Sting Sport LSA: Comments on Throttle Stops and Idle Speed

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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drdehave
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Sting Sport LSA: Comments on Throttle Stops and Idle Speed

Postby drdehave » Wed Sep 10, 2014 7:06 pm

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Folks:

This pertains to the Sting Sport S-LSA. I love this airplane (now just called, “Sting”). As I’ve elaborated here and via my YouTube channel, it has excellent performance and numerous sweet features. As a result, I’m probably the best advertising TL Ultralight (manufacturer) and SportairUSA (US distributor) have going.

Nevertheless, when something sucks on the airplane, I illuminate it, too! One such deficiency on early models, such as my 2007 (I’m not sure about more recent models–maybe dstclair and others can report?), is the lack of positive cockpit-side throttle stops. Currently, Rotax (I forget just where, but I’ve seen it) “recommends” idle-and full-throttle stops (screw-adjustable ones, actually) for the cockpit throttle-slide. At minimum, an idle-stop is needed, because it provides insurance. It ensures you can’t pull the throttle back so far you bend the throttle stops on the carbs (at carb valve levers), snuff out RPM, and kill the engine. Yet my Sting Sport came without any positive stops on the cockpit throttle slide; perhaps TL engineers determined that the carbon fiber “cut-out” for the throttle slide provided its own “natural stops” fore and aft, and that was sufficient (it’s obviously not), I don’t know.

I just know that when I got my baby, with 250 hours (1,525 now), she already had badly bent stops at the carb valve levers (where the idle screws make contact). When my current (and best) mechanic (during the first Annual he performed for me) saw those bent stops and my cockpit throttle setup, his first words were: “You need an idle stop there! I’ll make you one, right now!” And that positive–-but non-adjustable stop–-is the stop you see in the attached photo. (That was over 2 years ago; I’ve been meaning to discuss this issue ever since.)

Now, it would have been really nice if he’d made that stop adjustable, instead of “hard.” (I’ll get to “why,” shortly.) Anyway, with the ‘hard’ idle stop, proper setup entails setting idle speed via the Bowden cable adjusters at the carbs, while the throttle-slide is against the idle-stop, then setting the idle speed screws on the carbs so they are just barely off (or barely touching) the carb valve levers. (Am I in the ballpark on all this, so far?)

Once the proper idle speed of about 1850 RPM, or so, is achieved and set, the other Sting Sport anomaly invariably begins: idle speed slowly creeps back up over time! (Do you guys with other Rotax 912 ULS applications see this, also?) Recently, mine had crept back up to about 2050 RPM and sure enough, I began noticing the results: more bounced landings (from coming in with more thrust) and ugly-sounding(instead of soft-and-quiet) shut-downs.

So I called mechanic #1 (my main mechanic)–-who was too busy to get to me for a week, but did remind me that “idle-speed is adjusted at the cable adjusters.” Next, on to mechanic #2–-who was currently available, but who vehemently insisted that mechanic #1 was ill-informed and that idle speed is adjusted using the screws at the carb-lever stops. So I politely put mechanic #2 on hold (possibly permanently, for his clear lack of knowledge of this issue) and flew my baby over to mechanic #3, who, within 5 minutes had turned both Bowden cable-adjuster nuts ½ turn, leaving the idle at a perfect 1850–and recorded the gratifying event in my logbook! (And of course, he checked the synchronization of the carbs, which were still spot-on perfect, too!)

Now here’s an actual true-and-honest question, finally, to start to end this treatise. If that improvised ‘hard’ idle-stop my mechanic put on the cockpit throttle 2 years ago had been adjustable, I would have just been able to lower the idle speed right there–-with one adjuster, instead of with the two cable adjusters, right (yes, assuming the carb-idle screws were first backed off)? Okay, just checking...

Excuse me now, I’m going to practice my low-thrust landings--and silky-quiet shut-downs. They are so fulfulling!
Last edited by drdehave on Thu Sep 11, 2014 6:24 pm, edited 6 times in total.
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby roger lee » Thu Sep 11, 2014 12:26 am

Fixed throttle stops in the cockpit are okay. They don't need to be adjustable. The adjusting should be out at the carbs. That's part of the carb sync work. The throttle in the cockpit should hit its stop at the same time the carb comes to a stop at the idle stop screw. It's very easy to do at the carbs.
You can also add a cable stop on the throttle cable just where it goes inside the cable housing, between the Bowden cable adjuster and the carb throttle arm if you have no stop in the cockpit. This method is usually used for the Vernier throttles.

All the Flight Design's in the field are setup just like yours. Solid stop in the cabin. If a Rotax mechanic can't set the idle at the carbs to stop at the same time as in the cockpit then he doesn't know what he's doing on the carb setup side.
Roger Lee
Tucson, Az.
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby MrMorden » Thu Sep 11, 2014 10:14 am

The throttle setup for the 912 has cable tension constantly pulling the throttle open, so that if the cable breaks you go to full power instead of no power. I had the same "creeping RPM" problem in my CTSW, and it was very annoying, especially in cruise where I had to constantly keep retarding the throttle.

In the CT there is a throttle lever tension nut that can be adjusted (TINY adjustments are needed to make good changes, it binds up easily). I assume you have something similar in the Sting that your mechanic can adjust to your liking. You might have to have him mess with it a couple of times and test fly it; it's a fine line between "too loose and throttle moving itself" and "too tight and hard to make fine throttle changes when you need to".

Good luck!
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby drdehave » Thu Sep 11, 2014 10:26 am

"it's a fine line between "too loose and throttle moving itself" and "too tight and hard to make fine throttle changes when you need to".

Ah, yes! We found that out--and decided to just leave that tensioner alone. Now-a-days, it takes a good 50-150 hours for the idle speed (with throttle slide pulled against the hard idle-stop) to creep up to where a small (1/4-1/2-turn of nut) cable-adjuster movement is needed to bring idle-speed back down to the target level.--R
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby CharlieTango » Thu Sep 11, 2014 10:39 am

MrMorden wrote:The throttle setup for the 912 has cable tension constantly pulling the throttle open, so that if the cable breaks you go to full power instead of no power. I had the same "creeping RPM" problem in my CTSW, and it was very annoying, especially in cruise where I had to constantly keep retarding the throttle.

In the CT there is a throttle lever tension nut that can be adjusted (TINY adjustments are needed to make good changes, it binds up easily). I assume you have something similar in the Sting that your mechanic can adjust to your liking. You might have to have him mess with it a couple of times and test fly it; it's a fine line between "too loose and throttle moving itself" and "too tight and hard to make fine throttle changes when you need to".

Good luck!


When a CT throttle slips it can go WOT. This happened to me 2 times back in 06 and a pair of belvidere washers was the fix, never slipped again.

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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby drdehave » Thu Sep 11, 2014 11:31 am

For those still a little behind in the learning curve of such nuances, it may be worth noting the Sting (and I assume most other LSAs) has both apples and oranges, relative to idle-speed creep:

1. Apples: You're sitting there, engine running and throttle pulled all the way back to the (cockpit) idle-stop, with idle-speed properly set at the desired 1850, or so, and idle slowly creeps up. This is due to vibration assisted by the carb "sprung-open" condition pulling forward. This annoying trait and any straight-n-level throttle position creep may be controllable using adjustments to the cockpit throttle tensioner. But, as noted above, there is a "fine line" to tread, here. More often, I just keep pulling the idle back down--as it creeps up a bit, whilst I am sitting or taxiing.

2. Oranges: You set up the idle speed to the desired 1850, or so, with throttle pulled all the way back to the (cockpit) idle-stop. But over time, this set, minimum-possible idle-speed gradually increases. Within 50-150 hours, it may increase to over 2000 RPM. This, is the adjustment my mechanic just made--lowering that set minimum idle-speed back down to 1850, using the cable adjusters at the carbs. Question is: Why the heck does this form of 'creep' occur? I haven't a clue! Bigger brains--please elucidate for me!
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby MrMorden » Thu Sep 11, 2014 11:51 am

CharlieTango wrote:
MrMorden wrote:The throttle setup for the 912 has cable tension constantly pulling the throttle open, so that if the cable breaks you go to full power instead of no power. I had the same "creeping RPM" problem in my CTSW, and it was very annoying, especially in cruise where I had to constantly keep retarding the throttle.

In the CT there is a throttle lever tension nut that can be adjusted (TINY adjustments are needed to make good changes, it binds up easily). I assume you have something similar in the Sting that your mechanic can adjust to your liking. You might have to have him mess with it a couple of times and test fly it; it's a fine line between "too loose and throttle moving itself" and "too tight and hard to make fine throttle changes when you need to".

Good luck!


When a CT throttle slips it can go WOT. This happened to me 2 times back in 06 and a pair of belvidere washers was the fix, never slipped again.


I might have to look into that. Mine needs a slight tweak every 20-40 hours or so.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby MrMorden » Thu Sep 11, 2014 12:00 pm

drdehave wrote:
2. Oranges: You set up the idle speed to the desired 1850, or so, with throttle pulled all the way back to the (cockpit) idle-stop. But over time, this set, minimum-possible idle-speed gradually increases. Within 50-150 hours, it may increase to over 2000 RPM. This, is the adjustment my mechanic just made--lowering that set minimum idle-speed back down to 1850, using the cable adjusters at the carbs. Question is: Why the heck does this form of 'creep' occur? I haven't a clue! Bigger brains--please elucidate for me!


The idle is set with a set screw at each carb. I'd guess either the idle screws are backing out and need some Loctite or other locking method, or there is some "slack" in the system somewhere, either cables stretching, the carbs having some play on their mounts, or something similar allowing the entire system some free play to move over time. There are also some washers, pivots, and the throttle arm all right there that could be flexing/loosening/stretching somehow.

Just guessing here. On my CT the idle is set pretty low, like 1650-1700rpm. But at various points during warm up and pre-flight it will be higher, sometimes up to 1900rpm. It is lowest after flying a bit, I usually leave a little throttle in taxiing so that I stay out of the yellow arc below 1800rpm. I like the lower rpm idle because it leaves less power in when I go to idle in the air, even though it means more throttle jockeying on the ground.

Is the engine fully warmed up when you are checking your idle rpm?
Andy Walker
Athens, GA
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2007 Flight Design CTSW E-LSA

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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby sandpiper » Thu Sep 11, 2014 1:36 pm

I once tried an idle setting of 1800. As Andy says, this causes a problem in the air. On final, to minimize float in the flare, I like to see about 1900-2000 rpm at 55K. With the 1800 setting my engine was indicating about 2200. For me, that doesn't work.

So, I keep my warm engine idle at about 1600. On the ground I use the throttle as necessary to keep 1900-2000 rpm.
John Horn
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby MrMorden » Thu Sep 11, 2014 1:45 pm

sandpiper wrote:I once tried an idle setting of 1800. As Andy says, this causes a problem in the air. On final, to minimize float in the flare, I like to see about 1900-2000 rpm at 55K. With the 1800 setting my engine was indicating about 2200. For me, that doesn't work.

So, I keep my warm engine idle at about 1600. On the ground I use the throttle as necessary to keep 1900-2000 rpm.


Exactly...when my idle was at 1800-1900, that airplane just did NOT want to come down.
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby roger lee » Thu Sep 11, 2014 6:01 pm

Hi Andy,

The throttle doesn't control your speed or float.
Just pull the stick back and the speed will be reduced with a higher nose up attitude (which in our CT is very little) and the float decreases. I can land all day in 1000' (300m) with 2700 rpm. I know this because I had a bet with our friends from across the pond in the UK when they said you had to have the idle down around 1550 rpm to land in that distance. The idle rpm will not be down at 1600-1700 on approach or just before landing because of the air passing over it. It will be higher than ground set idle rpm. Now ask yourself and then test it; If my rpm on the ground is set to 1650 rpm one time and the next I have it set at 1800 rpm on the ground will the rpm be corresponding that much higher during approach or just before landing? You have wind driving the prop and you have engine compression trying to reduce it?

Who here has done the comparison?
This means flying two times back to back and stopping in between to adjust the carbs back up or back 150 rpm. Thinking it makes a difference and actually doing it as a real test may offer different results. You must absolutely make sure the approach and just before touch speeds are the same between test. Sometimes it sounds so logical it must be true.

I'll be perfectly honest I have not done this particular test, but I do have a little extra insight to the results thanks to our friends from across the pond bet.(2007)

I can say use your stick and don't worry about 100 rpm one way or another and it won't make any difference.
Roger Lee
Tucson, Az.
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby MrMorden » Thu Sep 11, 2014 6:15 pm

Roger, When my idle was 1900rpm, my idle rpm on a 55kt approach was 2200rpm. It's now 2000rpm. That 200rpm made a big difference in how steep the approach could be without me having to slip or add more flaps. You might think it doesn't make a difference, but I can feel it.

I could land fine with the idle set higher, but I have better options that suit my flying style with a lower idle.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby drdehave » Thu Sep 11, 2014 6:55 pm

"I could land fine with the idle set higher, but I have better options that suit my flying style with a lower idle."

Andy, I am with you, bro! Having that higher approach RPM--from a higher idle-speed--just turns my landings into snot!

Recently, about the time my idle-speed creep-up peaked at around 2050 RPM, I was making so many out-of-character landings, that I got to thinking it might already be time to hang up the airplane keys (well, if the Sting Sport had one!) and just start piddling around the hangar all day like so many of my older hangar-mates around here do! So, I went on a PTS (Practical Test Standards) practice binge for 21 (yep, 21) straight days. And every day, I was also reviewing and studying, like I was prepping for the Knowledge Test, again. Then, on the 22nd day I took a BFR (Biannual Flight Review), although it had only been 14 months since my last one. And on the 23rd day I took another BFR from another CFI, for another viewpoint and "exam.".

So, you see how crazy that little extra RPM at landing had made me?

Bottom line is: I had developed a lot of bad habits in addition to idle creep. Correcting the bad habits, brushing up on forgotten knowledge, and taking care of that little 'creep' has me back flying the way I used to!

I love Roger and respect his knowledge to the Nth degree, but you and I both can agree to disagree with him on this one!

PS (12 hrs later): Actually, all seriousness aside, I think I know what my recent crappy-landing syndrome was really all about. I come in with my hand on that throttle--ready to instantly flick it forward. Trouble is, sometimes in the heat of the landing attack, I think I do this subconsciously. So, if it is already a little high--from throttle creep--then, look out! One theory anyway.
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Re: Sting Sport LSA: Comments on Throttle Stops and Idle Spe

Postby drdehave » Fri Sep 09, 2016 7:08 pm

Wow, what you can find in the archives here! Today, I was out and about (and also doing "refresher" training on-line) accumulating several small parts for the Cockpit, Adjustable, Hard Throttle Stop I am finally going to build and install in my 2007 Sting LSA.

And low and behold, I found this old thread from earlier. I just wanted to add this little factoid: Newer models of my LSA (i.e., Sting S4) come from the factory (TL Ultralight) WITH such an adjustable cockpit hard stop! Heck, it may be because of this little thread! Now, this 'adjuster' could be installed in my plane, with some work, including welding.

But I don't like it, because it has too little idle adjustment "travel," in my opinion (as well as a nearby S4 owner).

So that is why I am making my own adjustable stop for my airplane. And, if you are about to ask, "Will you be getting an LOA from TL for it?" the answer is "Stand by, I'll advise you later about that."

(Post Script, 1-Week Later: Scratch that idea. I made a very nice adjuster out of aluminum, but there was just no way to safely & soundly attach it to the carbon fiber console, which is curved and rounded. So, I am back to adjusting my idle like always--at the cable adjusters on the carbs. It is quite amazing how a 1/6th turn of the nuts bring a 200 rpm change in idle!)
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