I need some suggestions, Please! I have a Gaint scale Ultra Sport with a brand new ST G-2000 on it (wore out the last one) . Bought the plane at Perry a couple of yrs back, ran it &amp; flew great. ST Carb was missing the low speed screw, so put a new large Perry Carb (5100?) on it, ran pretty good, but was kinda loose, i.e., would rotate a bit after running a while. So, sent it back to Bob Perry, &amp; he sent me a new one which fits tight.<div style="background-color: rgb(255, 255, 255); padding-top: 5px; padding-right: 5px; padding-bottom: 5px; padding-left: 5px; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font-family: Arial, Verdana, sans-serif; font-size: 12px; ">Got engine running very well, but after about 1.5 minutes of flight, engine quits. Re plumbed fuel tank, new tubing, etc. pressure checked, etc. Everything OK. It has a Rectangular box style Super Tiger muffler on it. Runs about a minute, quits..runs a minute quits,
THEN put check valve on pressure line. Runs good, for about TWO minutes, then floods out. Seems that the back pressure is not enough. Put baffles in ST muffler, no help....Bought brand new engine, does same thing, tried brand new original carb, same thing, one, runs good on test stand.
Fuel pickup line level with carb inlet.. etc. bored out pressure nipple on ST rectangular muffler
Running 5 % Cool Power or Morgan fuel, no difference, Engines (s) are definitely not tuned too lean,
To top it all off, I've got a St 90 on a warbird &amp; a ST 75 on a 4 Star with Bison &amp; stock muffler doing the same thing low end set correct, spray bar @ 45 degrees,
What am I doing wrong??



Got ME STUMPED! Help Please!</div>

is it the 2000 or the 2300 ?

Why the check valves???? They cause the tank to keep too much pressure and the engine will start to run rich once throttle is reduced. The tank must vent its surplus pressure back into the muffler if the engine load is reduced.

Interesting, multiple engines doing the same thing. Both on the same fuel?

You say the engine runs on the test stand, not in the aircraft. It's gotta be tank or plumbing, could be vibration making the fuel foam as the tank gets partially empty.

Using a check valve will increase tank pressure, but when you idle down throtle, tank pressure will drown the venturi. Usually a Cline or Iron bay fuel regulator is used in conjunctionj with the one way valve; I have heard of where the low end could be leaned out with the use of a check valve only but I haven't tried it. Pardon me if I missed something here.

I'm having a hard time with my ST S90K too.. Some of the ST "Experts" claim the carb is easy to tune but I disagree. Perhaps If we all had 50 years of experience, things would be different. I've ran Glow engines for 14 years and never have I had a harder time than with my ST S90K and K&B .65. My TT .46 Pro runs like a top 99% of the time.

I have tried my spraybar in either direction and no matter how much I turn it, the midrange is consistently rich and randomly quits. If I turn the fuel nipple on the spraybar towards the engine mount lug, it will not idle below 4500rpm. Going that far makes the high speed mixture too lean and readjusting both needles is in order.

I am going to try the spraybar 90° from "straight in" and see if that makes a difference. Seems to my non-college educated brain that there would be better vacuum to pull fuel in this position than straight in throwing globs of liquid fuel into the engine. Perhaps if it has a slight chance to atomize the fuel before sucking it through the crankshaft it'd mix better and be more consistent. BUT for all I know it may not even run in this fashion. Will have to experiment more.

I do know checkvalves are not good without a regulator since the tank needs to be pressurized for most regulators AFAIK. I tried them and tossed them out within 5 minutes. (TT .46 inverted 8" away from tank.. 6" below and 2" behind engine) I changed over to a Perry pump on the TT.

It is not you. It is the carb, which probably has a dog-legged stranslation guiding groove. At the time, much confusion was everywhere, because not all the carbs had this correction issue. Some had straight translation grroves.
There also were multiple smaller issues like dull tools and insufficient fuel flow capacity of the fuel needle bar.
I wrote an incomplete rundown here.

Thank you for the prompt reply. Do you have a picture of what the translation groove should look like?

Somehow the link in my post got inserted wrong. I updated it, so please follow the link once more.
I must have a picture somewhere . If so, I will post it.
I posted the complete pdf file. The 90 carb is no different.
Edit:
I updated the web page with image and drawing link. The drawing shows all needed mods to the ST carb.
Here is the drill about the amount of correction in the translation groove that is needed
1) set idle mixture (fuel nipple pointing to rear carb lug)
2) set high mixture
3) go to exactly half throttle
4) at half throtlle, use the IDLE fuel adjustment to get smooth running. Note the required amount of adjustment
5) The amount of adjustment in step 4) is the amount of the groove correction that is needed for PERFECT carb tune.
6) after grinding the groove, re-adjust idle for best transition

With my limited computer skills, I can't keep in my head how to post a link; don't laugh, I earned my memory lapse. If you take off the carb and look down the open rotating venturi drum, you will see the little brass rod running across the opening. The slit in that rod needs to be facing straight down the venturi regardless of altitude you are flying at. Now, with the engine running, if you transition the throttle up and small droplets of fuel seem to dance out of the carb, your bottom end needle is too rich. If you transition up and the engine sags and maybe dies in the throttle up without the fuel popping out of the carb, your bottom is too lean. Do turn in two maybe three clicks at the most before trying the fireup again. You will need to click the top end needle out as many clicks as you click the bottom end in since they work inversely proportional to each other. If you are holding the carb in your hand, you might be able to see the bottom end needle in the slit since it has a blunt end; the top end is tapered to a needle point. I apologize here for the wordiness, there are some very good links to ST tuning nomenclature out there. I have run these guys since around 1980 and I recognize they weren't ever meant for easy tuning, but they do run very well when you find the sweet spot; just demanding patience from you, I hear you.

Posting links is easy.. Highlight the link you want to post; click with right mouse button and click copy (or copy link location). In your forum reply box, click the "link" button under the smileys. Left mouse click in the "URL" box and follow that with a Right mouse click, then click paste. In the Link name box, Type the text you want the to show up as. Click "OK" and it will fill put your link into your message. Just be sure to start typing your reply after the very last "]".

On the original subject - I know there are many that have zero problems, as there are many with many problems tuning these carburetors. I am confident I can make it happen, but Getting past the random quitting is a milestone in itself. Based on Pé Reivers information, I looked at the barrel in my S90K carb and this is what it looks like:
Seems to be one of the "corrected" ones, though I did not map which angle the translation groove follows, but it looks far more linear than the "dog-legged" translation groove found in some engines. I dont perceive this to be my problem.

I used RTV silicone on top and bottom of the carb base's O-Ring, putting pressure on the o-ring using a medium sized vice grip fitted with rubber hose on the jaws to prevent marring of the carb surface. Not a huge amount of force applied, just enough to have the carb firmly seated. I doubt there are any air leaks anywhere on my engine. The muffler is of the perhaps "newer" variety; Its rounded off and sort-of funny looking as compared to other mufflers I've seen. It is not the square boxy older-style muffler. I had the muffler installed tight to the engine for break-in and after installing it on my boat, I then moved the muffler out so the end of the header protrudes into the muffler only a fraction of an inch past the flange/clamp area.

My engine retains its original fasteners, which are tight and stay tight when warm. I've used 2 different 16oz Texon tanks with 2 different standard clunks, and a 8 or 10oz Sullivan tank with an OS Sintered Clunk. No difference in running characteristics between the two tanks. I dont think the problems I've had are related to the fuel tank or its plumbing for the most part. There are some bubbles getting through still, so I think a Bladder tank is the answer to the bubbles, however the bubbles are not causing the quitting. The engine is not overheating. Car guys will spit on the cyl. head and depending on how long it takes for the spit to sizzle off will tell you how hot the engine is. Bubbling off immediately means its too hot, taking 3-5 seconds is in the "butter zone" and any longer than that and its not up to operating temp. This is kind of a general idea, and usually isnt too far off. Mine took about 6 seconds to bubble and 10-12 seconds to completely dry up. That tells me the engine is not scorching hot.

I really think there is something amiss with fuel delivery inside the carburetor. I will remove the spraybar and measure the I.D. of the spraybar nozzle, as well as the ID of the inlet nipple on the spraybar. If these are indeed smaller like Pé suggests, then perhaps there just isnt enough fuel flow to get a good mixture setting. It makes sense that if there is a lack of flow, opening the needle too wide to compensate would drown the plug going WOT, and if the idle needle is too rich for much the same reason, its just drowning in fuel.

I'm no expert as you can tell, but I'm trying like hell to learn to be one. If I ever get this carb dialed in A+ perfect, I think I'm going to submit a request to be the youngest person to get an original ST carb dialed in without replacing it with a Perry. I'd like to think its in the near future, but who knows. (I do try to add a small amount of humor into my posts every now and again..)

Happy 4th of July, fellow North Americans!

This type of carb with the straight groove still has a rich midrange, though not as rich as to make the engine die. Lots of nitro in the fuel will blanket the design flaw.
Idle adjustments are critical. 1/16th of a turn is sometimes too much, and it is easy to overshoot best settings. Use the throttle acceptance method. When satisfied, use the fuel line pinch method to check rpm rise before the engine dies. If there is no rpm rise, you are heading for deadstick landings.
The almond eye in the spray bar should have nice pointed ends for easy idle tuning. If the end has a small radius, it indicates a dull tool in the milling operation, and idle setting ease suffers big time if you ever can get it right.
Once you know how not to overshoot correct settings, a carb can be tuned well in about 5 minutes time. Once set, you'll never have to touch that idle needle again.

My SuperTiger S90K I've found to not tolerate any nitro. It wont run for very long using even 5% nitro. Not worried about this. I know when using the fuel line pinch method you want a slight rise in RPM without killing the engine. If it dies very quickly pinching the line its too lean, I know.. been through that. I like going off using a tach to set the high end, but I'll revert back to the pinch method for a time or two and see how it fares. Obviously there are flaws in the design. I do think the knurling of the needle detents to be rather coarse, perhaps a problem with some ST carbs where the optimum setting could very well be between clicks. My retention clips hold the needle pretty securely but it will not hold it between "clicks".. any suggestion to this?

The spraybar orifice has nice sharp corners and is diamond shaped. It does have the older flat machined spraybar which did have a very minute amount of swarf on the edge of the spraybar orifice. I gently smoothed those off with an Xacto knife so the edges are smooth and clean.

Should I be concerned with the size of the hole through the spraybar and fuel nipple despite the carb barrel being of the "corrected" or "more-corrected" variety? My fuel inlet nipple measures 1.68mm I.D, but I didnt pull the spraybar out to measure yet. I'll have to do more experimenting when my wife isnt home. I was driving her nuts yesterday running my ST...

First of all I want to thank you for the comp. input; I am going to get my wife to copy the stuff you typed here, again thank you, will practice this skill. I thought of another thing and it has to do with the vintage of the carb. The newer ones have a rubber boot on the side of the barrel since the running of the steel barrel in the aluninum body would eventually create tolerance issues, vacuum leaks. The older ones did not have the black rubber boot that was wrapped around the barrel and attached to the body. Will get back with any more info.

The rubber boot serves no other purpose than to keep dust and dirt out of the carb. I thought it could be a place for an air leak but according to Bax @ Hobby Services, its only a dust boot and nothing more. The oil in your fuel will form a nice film that will actually float the barrel in the carb body. This oil film is what creates the air seal around the barrel, not the boot.

Hi!
Sounds like a fuel problem to me!
Try 20% Castor oil and 5% nitro fuel. Personaly I blend my own fuel so I know whats going on...
Then use a Enya 3 glow plug and a APC 14x6 or 15x4 prop.

16oz tank is ok. But have you mounted it according to the "tank mounting rule"? Also check that the pressure nipple on the silencer is not clogged!
Experienced that myself a year when I was starting one of my .40 engines and the engine ran for just a couple of secondsbefore quiting.

Could you please send a picture of your engine /plane?

If ANY model engine will run "all day" on the test stand, but not in the airplane, it's not the engine's fault. Something in the engine, airframe, and fuel system combination.

Usually, when an engine quits shortly into a run, it's going too lean for some reason. Airframe vibration is a common cause that agitages air into the fuel and causes a lean mixture. Sometimes, it's a plugged vent line. As fuel is drawn from the tank, air cannot replace it, so a partial vacuum builds in the tank and opposes fuel flow to the engine, which quits from fuel starvation. Of course, leaky fuel lines, cracks in the metal tubing in the tank, and so on are also causes, and everything needs to be checked.

The spraybar in the carb can easily be clogged by all sorts of trash. Remove it from the carburetor and run a fine wire through it to push out anything that may be there.

Please see the SuperTigre Direct Support Forum for how to set the carb. We've posted that in a "sticky" thread at the top of the forum. Setting the carb as we describe is essential. Also, don't try to get a too-low idle RPM. If you do that, you'll have problems with transition and midrange.

If the engine does not tolerate nitro, add shims under the head.
Remove the clip, and use silicone tube as spacer. This will hold the needle securely.
1.68mm will be fine with a .90 engine. Standard was fine on my .90, but on my 23cc engine it did not allow sufficient fuel flow. Use large ID fuel tubing, and good exhaust muffler pressure. Be sure there are no burrs inside the spray bar. (been there!)
Fitting the spraybar off a .40 engine works wonders as well. Slightly less power, but much improved tunability..

@ Bax:
You are absolutely right, especially the tuning and spraybar cleanliness items. I use a drill bit that fits the bores exactly to clean out all debris and gum. The stuff I found in engines brought to me makes one wonder how the engine could have run at all. I run three filters in my glow planes: 1) fueling canister 2) fuel on board fill line 3) felt clunc to carb. (I never use any T-lines for fueling. They are a source of lint contamination and air leaks.)

Did you guys miss this or did i miss something?


"To top it all off, I've got a St 90 on a warbird & a ST 75 on a 4 Star with Bison & stock muffler doing the same thing low end set correct, spray bar @ 45 degrees,
What am I doing wrong?? "

No, the boot was put in place after extensive run time on the earlier vintage throttles revealed slop in the rotating barrel in the body. It could be moved from side to side, creating vacuum leaks, hence the boot.

right, but that's the only way to get it to run!!!

2000, have 2 brand new ones, could it be the Perry 5100 carb?

missed what?
The ST carbs that have a rich midrange are flawed, but can be corrected very well if you have the required skills. Read the previous posts please.
The latest China assembled engines seem to have the issue solved if I read the posts about them well.

<span style="font-size: medium"><span style="font-family: Arial">Pe,
you're correct on the older carbs. I changed the Perry out to a newer ST carb with out the dogleg &amp; it runs fine. Same for the 75 &amp; 9o's. The dogleg is the difference. But wonder why the Perry won't work/run? Venturi too large, reckon? Or it could be one made for a pressure not a vibratingpump. Wonder how you tell the difference on the 5100 Perry?
</span></span>

A Perry carb is very delicate in it's idle fuel cirquit. Unlike the ST carb, the Perry is not self-cleaning. The internal metering slit is very narrow, and may need some cleaning attention if the carb does not tune well. With carbs like this, the fuel suction line MUST have a super filter (felt clunk etc), and the line must never be disconnected for any reason except dismantling. NO T-fill connection either.