oldtyme

Hi all,

I was just in the Scale Aircraft forum and someone there mentioned that the newer SuperTigre G75 engines were having problems with mid-range fuel problems in that they ran rich in the mid-range band which could make the engine un-reliable in the event you have to throttle up to do a go-around or other maneuvers after you have throttled back. I have purchased 4 new G75's for various projects because the kit manufacturer suggested that these would be a good fit for the
planes using in-cowl headers and mufflers. Of course in warbirds such as the TF P47 and AT6 and the H9 P47, I wanted realism so I went with a substantial investment here. I'm hoping that this is
not a chronic problem and can be dealt with.

I am not new to the hobby, I have been flying since 1980 but have never owned anything except K&B and OS............and I have a lot of engines.

All input is welcomed and appreciated. I'm commited to these engines now so just wanted to know if there is any basis for this statement and if so how have you dealt with it........I'm not going to sell them so I need to know...........can't afford to buy anything else either.......getting ready to retire and I thought I was set.

Thanks in advance,
Andy

NM2K

My Feedback: (14)

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Was the person that posted about the rich midrange problem running a pump? That could cause the engines to run rich in the midrange if the pump pressure was turned up too high and the high and low speed mixture needles were adjusted to compensate for the condition.

Proper glow plug selection can also influence how the engine runs. The high and low speed needles do influence the mixture strength of the midrange in most two needle carbs. If the glow plug is too hot, the operator will adjust the high speed mixture too rich in order to retard the timing and cool down the engine. This rich high speed mixture will influence the midrange and will show up as a rich condition.

Too much nitro in the fuel will also cause the operator to try to retard the timing by running a rich high speed mixture, again, richening the midrange.

There are a lot of factors, such as prop size, engine cooling, fuel tank height, etc. that will influence the midrange mixture. Don't be discouraged by just one or two folks having some difficulty.

Lately, the trend has been for everyone to run too much nitro. This is probably a spill over from 3D flyers running high nitro in their four-strokes and not knowing any better than trying to run the same fuel in high compression two-strokes.

Those modelers that cut their teeth running OS engines have been spoiled. The ST carbs are a little more ticklish to adjust. Small changes in needle settings can make a big difference. Change any other factor, such as prop size or type of glow plug, while trying to find the right needle settings and you have to start all over again.

Another thing is that ST ringed engines take a while to break-in properly. Before that break-in point is reached sufficiently, they can be a bare to adjust for civil behavior. Do yourself a favor, if you don't have one already, and build a portable engine test stand on which to run a gallon or more of fuel through each engine before even attempting to set the low speed needle for perfect performance. You will be far ahead of the game if you do. You will also have four of the sweetest running two-strokes ever made. Good luck.

DarZeelon

Andy,


I subscribe to everything Ed (Artisan) wrote!

I would only add:

1. Use fuel with 20% oil, half of which is castor and 5% nitro. Not 10-15% like everyone else and less oil.
Castor oil is more viscous and improves compression in ringed engines too.

2. Use a cold glow-plug. like the OS A5, the Rossi R5 (R4 is borderline), Enya #5, or #6. These will allow you to lean the high-speed needle as far as possible and the low speed range will be affected for the better.

speedster 1919

My Feedback: (3)

I think some of the reports you read are lean problems too. ST 's will run lean when other brands will quit. Thinking they are sluggest and loading up. Stay with 10% nitro or lower. Warplanes engines usually get inverted. A FOX IDLE bar plug is the proper heat range for timing and IMO will prevent a deadstick or two. If you could tune a K&B you should have no problem. ST do smoke a lot too. GOOD LONG BREAKIN and needles set right , they will last forever.

Cyclic Hardover

My Feedback: (3)

You may want to just go to the "2300 Testing and Experimenting" as this subject was really beat to death. The ST line does have carb issues and have simple fixes but you can decide. I like them and are my prefered sport engine of choice. To be honest here, I think its mostly a tuning issues but at the same time, I would not trust it for any 3d at all. I don't do 3d and never will. I run warbirds and speed type, so I really don't have issues. I just throttle up to take off and back down to land. I have the 2300 and some G90's, .45 and like em.

downunder

Super Tigre carbs have a midrange adjustment that isn't obvious at first glance. Loosen the two screws holding the fuel inlet nipple into the barrel and rotate the nipple upwards to lean the midrange and down to richen. Don't forget to retighten the 2 screws.

oldtyme

Artisan (and all others who responded),

I will take your advice on longer break-ins for this engine. Problem is I have the first
one mounted in a cowled situation and really don't want to remove it so will probably just
break it in in the plane. Yes I do have one of those PSP test stands and usually run in my engines
there but I got in a hurry on this one and the G75 is mounted in a 45 degree inverted position.
No, it is not a pumped engine.

I can't remember whether I have the stock ST glow plug in it or I put an OS #8 but it is one of the
two. I was told when I move here (Bend, Oregon) that because of altitude I should run 15% nitro
because of the altitude of the region (3500'). I can switch to 10% if that would be better.

Thanks again,
Andy

oldtyme

OOPS..............forgot to mention that the 15% nitro (Powermaster) fuel that I use has
18% lubricant

Sport_Pilot

That should be fine. Also the ST's sold here likely have a lower compression ratio than those sold in Israel, so I suspect it will work fine with 15%. Also try some differant plugs an Enya #4 or even #5 may help your midrange problem. High and low tank can also affect midrange, high tank rich midrange, low tank lean midrange.

Bax

My Feedback: (11)

Acutally, SuperTigre engines are the same worldwide. Only one version of each type is made.

Sport_Pilot

Bax,
So what is the recommended fuel for the ST?

Cyclic Hardover

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What kind of plane are these going in?

XJet

I've noticed with the ST carbies that there *is* a problem with the linearity of the low-speed fuel metering system.

Bax frequently points out that many people have problems tuning ST engines because they set the idle too low.

After much experimentation and analyzing the setup, it would appear that raising the ST's idle *will* improve the mid-range transition -- because it allows you to lean out the low-end needle a little more.

The problem is that many folks are using these engines to over-power models and a fast(er) idle just won't work -- unless you've got a *really* long field that allows the "hot" landings that a fast idle produces.

The STs really do idle beautifully at lower RPMs -- but because the carby is not designed for these low idles, it requires that the low-speed needle be opened up or the slow-idle is too lean. Unfortunately, when people open up the low-speed needle to support a slow idle, the mid-rnage goes to hell in a handbasket because it gets way too rich. The low-speed needle affects not just the idle but the whole range up to about 2/3-3/4 throttle.

Turning the spraybar has a small (but insufficient) effect on this -- a *low* idle will always produce a fat mid-range -- it's just a characteristic of this carb design.

So Bax is quite correct when he says that raising the idle will solve the transition problem -- but it's only a symptomatic solution, it doesn't address the fact that the carby over-restricts the fuel-flow at any *reasonable* low idle setting.

Of course this doesn't change the fact that the ST engines are incredible value for money and will run forever if you don't abuse them. I've got several and intend to buy more -- but I am now building my own carbs for them and it makes a *huge* difference. Throw a properly designed carby on them and they'll idle *really* slow and have flawless transition with no deadsticks.

Not24

My Feedback: (1)

What he said^^^^^^^^^^!!!!!!!

sigrun

Best post I've ever read on the ST carb issue Xjet. My experiences concur entirely. Particularly well constructed explanation. Analytically accurate, emotively honest without brand or commercial prejudice.

But to mention for the sake of clarification.

It's not even case of overpowering. Fitting the correct power range engine to a particular model such as a the average trainer where the effect of an excessivly high idle is exacerbated by very small changes (increases) in either angle of attack and speed in particular (V²) due to its wing efficiency leading to an inescapable characteristic that it will float or hold-off 'forever' chewing up the LDA if there is too much lift being produced due that combination of thrust (speed) produced by that high idle. ie: "Throttle controls rate of descent".

The trade-off, which you correctly identify, is to compromise by lowering the idle resulting in an unstable gurgling transition, with hesitatory and distracting engine acceleration during execution of the touch and go, go around or missed approach semi assuring a dead cut and EFFATO if the stud opens the throttle rapidly without understanding. That MAG carb design characteristic is exacerbated on smaller ST engine sizes (eg: 40) by the relative size of the carb throat, which is simply huge in comparison to brand peers. eg: nearly 10mm on my S40K ABC MAG in comparison with around 7mm on most Japanese .46s. Sure this makes for a stinkin' hot high end at full honk when that boost port is in full song, but also for excessively fiddly overly complex carb tuning and rubbish transitioning without an excessively high idle in a world where a majority of most McFlyers can't even hand start and haven't a clue whether an engine is lean or rich when it won't, nor have any have no idea how to diagnose or adjust the low end mixture needle.

I think it most tragic where excuses are proffered, typically "unrealistic to expect any engine (in the particular capacity range) to reliably ide below 2800rpm". No argument that certainly holds true for ST's 40 IME, but certainly not for ENYA's, O.S.'s TT's et al. In fact, for training, it's vital that one achieve a stable sustained low idle, with an equally reliable steady transition to full power, touch and go after touch and go after full stop, or after prolonged idle whilst appraising the student of particular points to take-off rinse and repeat.

Neither TT or O.S. nor ENYA's TN carbs present with these issues so typical of the MAG, and they also have a much small effective throat diameter apparently without detriment to high end performance. And of course, with the air bleed examples of ENYA and O.S.' LA training engines with their typically small throats ensuring good suction, their super reliable low idles and rock solid transition is the model of what is achievable. Why is it that one can achieve a stable idle and transition from as low as 1600rpm with any and all of these engine's carbs, yet anything below 2800rpm on the same capacity ST fitted with a MAG results in the 'gurgle 'n gamble'?!!! Two words. Carb design.

I for one am not only gobsmacked at the blind prejudice, but at simply beyond tired of hearing the perennial Secret Club of Superior Technique Tuners utter nonsense about MAG carbs. Blowing in tubes whilst consecutively reciting three "Hail Mary's", tweaking spraybars, "3000rpm idles = normal and OK".

Such a shame ST persist with fitting this indisputedly flawed design to their engines - which are otherwise absolute bull performers!

Not24

My Feedback: (1)

My 2300 idles very well at 1800 rpm, and transitions great from that condition. Oh yeah, I did change the carb to get it to do that.[sm=72_72.gif]

RCPAUL

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I'm running 2 60s, a 40, a 45, a 90 and a 3000. All idle and transition great. Oh, I also use an OS F plug in them all.

Paul

oldtyme

sigrun,

Yeah, like I said, these are going in warbirds that will have retracts fully cowled engines with headers and in-cowl mufflers and in the case of the P47(Top Flite 63" wingspan) I'll have tank
drops, flaps sliding canopy........so I don't think these birds will be over powered. These are the engines that Top Flite recommended for both the P47 and the AT6 because of the ease to keep them in the cowl and still have adequate power.

Andy

sigrun

Hi Andy,

Unlike many sport planes and almost all trainers, most warbirds wing loadings mean they can still achieve sufficient sink rates with positive throttle on demanded of their higher approach speeds for (notably tip) stall avoidance, so setting that higher flight idle RPM required to ensure an unstumbling transition doesn't present the same problematic issue.

None of this actually alters the MAG carb fitted engine's idle and transition characteristics, but just determines whether they present a problem in specific application or not. As you rightly point out, in your specific application = not.

Cyclic Hardover

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Should be fine, I run mine inverted with lowered fuel tanks and no problems. Have a G90 in a Shoestring for 4 years and no issues.

jgs77

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Who here has replaced a carb on a GS45 or G51? What carb did you use and does it solve the mid-range issues? In other words, can you now run it for 3-4 minutes at half throttle and then hit full throttle w/o a stumble or having it gradually load up? I've tried an ASP 40 size carb from Just Engines which they offer machined to fit the GS45. It works a little better than the ST carb - still stumbles a little. Anybody else? I'm wondering what has worked for people and what hasn't in terms of replacing the carb with no other modifications.

freeonthree

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I just bought a 1/4 scale Giles 202 with a ST 3500, and I just got it running today. It's funny I came by this thread, I found that exact problem with this one. It just doesn't like to idle slow at all, but now I know why. Good reading, thanks !

NM2K

My Feedback: (14)

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If your engine isn't broken-in yet, you will be running both needles a little fat, leading to the rich midrange. Just don't expect this engine to be super crisp going through the midrange. When the engines were designed, most people flew using the throttle like a toggle switch for idle and full throttle. So it didn't really matter much if the midrange was rich, as long as the engine succeeded in throttle-up.

I've never flown glow engines with R/C throttles at much higher than sea level, so compensating for altitude by adding nitro is an unknown for me. The locals should know.


Ed Cregger

Hobbsy

My Feedback: (102)

After much experimentation and analyzing the setup, it would appear that raising the ST's idle *will* improve the mid-range transition -- because it allows you to lean out the low-end needle a little more. (quote)

X, I just mentioned this very thing concerning an inverted Saito.

freeonthree

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Could you state that in clockwise or counter clockwise as looking straight at it please ?