Engines and Tanks

Castor?  Synthetic?  Percentage?
The perfect C/L Stunt brew got you puzzled?  Here’s the scoop.

It’s nice when your engine purrs like a happy kitten, and goes through the pattern smoothly, coming on and off, exactly when and where you want it.  Unfortunately, a lot of times, they growl and belch and are a total pain to deal with.  One of the biggest causes of this that I have seen is improper fuel.  Fuel is one of the most critical aspects in running model motors.  Use the right fuel and you will probably notice nothing; the wrong fuel will have you grumbling, or worse, will have your motor screaming, belching and running with absolutely no consistency whatsoever.

Most fuels on the market today use a synthetic base and are blended for the R/C sport flier.  These are typically very low on oil content, usually in the 12% through 15% range.  This is never acceptable for our use in C/L Stunt.  There are many reasons but the most important is the fact that we do not run our engines in a peaked two cycle, but rather a broad range of four cycle and rich two cycling.  Any time you run with the motor set to come on and off in the maneuvers (like a typical 4-2 break) you are not only asking the fuel to lubricate the motor, it also has to cool the engine.  The only way you can run in a 4-2 is to heat and cool the parts in the combustion chamber very rapidly.  This makes the oil content critical, because it’s the unburned oil that has to carry out the heat.

Years ago, most fuels had only one oil caster.  This is still a very good oil with many good but some bad points.  Some of its good points; it carries heat out of the motor and gives a good plating action on all surfaces, especially when they’re hot.  A few of its bad points; it burns and sticks to the piston sides and the ring groove and all other parts that are hot enough, and will carbonize the chamber.  This causes excess friction and heat and will ruin your motor in time.

The alternative to castor is synthetic oil and almost all fuels have these in them; the vast majority has all synthetic.  Virtually all fuel manufactures use one type of synthetic; these are polyalkylene glycols.  They are mostly made up of alcohol started linear polymers of oxypropylene groups.  These are made by several companies and are available in a large range of molecular weights and viscosities.

This group of oils is the modern version of the old Ucon oils and also have good and bad points.  Some of the goods points; they are very good lubes without containing any wax; they have outstanding load carrying capacity, film strength, anti-wear properties, are resistant to sludge formation, and will help keep your engine clean.  The bad points are they give no rust protection by themselves, they don’t plate hot surfaces as well as castor and they burn at high heats.

As you can see, both oils have advantages and disadvantages to them; it’s for these reasons that they work much better blending together than they could ever work alone.  All my years of flying and research have proven this to me beyond any doubt; plus you can see this for yourself.  Recently, a friend of mine had a motor that would go into the pattern and lean out and act very inconsistently.  The only change that was made was to substitute one tank of my fuel in the model.  The results were drastically different; the motor now ran very smoothly, going into a two cycle instantly when the nose was raised and back into a four cycle instantly when the plane was leveled.  This was tried back and forth both fuels; his and mine.  The results were always the same.

I would like to tell you there is on Stunt fuel formula to run in all motors, but this is not the case and will never by as long as we have such a wide range of motors and running styles.  What I will tell you is a good formula for the most common types of engines.  Make sure you pick a fuel supplier who will give you consistent fuel day to day and will blend fuel for your motor needs or has fuel to match your needs.  Stay away from any supplier who will not tell you the oil percent, age or who say one type works for all motors.

For motors like Fox .35s, OS Max 35s or the old McCoy’s and K&B’s, use a fuel with 28% oil content; preferably half castor and half synthetic.  Do not use the synthetic in an old motor that has a lot of time on it with all castor fuel; the synthetic will remove the castor gum off the piston and sleeve and will in some cases, leave you with the worn-out motor that had to start with.

For motors with larger bushings and bused rods like to OS FP and Magnum GP series, a 23-25% half-and-half oil mixture works the best.  For S.T. .46 through .60s and most all ball bearings Stunt motors, a 23% half blend works best.  The tuned pipe motors like a little more synthetic and I recommend a 15% synthetic, 7% castor blend.  This works very well in the OPS and Max VF engines, along with the new Super Tigre .51 engines..

You should also make sure you have the motor properly broken in.  This will range from six tanks of fuel for one engine to almost two gallons for others.  OS, for example, says two hours running time for their motors.  Use the same fuel as you will for your Stunt run and try to do your break-in on a bench; this is a lot better and an easier way to do a proper break-in.  A diameter, one inch smaller than you plan to run at, at a 3 or 4 pitch, should be the prop to use.  This let the motor turn many revolutions more per motor run time.  Start out in a very sloppy four-cycle for cast iron piston and ringed motors, slowly progressing to the fastest it will run in a four-cycle, then put it in a short two-cycle burst for short times.  After the correct amount of time it should be able to run in a two-cycle without heating up and going leaner.

For ABC, AAC and ABN motors, start out in a very fast four-cycle and about every 45 seconds, pinch the fuel tube to kick the motor into a momentary two-cycle.  If you can run the motor in a fast four-cycle and without touching the needle, pinch the tubing to lean the motor into a two-cycle for 20 seconds or so, then it should go right back to a four.  If not, it probably needs more running time.  When you first crank the plane, notice if it goes rich and sags slightly when the battery is removed; if so, the plug is normally too cold.  This is also critical to a proper Stunt run.

Most plugs are designed to provide a  colder range than we want and you should try to get the right range for the motor.  Many days of testing and much time and expense buying almost every plug on the market has yielded these results; Glo Devil RC #300 long, Rossi RC hot, Rossi #1,2,3, Enya 3 & 4, Fireball RC long, the Hobby Shack RC long, SIC RC long and some of the FOX long plugs are best plugs for our use.  In almost all instances, use a long plus, as they will be substantially hotter than the shorts, plus they are deeper in the combustion chamber and this tends to keep things hotter and cleaner.

A lot of times the plug problems show up as rich inside maneuvers and leaner outsides; this happens because gravity and centrifugal force forces the oil-fuel charge down on the element on insides, thus cooling the coil and pulls it away on the outside maneuvers, letting it naturally go leaner.  I have seen this problem instantly cured by simply changing plugs.  All of this assumes you have your tank height perfect (you did adjust your tank height, didn’t you), you’re right side up and inverted lap times are the same.  This is important; don’t skip this step.

A few tips when flying; if your plane goes lean in maneuvers and comes back to a four-cycle slowly, you most likely need more oil, or less back pressure from the muffler.  I have seen a lot of fuel with water in it (methanol attracts water) and this will cause erratic runs and needle settings.  Always use frosh fuel and don’t be afraid to try another fuel if you think this is the problem.

Never try to put a brand new engine in a plane and try to break it in, trim and fly at the same time.  I have seen this too many times with disastrous results.  Keep good care of your equipment and it will usually take care of you; abuse it and it will most times let you down.

Now, for you guys who absolutely gotta buy the bargain R/C sport fuel…No amount of persuading will convince you otherwise; you at the very least need to add a healthy dose of castor oil.  You can figure 1.3 ounces will raise the oil content one percent (i.e. 13 ounces of oil to make 15% oil fuel into 25% oil fuel).  This is not recommended and at the very best will usually be a guess, but I guess it is better than not adding anything at all. The final thing I would like to say in this session is to make sure you use an after-run oil between sessions and when you store the motors.  This is another must do, because of the nature of the fuel we use.  Then nitromethane or any nitropariffins burn, they leave behind, in your motor, nitric acid; this, along with the water carried in partly by the alcohol, gets together and eats your bearing and other parts for lunch.  Good quality after run oil is easy to get; don’t skip this step.  If you can’t find a good after run at your local hobby shop, there are many available that are made by several companies…then try Prather’s.  They make a good one and so does RJL.  Do not use motor oil, Marvel Mystery oil; this is not after run oil.

Marvel makes excellent oil that can be used and as an after run oil and it is available from most auto parts stores and is called Marvel Air Tool oil.  As a matter of fact, most air tool oils can be used as an after run oil; they are designed to fight corrosion in metal air tools and this is exactly what we are looking for.  Another good place to get these types of oils are the large home supply stores like Home Depots and Builders Square type stores.  Look in the department where they carry air compressor and paint guns.  There are many brands of these oils so you see you have no excuse not to use them.

As for fuels, there are many good companies out there that will supply you with a good usable Stunt fuel.  Red Max is one that comes to mind; they will custom blend almost anything you want and have been over the years very consistent.  Stan Rautkis at S & W will probably do the same if you explain to him what you want as will Doug Taffinder at Carlina-Taffinder.  I am sure some the west cost fuel companies will also blend fuel for your needs, but I have not had any personal dealing with them, so I’m not absolutely certain of this.  If you are using some of these suppliers, call them up.  I’m sure most of them will oblige you.  The model magazine are full of 800 numbers for fuel suppliers and the ones that I have mentioned come highly recommended; however, this is by no means all of them.

SIG for example has a very high quality fuel that is stocked by dealer all over the country; their fuel is 20% blended oil and with extra castor or oil supplements, such as PB-1, makes an excellent Stunt fuel.  Keep in mind things will vary slightly, so don’t be afraid to try something new, or your buddy’s fuel if you suspect you have a fuel problem.  Thanks for reading; keep ‘em tight.  I’m outta here.

Fuel/Additive Suppliers:

Carlina-Taffinders, 803-553-7169
8345 Delhi Rd
Charleston Heights, SC  29418

FHS Supply (Red Max), 800-742-8484
PO Box 9
Clover, SC  29710

Prather Products, 213-835-4764
1660 Ravenna Ave.
Wilmington, CA  90744

RJL Industries, 818-359=0016
PO Box 5
Sierra Madre, CA  91025

S & W Fuels, 215-252-2024
PO Box 208
Tatamy, PA  18085

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Chicken Hopper Stunt Tank
  by Fred Krueger

I have tried numerous tank designs on my modified Twister in an attempt to obtain a consistent run from start to finish.  I have found that standard uniflow and clunk tanks, even with muffler pressure, yield gradually quicker lap times throughout the pattern.  Some flyers prefer a gradual speed up through the pattern, but I find this speed up to be excessive on a profile with the large pickup offset.  The chicken hopper design offers a constant fuel head due to the engine drawing fuel from a small hopper tank, which is continuously filled from the reservoir tank.  The GRW chicken hopper designs have intrigued me, but the pickup offset is greater than the standard uniflow types.  Thus, I decided to construct my own.

I developed this tank from an earlier attempt at a shallow profile wedge tank; I simply added the small 0.3 oz. hopper tank and re plumbed it.  The tank is constructed from K&S tin plate sheet and 1/8" copper tubing.  It is uniflow vented (see drawing) and the vent is located close to the fuselage to keep it out of the higher velocity prop wash.  During the last several years I read material in PAMPA columns on vent restrictions to minimize pressure variations.  Earlier this year, I had conversations with two club members, Al Kelley (speed flyer) and Jim Renkar (PAMPA member) who furthered my interest in vent restrictions.  This venting restriction method was discussed to control tank pressure variations in the wind when using straight suction fuel draw.  I crimped several short pieces of copper tubing to various depths and selected one as a test.  This slips into one end of a short length of fuel tubing, with the other end of the fuel tubing fitting over the vent.  This restricts the vent adequately, resulting in minimal pressure variations in windy conditions, without using muffler pressure.

The results are quite pleasing, the 40 FP 4 cycles beautifully through the entire pattern (a result of George Aldrich's meticulous rework).  The tank holds lap times throughout the flight and gives about 1/2 lap 2 cycle burst before quitting.

The tank should be assembled in the order shown for ease of construction.  Be prepared to do a little "file to fit" as most of us do not have a shear or a brake press.

In the last several years, I have designed and built 1.5 and 2 oz. sport racing variations (with overflow pinch-off).  The most recent variations are 1oz for a .09 diesel stunter and a 1/2A size for a Baby Clown.  All provide uniform runs, start to finish; even the Cox reed valve powered Baby Clown.

OS LA 25 Setup
by Mike Pratt

After receiving a number of phone call and e-mails about how I was setting up the OS .25 FP & LA for the Primary Force I thought it would be a good idea to post it here on SSW. Please don't think I know all there is to no about these engines but I have found a few things that worked really well.
OS .25LA: I used the engine in stock form and was quite surprised how well it ran right out of the box after a short break-in. There were a few things about the engine I didn't like but the biggest thing was its lack of consistency (I am really fussy when it comes to engine runs but I believe that most sport flyers would be more than happy with the way the engine runs out of the box). I had to reset the needle before every flight and it was almost impossible get the same setting flight after flight. During some flights it would drop out of the rich 2-cycle and into a doggie 4-cycle for a few laps and then jump back to rich 2-cycle. In addition, the engine would go lean and almost quit in the second loop of the four-leaf clover (with plenty of fuel still in the tank).

I thought the rear mounted remote needle was the culprit and replaced the remote needle with a ST needle valve assembly (through the venturi). This was a little better but still far from what I wanted. I used different combinations of fuels, glow plugs, head shims, air filters, and muffler pressure with little or no change to the run. Completely frustrated, I switched to a tongue muffler and instantly most of the problems went away. The run was much smoother and more consistent cruising along at a rich 2-cycle but it still had a tendency to lean out in the four-leaf. Using a Bru-Line green air filter helped and almost stopped the leaning out at the end of the flight, but switching to a .250 dia. venturi (from Leonard at SSW) cured the problem completely. One other little change that I made was making a plug for the uni-flow vent that had a .050 hole drilled in the end to keep the engine from running a little fat (rich) at the end of the pattern.

Now the little .25 LA runs like a Singer sewing machine from the beginning to the end of the tank with a short power burst one lap before it runs out of fuel.

Listed below are the suggested changes and/or parts needed:

.250" i.d. Venturi.
S. T. needle valve assembly.
Tongue Muffler. Modified RSM or SSW
3 ounce Uni-flow tank (1" x 2" x 3") Brodak or GRW.
Props: APC 9" x 4"& *10" x 4" (*trimmed to 9-1/2").
Grisch 9" x 4" Black Magnum
Bolly 9-1/2" x 4-1/2" (re-pitched to 4").
Glow Plug: S-300 Sonictronics or Thunderbolt R/C.
Fuel: Sig Champion 10 or 15% ½ synthetic & ½ castor.
RPM: Set at 10,2 to 10,500 on the ground depending on your prop choice. It should be braking back and forth from a 2-cycle to a 4-cycle (more 2 than 4) and as it warms up it should be in a rich 2-cycle (by the time you reach the handle). After launch, the model should speed up during the first lap and as the engine cools a little you should hear it back off a little into a rich 2-cycle.

OS .25 FP: This was really easy to setup because the above modifications worked perfectly on the FP. There was no need to plug the boost port or angle the intake ports on the FP. It ran really sweet and I think it even has a little more power than the LA. The FP liked the Bolly 9-1/2" x 4-1/2" with the pitch set at 3.75".

One other thing that I will point out is I live at an altitude of 5000' so some of the changes and settings maybe a little different at lower elevations. When I travel to contests (at lower elevations) about the only changes I make to the engine is adding a Bru-Line green air filter and if it is really hot I switch to 15% nitro fuel.

I hope this helps,


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ABC/AAC Engines What They Are, How To Run, and Treat Them
by George Aldrich:

1. The basics of how these systems work.

a. The piston material, a high silicon aluminum alloy, has a far lower coefficient of expansion than the cylinder material whether it is brass or aluminum.

b. The cylinder material is either high tensile brass or aluminum that is chrome plated.

c. The combination of the highly abrasion resistant piston material running in the very hard-chromed cylinder assures long life.

2. Why does the piston have such a tight fit (pinch) when the engine is turned over TDC (Top Dead Center)?

a. The high tensile brass or aluminum cylinder expands a great deal more that the piston when the engine reaches operating temperatures.

b. If an engine is set up with what most consider the normal fit and the usual bore that has little taper, then when the engine reached operating temperature, it would have little compression! With a straight bore, when an engine gets hot, it suffers from what is normally called "bell mouth"! The symptoms of bell mouth are great cold starting, but poor hot restarts.

c. When I first started working with the first ABC engine ever sent in the U.S., (1966) the first thing I did was to lap the piston to the fit we had developed with cast iron/steel assemblies. This basically ruined it! Form this early beginning, it was found that a tapered bore fit much tighter, was required to get this system to work at its best.

d. Later technical advancements in being able to hard chrome aluminum, led to using aluminum cylinders, and the high silicon aluminum piston, for what we now call an AAC assembly today.

3. We now have an engine that often squawks, and makes other "funny" noises, when it is turned over. What is the best way to run these engines? Because of the tight fit, it's not hard to see that the moving parts that carry the loads will have extra stresses put on them until the engine reaches it's maximum running temperatures. Because of this, the most critical time is the First Two Minutes that are put on the engine. Therefore:

a. The first run on this type of engine should bring it to its maximum RPM's, just as soon as possible, or Wide Open! The idea is to get the engine to its maximum temperature quickly to relieve the extra loads imposed by the tight pinch!

b. After the first two minutes of running, you can set the engine however you choose, and just Go Fly!

c. You will see such an engine just get better and better with more running.

4. With an engine that is a true ABC/AAC type, I have found that a fuel containing a total of 22% lube mixed 11%/11% castor/ synthetic is ideal. This particularly true, when using a muffler, or piped set up. The castor carries the heavy loads, and the synthetic acts as a detergent, reducing the varnishing that occurs with all castor fuels. I have never subscribed to the special "hemi-head" and "stuffer" back plate offered for the Fox .35. While working for Fox, I ran tests with Duke on an engine that had a piston and cylinder mounted in the back plate. Duke would start the engine and I would vary the crank case volume by pushing the piston in and out while Duke recorded the results. Duke knew Exactly where his engine ran the best! This engine now resides on the top shelf of the Fox display in our AMA Museum in Muncie.


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Engine Care - Randy Smith
Adjustable Tank Mount

A picture is worth a thousand words. Use 3/32" balsa and plywood, coat with thinned epoxy. Piece of foam between tank and mount.
Adjustable Tank Mount
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Chicken Hopper - Frank Krueger
OS LA25 Setup - Mike Pratt
ABC/ACC Engines - G. Aldrich
Fox35 Burp Fix
by Dennis Vander Kuur
Clunk Tank - Nils Norling
Saito Carb - Bob Reeves
Reverse Running Saito - Bob Reeves