I have 2 ST GS-40s and the way I tune them (4-500 rpm richer at max.) cause them to exhibit a similar trait, i.e., idle creeping up about 200 rpm after cutting the engine to idle from mid to high rpm on the ground.

I set the idle to 2800 - 3000 rpm. The idle is set rather rich (3-400 rpm increase in pinch test before quitting). I've never had a dead stick and I don't want any in the future.

If I set the idle to 2500 - 2600 rpm range, the dip in rpm at a sudden transition from full throttle to idle sometimes kills the engine on the ground. I have a feeling that this dip may not happen during flights, but am not brave enough to try it.

The transition is very smooth although slightly retarded than a leaner setting. I'm using 5% Powermaster and 10% Omega, both show this tendency. I used to use 15% Omega and I didn't notice this but I didn't pay as much attention, either.

I'd like to set the idle lower, but i'm afraid of dead sticks. The current setting is OK for a windy day (most of the time) but makes the landing more difficult on a calm day.

Leaner idle settings seem to make this tendency worse. Do you have any suggestions?

Sounds to me that the idle is too rich. The idle needle controls more than just the idle, it also effects the acceleration and mid range of the engine. So over tuning the idle to a rich setting may not be to your best advantage. Proper tuning of the idle will result in a smooth acceleration with no hesitation.

Cheers,

Chip

If you are going to use the low nitro fuels, and pretty rich setings,
you might try a hotter plug. That will give a less tendancy for the
fire to go out.

Lean the idle out just a tad as well.

FBD.

Why are you afraid to lean out the idle mixture? You will have more dead sticks from a too rich idle than with it set right. You don't have as much worry about overheating with a lean idle mixture as you would with the main mixture.

Ditto that - life is much happier at low throttle settings with the leaner idle settings. The engine does not cook from running close to full lean at idle like it does at full throttle - there is simply not that much heat being produced. The engine should not speed up much at all if you pinch test at idle - perhaps just a few rpm then it should quit in, oh, 2-3 seconds [ever notice how things work when you do them but describing the procedure in writing is a challenge?]. Do you pinch the fuel line at idle to check this? Some folks only pinch test at WOT but I find it a vey helpful tool at idle as well, once you get an ear for it.

I too have had more engines quit at low throttle settings or acceleration from too rich an idle. Once the idle mixture is correct adjust the idle rpm with throttle setting, not mixture. The benefit will be that if you are on an approach and things go awry, you have a better chance of accelerating quickly wthout quittng rather than having the engine stumble from beng too rich.

MJD

Yes, I do this. It takes a few seconds before an rpm change if any, unlike the top speed pinch test which is instant. I've read somewhere the idle pinch test increase should be 50 to 100 rpm.

The factory rep says below 2500 rpm idle is detrimental for the smooth transition and mid range.

I noticed if I set the idle mixture too rich, the engine tends to quit when you try to rev up suddenly from idling.
Also if I set the idle too rich, the transition becomes rough. The sweet spot seems to widen with increased idle speed.

I'm very curious about the present condition, where the rpm creeping up after a sudden idling. I have a feeling this is something to do with the combination of the high speed mixture and idle mixture. I notice it when I rev up for half to one second , then cut down to idle. The rpm gradually increases taking about 20 seconds to reach a steady state. I haven't really tried to see what happens if I continue to operate at the full throttle for longer than 10 seconds, then suddenly cutting it down to idle. I'm sure I have done this before. Probably it's not worse than short bursts.

I'm setting the high speed mixture definitely richer than the recommended (4 - 600 rpm rather than 300 rpm) to make sure it's not going to be too lean when I go vertical. The problem might be something to do with this rich high speed mixture. The engine seems to operate more reliably when the idle mixture pinch test is higher (2 - 300 rpm).

In my club, no one else (at least the ones I meet when I go there to fly) uses SuperTigre. Most of them even don't touch their idle mixtures. Whereas I have to do it almost everytime the conditions change. To me, this is a challenge and fun.

If it takes a few seconds then its very rich. When you pinch the line fuel is cut off immediately, however if very rich it takes a while for the extra fuel clinging to the crankcase walls and bottom to disperse. If its off just a little the engine picks up in a fraction of a second.

Sport_Pilot,

hmmm....I'm talking about idle mixture setting. How do you explain if the engine just slows down and quit without increasing the rpm? Isn't it too lean? It still takes a few seconds. I'm not talking about the high speed mixture, in which case the result of pinching is instant.

Also in this "lean" condition, sudden opening of throttle results in engine quitting. And if this is "very rich", why would engine run better if it's further enriched?

I appreciate and value your comments/advice very much. So please don't take this as combative. I say this because when I read back what I wrote, it does look a little combative...but I don't mean that.

I usually pinch approximately 3/8" to 1/2" before the inlet. I wonder if we are talking about the same thing.

I'm quite sure I'm within 1/4 turn from the optimal, more like 1/16 turn, or 1/8 turn, unless there are more than one peak in idle mixture setting, in which case, who knows, I could be way off!

Flyboy Dave,

I changed the glow plug to Tower Hobby from SuperTigre (both of them make only one kind of glow plug) the last time I was at the field. It did make it better for both high speed and idle. btw, the top rpm was 13500 with MA 11x5 which is over 1.28 BHP, according to Thrusthp.

Tower Hobby's plug is equivalent to OS A8, A3 or in between, depending on who you talk to. I should get A3 and A8. so I can make a comparison.

I have found that the ST idle needle setting has an effect on the high speed. When I get the idle set properly, I get a very reliable idle with good transition and more top end rpm. You will actually have to fine tune the high speed setting after the adjustment of the low speed needle. Tach your engine at WOT before and after-- if it is as rich as you describe, you should see a BIG increase in the top end when you get it set properly. Do the pinch test on the low speed needle setting as well as the high speed as mentioned previously in this thread.

The ST carb is very critical of tank position. I would also check to see that the glo plug you are using has the correct heat range for the fuel/ nitro content being used. When those are correct, you should have a very good handling engine throughout the rpm range.

turbo

Sport_Pilot,

If the main needle is set properly, that is slightly rich, then it will be slightly rich when you slam the thottle down to idle. Thus if the idle mixture is too lean it will speed up before it quits, if it slows down without speeding up before it quits it is too rich. An overly rich idle mixture can also cause the engine to quit when throttled up just not as cleanly, however there is a point slightly leaner from that where it will not quit but the engine will have poor transition but not quit. It does not matter where you pinch the line, before the needle, after, close to the carb, or close to the tank, they will shut down the fuel flow just as fast at any point. There is a small amount of fuel that will drain from the line if you close it far from the carb, however this is small enough that the engine should respond withen a fraction of a second. Unless the idle needle is open way too much, which would be too rich in any event. I think you need to close that needle and observe how it responds to a lean setting.

Another advantage to a properly set low end is that starting is much easier and more reliable. I find that once I get my engines correctly tuned, I can normally start them with just a quick backward flip of the spinner nut (don't even have to touch the prop).

Most engines are very sensitive to small changes in the low speed mixture. Make adjustments of 1/8 turn or less until you get a nice smooth transition on the punch test, a very slight rise on the pinch test. You'll also find that your reliable idle speed will drop as you get closer. Target a reliable idle speed about 2750 rpm.

A few minutes spent on the ground getting the tuning correct will save you hours in the future, and deadsticks will become a rarity.

Brad

I get the same type of idle changes with
my ST 45. Ill try some of the tips posted.
But I know just what you talking about
TestPilot6!

decay,
I'm glad at least someone else is going through the same problem