Over the years I’ve been asked a lot about how to determine if your engine is running correctly, or if it has been running correctly. The reality of this is, it is a highly subjective topic to share ideas on and to a large degree a lot to do with the eye of the beholder as to what a “good running engine” is for RC heli application/standards; but I’ll share my theories and experiences.

There are a few things I expect out of my engines, and they are listed in prioritized order below:

1. Running SMOOTH – or as smooth as a glow 2 stroke single cylinder engine can run… Vibration is an RC Helicopters inevitable but worst enemy. Vibration kills gears, electronics, rattles bolts loose, and just plain wears your helicopter out. Constantly changing non-expendable parts between crashes due to wear is expensive! In the last 10 years, vibration also effects how the helicopter feels in flight and performs since flybarless helicopters are using electronic sensors and PID rate correction control loops to stabilize both roll and pitch rotor head axis and yaw (tail rotor). However, there are many steps you can take to reduce vibration:

• Engine maintainence: Keeping the bearings fresh – specifically the rear bearing which takes the most abuse from use. Most often, rust is the culprit to a rough bearing.
• Cooling fan and clutch assembly: Checking that the fan and clutch are running true on the crank shaft is important. A run out of about 0.001″-0.0015″ is pretty good. Less than that, the stars have aligned, above that you might feel and see fuel foaming in the fuel tank, the tail fin buzzing, and feel a rattle in the frames and rotor head while it’s idling. ***One tip is to also leave a tiny click of play between between the fan/hub assembly and the front bearing washer. As the engine parts expand with heat, the steel and aluminum parts may not expand at the same rate, so a slight bit of wiggle room keeps things from binding up causing excess drag and rough bearings.
• Balance: Static assembly balance can be problematic if it is signicant, however, aligniment as mentioned above is more critical.
• Mixture (tuning): A drastically rich or lean motor will shake the helicopter… a lot. Each a little differently. A rich condition will have a pulse or bump where it jars the equipment, where a lean condition is a chatter type vibration.
• Fuel: Having good fuel is a key part of a smooth running engine. High oil content of a low viscosity will run the smoothest and cool best at high RPM. Most fuels on the market offer a low viscosity heli fuel with 20% or higher oil content. VP Fuels offers a 23.5% nitro, 23% Low Vis fuel that runs GREAT in the O.S. DRS (61G) regulated carburator and 1.05 engine. Byron’s 22.5% and Cool Power’s 22.5% also run well. Byron’s will keep a very clean motor like VP. Cool Power’s dye does tend to stain the piston and insides of the engine which does effect a few of my “disecting methods” explained later in this post, however, the staining has no effect on performance.
• Compression: If the engine is in stock form, and you are running the suggested fuel (in this case for the O.S. 105 15%-23.5% nitro) the engine should be set-up just fine with a single 0.008″ head shim. If you are using 30% nitro you will want to add a 0.004″ shim, or if you don’t have access to low viscosity oil fuel, it may want less compression as well. If the engine is over compressed it will sounds extremely harsh and pingy, and if you peek in the glow plug hole with a flash light, you may notice pitting on the top of the piston or inside the bottom of the head. If you don’t have enough compression you’ll notice the engine tends to make little power and will go consistently lean under load.

2. Consistency: The engine running consistently greatly helps tuning the flight parameters of the helicopter, as well as, how the governor performs. Consistency comes from maintainence and also break in (which is explained later in this post). By nature, the O.S. 1.05 DRS will run extremely consistent due to the regulated carburator system. It is a demand regulator and constantly has fuel delivery by a pressurized fuel tank. If a DRS system is not running consistently (even consistently wrong) these might be the reasons why:

• Bearings: If the engine was running OK then suddenly changing from flight to flight, likely the rear bearing is bad. Excess heat and drag from a rough bearing effects the consistency from run to run and from the beginning to the end of the flight.
• Fuel tank not holding pressure: after each flight the tank should hold pressure indefinately, so it is important to the release the pressure to not wear out the seals on the tank. After each flight the pressure release should make a strong “pppppssshhhhh” sound. *** One tip is NOT to use fuel clips on the pressure release fuel line, RARELY do they hold well enough to not leak. Best to use a T fitting and a fuel line plug that holds strong.

• Fuel pickup: Needs to be fresh and clean. If using a foam Fuel Magnet (or similar/current brands) be sure it is not falling apart or pinched around the fuel holes in the holder. The fuel pickup should also nearly reach all 8 corners of the tank without getting stuck or without touching the corners.
• Floppy Fuel line: This can be a BIG problem, as flying fast backwards, forwards, and every which way, floppy fuel line can fold over in flight and cause lean issues, or cause the engine to even die. Try to shorten the fuel tubing as much as possible and mount rigidly if you need to.
• Fuel Line: You want to run the biggest I.D. fuel line you can find that will fit securely on the carburator and fuel tank nipples. You don’t want to bottle neck the fuel flow to the engine. Fresh fuel line also helps keep from cracks/leaks/expanded areas around the connection points from happening. I have found I really like Losi’s RC car fuel line. I use it everywhere, even the clunk, as it’s very smooth flexing.
• Check Valve: The pressurized tank requires a good one way valve to hold good tank pressure and pass crank case pressure effectively. If the check value is not working correctly (either way) the engine will run cronically lean. You can test by blowing into each side of the valve through fuel tubing. When you try to pass air through one side, it shouldn’t ever go through, if you flip it around, with little pressure the air should pass through once you reach a certain force.
• Glow Plugs: The glow plug is a vital part of how the engine runs, if the engine starts to sound “harsh” but isn’t showing signs of being lean or rich, a glow plug with build up on the element is likely the cause.
• Exhaust Leaks: Air leaks are a major issue with consistency or having lean lean spots across the throttle range. After a flight, the engine should actually be the most dry and clean area on the helicopter. I personally much perfer fiber gaskets over RTV to seal the exhaust to the engine.
• Fuel Filter: Filter needs to stay clean. If there is any debri inside, it will drastically change how the engine runs.
• Clutch Slipping: If the clutch is slipping, this will cause the engine to run lean and also fight the governor. You will hear a pulsing noise or studder in the engine under load if the clutch is just barely slipping. Of course if the clutch is completely letting go, you will hear the engine overspeed the blades, then sound like it catches up when you unload the rotor disk.
• Dirty Regulator: Regularly flush out the DRS regulator by removing it from the carb and taking the back off. To clean throughly use 99% alcohol and squirt the alcohol through the regulator holes holding down the butterfly. After cleaning, idle the engine for a bit to get fuel flowing through the carb again. If the regulator is dirty, you’ll most likely notice the engine running rough shaking the fuel in the fuel tank, and also lean spots that can’t be solved with mixture settings.

3. Power: Power is actually the lowest priority on my list because if #1 and #2 are working correctly, #3 a lot of times comes naturally.

• Mixture: If the engine is too rich, it will load up with excess fuel it can’t burn and not make power. If the engine is lean, it will starve for fuel and labor when under load because it is lacking fuel to burn and be down on power. The best mixture setting is when the engine sounds as if the sound is one consistent drone, and when you load and unload, the change in sound is purely in it’s tone; the engine doesn’t start to gurgle and sputter (rich condition), or become raspy and pingy (lean condition). Checking the high end is pretty easy, just keep it loaded without bogging the engine and listen, it should sound like a consistent “ppuuurrr”. If it is sputtering or gurgling, it’s rich, if it gets a “whine” or pingy raspy sound, its lean. Mid range is a bit more tricky, but can be checked rather consistently by loading the engine, then backing off the collective some and seeing how it recovers when you load it again. If it gurgles and then clears out, the mid range is rich, if it labors and pings and then clears out it is lean. The low end can be mostly tuned mostly at idle, but does transfer into flight a little. When idling if the engine starts, slowly seems like its slowly ruducung RPM smoothly, then gurgles and quits it is too rich. If the engine starts to increase RPM and ping, it is too lean. The best way to test this mixture setting in flight is do a pretty agressive upright descent and listen for the governor to pull the throttle back. As you hear it throttle back listen for pingy or raspy noises, and richen, or if it loads up and gurgles as you apply power, lean it out. Useful ranges on the low end adjustment on the O.S. 1.05 DRS carb are the flat head screw driver slot pointing between 1 and 2 oclock when mounted on the engine with the helicopter sitting flat – make SMALL adjustments at a time.
• Good air: Point blank, a glow engine will not make the same power everywhere, everyday. So don’t compromise the tune to run on the ragged edge to try to get more power if it is a day where the engine simply isn’t going to make the best power. (high altitude locations/ humid days) Rather, try to build your flying style and manuevers so that you do them in a shape, speed, and size that you aren’t requiring full power to complete. That gives you some head room as conditions change.

Breaking an Engine In

Whether it be a brand new engine or freshly rebuilt with new ring and/or other parts, the engine needs to be run in and seated so that the ring seals shape to the sleeve as much as possible. While there is no guarenteed way to make sure the ring seats, the best method I have learned from various people and my own tests are to simply run the engine at normal RPM and normal mixture from the start, however during this time period it is extremely important not to bog down or load up the engine in flight where it isn’t able to hold RPM, nor overspeed the engine past it’s targeted RPM unloading it. The second part likely being the most important. When an engine overspeeds or runs at a high rpm unloaded the piston is no longer driving the connecting rod and crank shaft under constant load, but rather all the peices are times are floating independatly which will allows the piston to slap and chatter in the sleeve.

If a engine has been oversped: It’s very clear looking at a piston that has been slapping as the skirt and usually right under the ring are polished to nearly a mirror finish.

If and engine has been loaded up bad or ran too rich: If a engine never reaches operating tempurature, the ring and parts never seat. In turn, the ring no longer seals and the excess pressure from combustion blows by the ring, reducing power.

How to tell when an engine is broken in: One thing I’ve always seen is when an engine is in the break in process, it continually gets richer. Once the engine seems to stabilize flight to flight, and from beginning of the tank to the end of the tank, I feel at that point things have stopped wearing in and the engine is what it is. Usually this takes about a gallon or slightly more of fuel (around 7-10 flights on a .90-1.05 size designed helicopter).

***A key tip is once the engine reaches that stable point, change the glow plug. When you take the glow plug out, you’ll likely notice little black specks stuck to the element coil. That is debri that stuck to the element from the brand new engine parts. Throw that one out or use it as a glow ingnitor battery tester.

Disecting if an engine was broken in well/has been running well

Unless you take the head of the engine off, and remove the piston you really can’t see all the areas of the ring and piston that would show the signs, but a very good gauge is by taking off the exhaust and looking in the exhaust port, as well as, removing the glow plug and shining a light on the top of the piston from the glow plug hole.

• On the exhaust side above the ring towards the engine head will turn a golden brown. This is normal and doesn’t hurt anything, however, if a ring is sealed well UNDER the ring towards the crackshaft should look brand new and no golden brown, and the silver should be a consistent dull finish, not polished. 1) If there is golden brown consistently past the ring, the ring isn’t sealing well under pressure. 2) If there are scratches the engine is running lean and lacking lubricant. 3) If the area under the ring and near the bottom of the piston is polished, the engine is running too high of RPM or the helicopter has the wrong gear ratio causing the piston to slap.
• On the glow plug hole, you should see a perfectly smooth top, with after some gallons likely a golden brown finish, likely, significantly deeper on the side of the exhaust port. 1) If there is pitting on the top of the piston, the compression is too high and the engine is predetonating, there is no real fix for this, replacing the head and piston are needed to ever get full potential – going forward add a 0.008″ head shim when rebuilding, or use a lower nitro content fuel. 2) Looking at the glow plug, a similar brown tint to the glow plug casing is good. The element or little coil that is heated inside should be mostly shiney silver like it started; if the coil is pretty brown, the engine is running rich; if the element is white or a blunt silver, the engine and plug have been running lean.

Signs an engine is damaged or worn out: All engines, even the ones perfectly maintained and never abused, wear out.

• Ring is no longer solid black: When the ring is no longer a sold black line, the ring has been pretty worn and it is likely time for a new ring. If there are streaks of lighter black then the rest of the ring is solid black, it’s still likely ok, but is soon going to need a ring. That doesn’t mean you need to replace the sleeve though. Check the sleeve for cross hatch, and if it is still visable with no scratches, it’s fine and a new ring will seat to it.
• Minor Scratches: If there are small scratches on the piston, ring, or liner, yes, the engine likely isn’t going to be 100%, but no engine ever is… it’s a matter of getting as close to 100% as possible. So if you see a few small scratches, don’t throw your engine guts out!! They likely still have lots of life left in them and honestly the little bit of performance for our application we’ll never feel or be able to measure.
• Large/Deep Scratches, Dents, Pitting, or Chunks: If there are deep scratches that you can put your finger nail in, pitting on the top of the piston or inside the engine head, dents or chunks missing, I would suggest rebuilding the engine, cleaning out the exhaust well if there was chunks, and replacing the bearings. No use in possibly doing enough damage to where the engine seizes while running ruining much more than the guts.

General Maintainence: Over all at the end of the day there is not a lot I tend to check for other than I make sure the engine is still clean to make sure on the next flight there won’t be any dirt/debri that could be passed through the carb as that will instantly ruin an engine. It helps to take note of your take off/landing pad when you fly nitro. And also make sure there is no pressure in the fuel tank to try to preserve the tank seals.

If the engine is going to be sitting for a long time (more than a few days between weekend flying), it helps to put a bit of automatic transmission fluid in the rear bearing to prevent rust. For more information on this techique, you can watch this video. DO NOT put a lot of ATF in the engine, you don’t want it running into the carburator/regulator. Also, this will NOT work on YS Engines.

If there is any questions, comments, or techniques you’d like to share you’ve found work good, please leave a comment below and I’d love to try your idea as well as edit this post as new updates and techniques come about!