throttle until the engine is idling close to the desired speed, generally around 2,800 to 3,000 rpm. Very carefully adjust the air bleed screw in small amounts and listen to the engine. If the rpm increases and the engine stops then the mix is too lean - wind the screw in a little.
If the engine slowly loses rpm and stops then the mixture is too rich - wind the screw out. Adjust the (main mixture) needle until you have the highest rpm the engine can retain without stopping, then wind (the screw) in about 45 degrees [1/8th of a turn] to give a very slightly rich mixture.
Except in the most extreme circumstances, this is a set and forget area as it is very tolerant of prop loads, plug changes and extremes of weather.
(Air bleed) is a very simple and reliable mixture control so why isn’t it used on more carbies?
It has two drawbacks! To choke the engine for starting the throttle must be open otherwise air is drawn through the hole and it drastically reduces the suction on the fuel supply.
The other problem is that it controls the fuel mix for a very small amount of throttle movement, so that the engine is slobbery rich in the mid range.
The method of overcoming this to some extent is to fit a carby with a small venturi and this is the case with the Enya and OS and this has a side benefit in that the suction from the tank is greater so tank position is not so critical with most of the engines fitted with these carbies.
Before we move too far away, on both these carbies you can see another spring loaded screw, in both cases, on top of the carby. This is the rotor retention and idle speed screw.
Serving a double purpose it retains the rotor in the carby body and can also be adjusted to set the speed of the idle. In most carbies, if this screw comes out, the rotor follows quite rapidly so keep an eye on it occasionally.



The main mix for these carbies is the same as most others and that is a tapered needle in a stepped hole tube commonly called the needle valve. The end of this tube is exposed to the air flowing through the venturi section - the main hole through the carby into the engine, - and there it does the job of a fuel jet or spray bar.
The needle valve is drilled to a diameter that is a neat fit on the needle and, close to the jet end, the diameter of the hole is reduced.
As the needle is wound in, the pointed end enters the stepped down hole and gauges the flow of the fuel. Further in, less fuel and vice-versa.
Adjusting this mixture is common practice gained by experience and the experienced modeller knows well to tune a little on the rich side as the engine unloads in the air and a fully tuned engine on the ground is a lean engine in the air.



The other adjustment on the carby is the throttle arm and this is the most neglected adjustment of all in most cases. I am certain you know well the methods of setting differential aileron travel, for example, by using a disc on the servo and connecting the aileron rods behind the centre of the disc.
Also, when installing a control horn on a surface such as elevator or rudder, the importance of having the connection pivot in line with the hinge line to obtain equal travel.
The same problems crop up with throttle connections where we need a Set your engine servo also in the halfway position and then make the connections between the two. Now you will find there is no "ganging up" of the throttle movement and the changes will be reasonably even as you move the stick on the Tx.
The big benefit is that you will have very nice control at low throttle which, as any scale flier knows, is extremely important when coming in for a landing talking of which, it is about time for me to come to the final leg.



This type of carby is so called due to the high speed and idle adjustments being controls over the flow of liquid fuel. The air is the absolute domain of the aperture set by the opening of the rotor. Typically the main mixture is adjusted by a needle with a fine point or acutely angled end and its rotary movement is controlled by a spring detent (ratchet), compression spring or friction provided by a tight fitting ‘O’ ring.
Between the main needle and the venturi will be a fuel nipple leading to a fuel chamber. A fuel chamber is a small reservoir that maintains enough fuel to supply the demands as the rotor is opened, in order that the increase in engine speed is instant.
This is as close to a float chamber as you get on a simple carby. It surrounds, or is adjacent to the needle seat through which the metered fuel flows. [I’ll wager not everyone was aware of that?]. The seat is a small hole, large enough for most of the tapered section of the needle to enter. The further the needle enters the seat, the less fuel flows. This is leaning the mixture. The basic principle is the same for all types and that is the supply and metering of the fuel.
It goes without saying that any interruption in this area means engine failure in some form. Either the engine will stop due to lack of fuel, overheat due to a lean mix or run erratically due to an uneven fuel supply. Luckily this area is generally trouble free mechanically but the human element is hard to beat. The main cause of any problems are foreign materials. The final aperture(s) for the metered fuel is extremely fine, it takes only a minute particle to cause problems. The first problem is dirt - a word encompassing dust, grass seed, lint, animal hair, rust flakes, paint flakes wood dust and fibres, particles of glue, glass fibres or just unrecognisable grot.
There is only one way of preventing these ‘nasties’ from getting into the fuel stream and that is effective filtering. If you use only one filter it has to be as close to the fuel nipple of the carby as possible.
Why take chances? Filter the fuel as it is mixed (if you mix your own). Filter it as you fill the tank and filter it between the tank and the engine. Would you run your car without a fuel filter? [a filter on the pressure line from the muffler is also practised by some]
The other type of blockage in the fuel chamber is the ‘mouse’s eyelash’. This is a tiny crescent of silicone about the size of a ‘mouse’s eyelash, and it can be a real ‘bitch’. You might have no problems for several runs then an engine failure. The engine will often start again and run for several tanks then fail again - very frustrating.
Inside the chamber the eyelash floats around and every so often, one will enter the fuel system and disrupt the flow. When the engine stops, the eyelash floats back into the chamber until it is in position again to sometime cause another disruption.
This ‘eyelash’ is a tiny sliver of silicone fuel tubing caused by a sharp edge on a metal fuel tube cutting it off the inside of the silicone tubing as it is slipped on the metal tube. Smooth all metal tube ends and nipples with super fine wet and dry paper and polish with metal polish to be certain there are no sharp edges. To clear the ‘eyelash’, backflush through the jet tube with fuel, having removed the fuel nipple if possible.



Follow the flow of fuel through the needle seat, along the jet tube and out of the aperture. This aperture is our carby jet and can be in the form of a crescent shape slit, a straight and narrow slit, a rectangular slot across the diameter or just straight out the end of the tube. The fuel is sucked out of the aperture by the action of the air changing pressure and speed through the venturi. Air is sucked into the carby and down the venturi which narrows at the middle causing a change of air pressure and subsequent speeding as it passes the jet.
In so doing, it sucks the measured fuel supply with it to supply the demands of the engine. Too much and the mixture is too rich. Too little fuel and the mixture is lean. The engine will run on a disproportionate air/fuel ratio until the mix is so far out the engine stops. Stopping rich is no great problem but stopping lean can kill an engine in a remarkably short time.



On the opposite side of the jet tube to the main needle is the idle mixture control and this can be in the form of a tapered needle, parallel needle or encompassing tube all of which impede the flow of fuel from the jet. A fuel metering carby of the common type has a helical slot in the rotor so that rotary movement induces side movement. This causes the idle needle or tube to slide into or over the main jet and ‘meter’ the amount of fuel in proportion to the now reduced venturi opening.
This adjustment needs considerable care and checking as it also controls the change from idle to mid-range operation. In all types of idle metering devices the amount of movement for a considerable result is very small - 1/10 of a turn at a time. Fortunately, once set they are not prone to changes in weather like the main needle so it is rare that you would need to readjust once it is set provide you do not make radical fuel changes.



Generally, once the high and low mix is set, the engine is brought to mid-rpm and adjustment made to mid-range to obtain smooth running. Other bits will be a spring loaded bolt or bolt with a collet nut such as in OS carbies. These are idle speed adjustments and are a simple mechanical adjustment to stop the travel of the rotor. Unless it is a large engine, you should set your idle speed between 2,500 and 3,500 rpm. Under this and the engine might stop as it cools down on the landing approach, higher than this might provide too much speed for landing. Some carbies do not have this adjustment, so set it with your servo.
The usual method for reliable idle for back stick - high trim on the transmitter and low trim to kill the engine. These carbies will have a slotted or hexagon head bolt to retain the rotor which must remain firmly tight at all times. If the carby has both adjustments and retaining bolt you might find that removing the adjustment bolt will provide more control from the T/x.
It would be wise to replace the bolt with a shorter one screwed snug to prevent air leak.



Connect a length of fuel tubing to the nipple and close both needles gently. Open the rotor fully and blow into the tubing as you slowly open the main needle. As soon as you can feel the air flowing, stop adjusting.
Do the same thing with the idle (needle) after closing the rotor so that just a fine crescent area is open. Now open the main needle two more turns so we don’t start lean. Start the engine, take it up to full rpm and let it heat up for a minute or so.
Two clicks at a time close the needle valve (assuming the engine is running rich, if the engine is a Saito then open the needle four full turns) and listen to the rpm. Wait about 10 seconds after each adjustment for the engine to settle, then close down another two clicks and wait. Continue doing this until you do not hear a change in the engine speed.
At this point the engine is fully adjusted for static running. When the model is in the air, the static load on the engine is reduced and the rpm will increase. For this reason we are going to tune slightly rich, so wind the needle open four clicks.
The absolute final adjustment will be achieved in the test flight. By now the engine is nicely hot so we can set the idle. Take the speed down to about 2,800 rpm and listen to the engine. If it stops or loses rpm it is too rich, so wind the needle in two clicks. If it speeds up and stops it is too lean, so wind out two clicks. Keep adjusting until you obtain the maximum steady rpm at which the engine will keep running.
You might have to readjust the idle stop screw during this operation. Very slowly take the engine up to full rpm and let it run there for a minute or so. This will clear any residual fuel in the case, then let the engine return to the optimum operating temperature as this generally lowers during idle. Now back down to idle and try the transition from idle to high speed, moving the throttle lever at a speed similar to which the servo will move it. If the engine falters after idle, make a small adjustment to the idle mix as it is too lean. If the engine chokes and coughs it’s too rich.
From now on, using the same fuel, prop and plug you will need only a very small adjustment of the main needle for extremes of weather. Do not wind the needle back and forth each flight, as only minor adjustments will be needed. If you have a bit more than adequate power for the model, set richer at the start and never touch the needles. If the engine runs richer some days it won’t matter as you are not after every last drop of power.



You can fit a remote mixture control for inflightmixture changes. Before you consider this, make sure you aren’t having yourself on that you are a beaut engine tuner. You need a very good ear to understand what the engine is telling you in the air, so don’t fit a remote tuner just for the sake of having a complex gadget or one-up-man-ship. If you must, I recommend you consider the Varsane Inflight Mixture Control due to the precision manufacturing and reliability. These units work very well and will last. The instruction sheet is comprehensive for the simple method of operation.
The other remote to consider is the needle valve. Any engine will run from a remote needle valve and it is worth considering for the sake of easy operation with awkward cowl situations, also for finger safety. Again, I can recommend the Varsane unit for the same reasons. Just keep them mounted and all connections airtight. The new OS Fx series engines come complete with a remote needle that is part of the backplate.



Sucking dust and even grass seeds into the engine via the air intake rarely bothers the fuel supply or causes running problems. It does cause accelerated wear (in the case of four strokes, power loss if it upsets the seat of the intake valve). Another consideration is the supply of air to the engine and the problems of hot air, tight cowls and air suction pulling the fuel out of the carby.


Should you wish to ask Brian a question on these topics please send a self addressed envelope and print your name, to Brian Winch, 33 Hillview Parade, - LURNEA NSW 2170

by David Foster

On the shelves of most hobby shops you’ll find fuels labelled “2-stroke” and “4-stroke”. Is there really a difference, or is this a big con by the fuel manufacturers to sell you more fuel?

The main difference is that most 4-stroke fuels contain less oil than 2-stroke fuels - typically 15% for 4-strokes and 20% for 2-strokes.

You’d think that because 4-stroke engines have more moving parts they would need more oil - not less. The number of moving parts has nothing to do with it. What is important is that, with very few exceptions, 4-stroke engines run substantially slower than comparable 2-strokes...most in the under 10,000 rpm range vs 13,000 and more for a typical 4-stroke of the same size. They are designed to deliver maximum power at slower rpm’s with bigger props.
The more slowly an engine turns, the less heat it generates from friction. If you don’t believe me, rub your hands together slowly, then as fast as you can.

So...lower rpm’s = less heat = less need for oil.
Four-stroke engines fire every alternate stroke, vs every stroke in a 2-stroke engine. Firing, or combustion burns fuel, which creates heat. Logically it may be deduced that if the engine fires every alternate stroke the engine has time to cool-off a little between combustion cycles.

Using, for example, a hypothetical 4-stroke engine turning 10,000 rpm = 5,000 combustion cycles a minute, vs. a hypothetical 2-stroker turning 13,000 rpm with 13,000 combustion cycles per minute, you can see that the 2-stroke has 160% more combustion cycles than the 4-stroke. Ergo: 4-strokes remain cooler, and therefore need less oil.
Because the 4-stroke is only firing every alternate stroke the glo-plug element cools down between strokes. Excess or unnecessary oil constantly dousing the element is going to make it more difficult to achieve a slow, smooth idle. If you’re one of those who say, "Too much oil can’t hurt anything”, you’re wrong.
In addition to causing undue friction in the engine, keeping parts from properly mating etc, too much oil in a 4-stroke fuel is constantly trying to cool a plug element that is already having problems. Like pouring a bucket of cold water on a bloke who is already shivering.

Since oil doesn’t (or shouldn’t) burn...it simply lubricates and goes straight out the exhaust all over everything...it does nothing to help us deliver power. However suppose we replace that 5% or whatever of unnecessary oil in the fuel with methanol which does burn. What do you know...greater top end power.
The conclusion to be drawn from all this is that a a properly blended 4-stroke fuel containing 15% oil will give better all-round performance than a regular 2-stroke oil containing 20% oil in the same engine.
While it’s not actually going to harm anything by running a 2-stroke fuel in a 4-stroke engine, don’t do it the other way around. It’s not going to have enough oil.

The manufacturers of YS engines - the most powerful 4-stroke engines available - recommend their engines be run on fuel containing 20% oil. These engines are quite unique in many ways and the manufacturer’s recommendations should be followed.

by David Foster

I can’t remember ever buying ready-mixed fuel from a shop. I’ve always mixed my own. Not only do I know what I’m getting, ‘home brew’ costs a fraction of proprietary fuels. My ‘straight’ (i.e., no nitro) glow fuel costs $3.70 a litre; the hobby shops charge around $13.00 (fuel prices went up 10% with the GST because there was no wholesale sales tax previously). I use about 75 litres of fuel a year so the savings are really worthwhile.

How do you do it?

Simple enough. For an everyday 2-stroke sport fuel you can mix 1 part Morgans Coolpower or Klotz oil and 4 parts methanol (20% oil, 80% methanol). That brew that will give good performance with most 2-stroke engines. I usually use 15% Coolpower, 5% Castrol M (castor oil) 75% methanol and 5% nitromethane. The castor provides an extra margin of protection in case of lean runs and the nitro improves performance a bit as well as making starting easier. For my 4-strokes I use 15% Coolpower, 75% methanol and 10% nitro. The higher nitro content ensures better idling in 4-strokes.

The castor versus synthetic oils argument still rages. I have been using Coolpower for years and never had any problems. Also Coolpower is much cleaner - it doesn’t turn to varnish and clag-up the engines’ internals (particularly the ball races) like castor oil does and the models are much easier to clean.

Having said that I still like a bit of castor for the extra protection it provides. Some of my engines have been in constant use for more than 15 years without any problems or parts replacements, so I must be doing something right.

Just as important as what you use is the way you use it. Cleanliness is the key. Ensure the containers and measures you use (if plastic make sure they are not styrene) are perfectly clean before you start. Ialways strain my fuel through several layers of old nylon stockings in a funnel when decanting into the final plastic jugs. It doesn’t take much to block jets in your engines’ carbies, so eliminating any foreign bodies is a worthwhile step.

I’ve heard getting methanol on your skin is not a good idea, so I always wear rubber gloves when mixing fuels. Also methanol is hygroscopic - it absorbs moisture like a sponge. So always keep methanol containers tightly capped when not in use - that applies to fuel containers too. Water-contaminated fuel degrades engine performance and causes glow plugs to burn out quicker.
If you suspect your methanol or fuel is water-contaminated, leave it in the freezer overnight. The water will freeze and the other components won’t - so you can pour off the de-contaminated fuel into another container.

Where do you get it?

Most hobby shops sell Morgan’s Coolpower in 1 US gallon (4 litre) jugs at around $50.00. Castor oil (Castrol M) is available through Castrol stockists or direct from Castrol at Guildford. There are several sources of methanol. Incidentally, make sure you are getting aviation grade methanol. Warren Lewis, one of our members, organizes a bulk purchase one or two times a year and you might be able to get in on that. Phone Warren on 9417 0269. I buy mine from Whitey’s Workshop, Cnr Kenyon and Fowler Roads, Merrylands (phone 9637 3097). They charge $1.21 a litre inc GST, (take your own drum) which I think is the cheapest anywhere. You can buy Shell A Racing Fuel in 20 litre drums through Shell dealers or direct from the Shell Terminal at Clyde. Shell A contains about 5% acetone, which I’ve never found does any harm - in fact the acetone acts as a performance improver. Klotz oil and nitromehane are available from Hobby Headquarters at Kareela, however if you know someone who’s intro drag racing he can probably get nitro for you much cheaper than there.

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