Engine Tuning How-to

Engine Tuning How-to
last updated 4-24-2005

Credit goes to my friend Bill Meador. I could adjust an engine fairly well, but my adjusting was more experimental then scientific. I had wondered if there was a step by step procedure to correctly adjust them, but could not figure it out. Bill taught me how and now my engines pur like a kitten <grin> Thanks Bill :) Bill knows more about r/c helicopters then the manufactures do and that ain't no joke!

The Beginner Engine Tuning page can be found <here>
This page describes tuning at a more advanced level.

So what makes tuning so difficult??? It is due to the combinations of pitch curves, throttle curves, and mixture curves.

There are two major things to tuning. One is to get a constant head speed at every point on the throttle stick. And the other is to have the ability to recognize when there is a problem instead of just continuing to turn the needles.

Here are the steps I go through in setting up the engine for running 3D
1. Confirm full travel on throttle
2. Setup cyclic to throttle mixing <click here>
3. Confirm full travel of pitch range
4. Set blades to zero degrees at center of pitch range
5. Set flight mode 1 throttle curve to 100,60,45,60,100
6. Set flight mode 1 pitch curve to 0,inh,50,inh,100
7. Test fly and check tracking
8. Check upright climbout versus inverted climbout and adjust links to get the climb rates the same
9. Adjust 3/4 throttle point to get the hover head speed I want to run then set the 1/4 point to the same value.
10. Check the head speed during a zero degree descent and adjust the middle point to maintain the same rpm as during a hover.

Observations:
air bubbles come from carburetor into fuel line after engine is stopped - engine too hot

Common examples and questions listed <here>

The first step is to set up the correct Throttle Linkage Geometry. I know you are thinking that this has nothing to do with whether the engine is too lean or too rich, but IT DOES!!!. If this is not done then you will be chasing the needles and never find a steady point. The proper geometry will allow the throttle servo to lead ahead of the collective pitch. The best example is if you are driving up a hill and you wait until the car starts slowing down before you increase the throttle then you will have a more difficult time getting over the hill then if you had added throttle just before the car started slowing down. By setting the geometry right then your throttle servo will reach its position before the collective servo and thereby provide the necessary engine power just before the new pitch angle increases the load on the system. If you are running a faster throttle servo then your collective servo, this will effectively do the same, but I still encourage setting the linkage up right. Click on Throttle Linkage Geometry and read how to set it up. PS: I can't tell you how many times I have had people tell me their fancy deluxe computer radio will do the same thing by adjusting the throttle curve. It will NOT. No radio can make your servo faster then what the servo was designed. The throttle curve in the radio only tells the servo what position to go to, not how fast to get there.
Next you make the pitch curve straight. You should only have 2 or 3 points at most. Two points, the bottom and the top, will be set for all flight modes for the pitch you want to try to run. The only time you use a 3rd point is in the normal mode to set the mid pitch. Once set, for the most part you won't adjust it again. The goal is to have a linear pitch from top to bottom so you can use this as a reference point for the engine. The loading made by the pitch changes needs to be as linear as possible to provide a linear loading on the engine. That's why you only set the top and bottom pitch in flight mode. If you had 'curves' instead of straight lines, then you would not be able to get a consistant mixture setting and the heli would not fly as smooth with a non linear pitch curve. All adjustments will be done to the throttle curve. This is where you really need a radio with at least a 5 point throttle curve. The reason you don't use the pitch curve to compensate for the engine is because that would change the flight characteristics. You want to keep the way it fly's the same. Let's say you added more pitch on the top end to compensate for the engine rpm increasing in a full power climb out. Now say you're doing some tricks. While the throttle is around 75%, you will have a different rpm then when the stick is all the way up. This is not easy for me to explain, the engine has a saturation point. The pitch and engine together have an optimum power curve that has a linear increase in power up to near it's maximum power at which point the power curve flattens out. The optimum power is found by trail and error. With your current top (or bottom if inverted) pitch, adjust the engine for a constant rpm in full power climb out. Try a climb out at 80% throttle and confirm the rpm remains the same as when you did a full power climb out. Increase the top pitch a little, then readjust the engine for the full power climb out. Check the 80% climb out rpm again. If it's the same, then increase the pitch a little more. Eventually you will reach a point where the rpm at the 80% climb out is faster then the 100% climb out. This means you have reached the flat spot of the engine to pitch power curve so you need to back off on the pitch.
High speed needle Clockwise leans it, counter- clockwise richens it.	Next you set the high speed needle. You have to set it first because the low speed needle is dependent on the high speed needle. The low speed is only a ratio of the high speed. When you richen the high speed, that also richens the low speed. Bill calls the high speed needle the gate keeper because all fuel flow into the engine is determined by how much you have the gate keeper open. Start the helicopter and do a little mild flying/hovering to let the engine warm up. All tuning will be done in normal mode (not in flight mode) with the rpm around 1650. Come to a hover, then punch full throttle and watch your RPM. If the rpm increases, then the high speed needle is too lean. If the rpm stays the same then your high speed needle is set right. If the rpm decreases, then either the high speed needle is too rich or the blade pitch is too much for that engine. To determine if it's too much pitch, do a climb out with the throttle about 80%. If the rpms remain constant, then decrease the top end pitch. With an engine in good condition, you should be able to run +9 on the top end.
The low speed needle is inside the throttle arm barrel. It uses a flat blade screwdriver to adjust it. Clockwise leans it, counter- clockwise richens it.	Now that you have the high speed set, you can check the low speed. The low speed effects your hovering. Start by punching full throttle as you did before to check the high speed needle, but this time when you go back to mid stick and hover you should not hear much rpm change. It's okay if you get an increase in rpm for a brief moment during the transistion from full throttle to hover, but it should settle back within a few seconds to the same rpm as when punching full throttle. If it's higher, then the low speed needle is too lean. If it's slower then it's too rich. Note: In some cases the correct adjustment requires the low speed to be set so rich that you have to keep the throttle a little high to keep the engine running right after you crank it up. That's okay, your main concern is to have the engine adjusted right. After the engine gets warm, it will idle just fine.
Final step	The prior adjustments were just to set the carb. The final step is to make adjustments to your throttle curves in normal and flight mode to maintain a constant rpm throughout the whole flight envelope.

Symptom	Cause
Note: If you had everything set just right and then it changes, one of the first suspects is the glow plug. A bad plug will cause the engine to become rich on the high and low speed. Also it won't idle very well. Also I have fixed several Raptors that had a problem with the fuel mixture changing during flight by simply changing out the fuel tubing. What happens is the stock tubing deteriorates quickly. An example of what happens is the clunk line (tubing inside the tank) can collapse from the exhaust pressure. Another thing is the tubing crystalizes and gets a crack that lets air into the line. If you have engine problems, changing the fuel tubing is an easy thing to try.
You have tons of smoke, but the engine still gets hot and sags (decreases rpm).	A. This could be an air leak somewhere __1. Tighten the muffler bolts. __2. Check head and backplate bolts __3. Replace all fuel line (clunk line inside the tank, pressure line from the muffler, and the one to the carb). __4. Leaky o-rings B. or a bad engine __1. Turn the crank shaft, if you feel resistance or a notchy feeling, then the front bearing and/or rear bearing is bad. __2. The piston/sleeve is too tight either from the factory or because it's distorted from too much heat.
If you have richened the high speed beyond a normal amount (around 2 turns open on the TT36 engine, past 3 turns, then something is wrong), and the engine is still increasing rpm on climb out	1. Air leak on the rubber o-ring of the main needle. 2. O-ring between carb and engine is leaking air 3. Trash in the carb 4. Replace all fuel line (clunk line inside the tank, pressure line from the muffler, and the one to the carb). 5. Muffler is loose. If you have the stock muffler check the long bolt that goes through the center of it. Also check the bolts that hold the muffler to the engine.
Your hovering speed keeps increasing no matter how rich you set the low speed needle	1. The little o-ring on the low speed needle is leaking air. 2. O-ring between carb and engine is leaking air 3. Trash in the carb 4. Replace all fuel line (clunk line inside the tank, pressure line from the muffler, and the one to the carb). 5. Muffler is loose. If you have the stock muffler check the long bolt that goes through the center of it. Also check the bolts that hold the muffler to the engine.
Engine idles fine, but as you accelerate to almost hover speed, the engine dies immediately like someone hit a kill switch.	1. Glow plug. I had this happen and helped a friend with the same problem, both times a new plug fixed it. 2. You have bad fuel, high humidity areas can cause moisture to collect in the fuel jug if it's not kept sealed all the time.
Won't idle good, too rich on low end.	1. Bad glow plug. It may not be heating up hot enough.
Inconsistent settings. You get everything set right, but either in flight or on the next flight it changes.	1. Glow plugs 2. Trash in carb. Flush out the carb. 3. Replace all fuel line (clunk line inside the tank, pressure line from the muffler, and the one to the carb).
Engine dies as if it's out of fuel, but you still have plenty.	1. The clunk line is bad. The stock fuel tubing is not any good, replace it asap
Engine idles good and is good in a hover, but at about 1/3 throttle the engine loads up and almost dies.	If this is doing it more then usually then your glow plug is bad. However do keep in mind that this rich spot is normal for some carbs.

Note it helps to know what a lean running engine sounds like. Maybe some day I will make a wave file of one that is pre-igniting. This is kinda like a pinging type of sound. You don't want to run an engine lean. If you ever run the engine very long with it so hot it's pre-igniting then this can damage the seals.

Another thing that causes leaning is fuel foaming. You need to make sure the helicopter is well balanced. Vibrations are also bad for the electronics.
A second area for leaning is during 3D flight, the clunk inside the tank can sometimes come out of the fuel. You can use a header tank to stop that. A header tank acts as an air bubble trap. Make sure you get the header tank installed below the center of the carb.

One more thing, the carb has a rubber o-ring between it and the engine crank case. If the carb is not pressed down good then it can leak there. Loosen the nut and press down on the carb then re-tighten.

How to check a glow plug

How to flush out the carb

The carb can get clogged not only from trash in the fuel but also from the fuel line itself breaking down.

When suspecting trash in the carb, first unscrew the main needle out then use your fuel pump to squirt fuel through the hole the main needle was in. Next wipe the tip of the main needle clean and re-install. If that does not do it then you have to take the carb apart.
Take the engine out of the helicopter. Loosen the nut on the engine that is just under the carb.This nut moves a shaft with a curve cut into it. When installing the carb make sure the shaft is rotated so
the curve matches the inside of the carb hole, otherwise the carb will not go in. Unscrew the main needle. Remove the tension arm (the clicker) Unscrew the nut that the nut just behind the tension arm.
On the other side of the carb, unscrew the flat head screw it has a small washer under it, don't lose it. Grab the throttle arm and pull unit K out. Also remove unit D. Also remove the low speed needle from unit K. Flush everything with fuel and wipe clean then put it all back together. Be careful not to let any o-rings get kinked when re-installing.

Common examples and questions

Typical question: I am getting rather confused about the needle setting, the manual says to set the main needle and low-end at one setting, the TT newsletter tells another setting, and when I asked others what theirs is set to, they are all different. So what should I set them to???

Yes, I know how confusing it can be with everyone running different settings. That's why you really can't go by what everyone else has. Each engine/muffler combination reacts differently and even if someone else has the same combo, they are probably in a different environment (altitude above sea level, humidity, and temperature). You can start with the high speed needle at 2 turns (Turn the needle fully clockwise until it stops, that is the full closed position. Then turn the needle counter-clockwise two turns) and the low speed at 1.5 turns (The key to this adjustment is that the throttle arm has to be full clockwise and kept there while you turn the low speed needle clockwise until it stops, then turn counter-clockwise one and a half turns.), BUT keep in mind that that is ONLY a point to start from. To set it the way your engine/muffler/enviroment requires, you have to continue reading. Note: If you have set your low end to the 1.5 turns and it will not crank, then go to the troubleshooting chart step 5

Can I tune the engine by observing the smoke emission?
You can't use the amount of smoke as an accurate indicator, but for the novice pilot this is something to observe. You need to get used to how fast the head spins, what the engine sounds like, and how much smoke you see. As you get used to how things should be, then you will notice when something is not right. For example, lets say you go out to fly, you lift up into a hover but notice you have to hold the throttle stick nearly all the way to the top. So look at the smoke, if you have a lot, then you know it's too rich, if you don't see as much smoke as you normally do, then it's too lean. Another example is say you've been hovering a couple minutes and everything seemed ok, but the helicopter starts sagging. You have to give it more and more throttle to keep your altitude. If you see there is less smoke then you know the engine is over-heated.