Help! Overheating/Lean ST G90 (Full Version)

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KojakDave -> Help! Overheating/Lean ST G90 (7/10/2006 8:23:43 PM)

I have a brand new ST G90 that I just put on my H9 P-47with a Slimline Pitts muffler. I cut the metal exhaust extensions approximately 1/4" to 1/2" each and then mounted silicone exhaust deflectors so that the cowl could be mounted but the exhaust would exit the cowl. The engine was run in according to the directions in the manual and seem to be doing wonderfully. I also took the plane up for its maiden and it ran like a top; plenty of power, great transition, and a healthy amount of smoke.

On the second outing, however, I ran into a problem. The engine was running extremely lean and I could turn the high speed needle valve at least 6 or 7 turns out (3 is the recommended rich setting for this engine). Also, I had a hard time getting the engine to shut down, even with the carb completely closed. I suspected an air leak so I changed out all of the fuel lines and reseated the carb with some red silicone high temp gasket maker.

I started the engine up again today and it is still overheating and running lean (it fails the pinch test even at 6 turns out). I noticed that a lot of exhaust and oil seemed to be coming out of the joint between the engine and the muffler. I pulled the muffler off and the gasket that came with the engine was torn, so I removed it and made a gasket out of the red high temp silicone gasket maker.

Is it possible that the pressure leak between the engine and muffler was causing the engine to run lean (i.e. not enough back pressure)? Also, would this still cause the engine to act like there was an air leak, as I again had trouble shutting the engine off? I am getting very tired of fiddling with a brand new engine, and am wondering if I should just send it back to the manufacturer for a new one. I have not made any permanent modifications to it, simply applied some silicone around the carb and muffler joints.

I have also thought about plugging one of the exhaust outlets to see if that helps, but it ran so well on the maiden, why would it suddenly change like that?

Also, the idle stop screw fell out during the maiden, but Bax (of the ST support thread) said that there shouldn't be any air leaks caused by that because it only catches the outside of the carb barrel and doesn't have anything to do with the actual air intake.

Can anyone think of anything else? I wanna fly this plane, not tinker with the engine!

Thanks, Dave



w8ye -> RE: Help! Overheating/Lean ST G90 (7/10/2006 10:32:53 PM)

There is too much restriction in your low speed needle circuit. Screw the LS needle out a little and try again.

The starting point for the low speed needle is to have its end half way across the cat eye slit of the spray bar when at wide open throttle. You can only see this with the carb removed.



bla bla -> RE: Help! Overheating/Lean ST G90 (7/10/2006 10:38:49 PM)

Do have one of those crappy refuiling valves?
If so, drop it... they're notorious for creating problems... going lean is one of them.
A leak between the engine and muffler won't be causing you're problems... just the dirty one you'll see around the cowl!



KojakDave -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:41:26 AM)

No refueler valve on this bird, I have a three line tank with a fuel dot. I will take the low speed needle valve out a few clicks and go from there. Thanks for the input. One question, though...would the low speed needle valve affect it at WOT that much? That is where I'm doing the pinch test and having it drop off, even at 6 or 7 turns out on the high speed needle.

Dave



w8ye -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:45:33 AM)

Yes, the low speed needle being in too far will almost do away with any effectivness of the high speed needle



KojakDave -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:56:32 AM)

Ok..I will give it a try tomorrow and see if that helps...if not, then it's back in the box and let the folks at Hobby Services figure out what's wrong with it.

Dave



Cyclic Hardover -> RE: Help! Overheating/Lean ST G90 (7/11/2006 3:36:30 AM)

Whats the wingspan on that H9? Also consider the routing of the fuel line to the carb. I had this happen on a G90 a couple years ago. If the line makes any type of a short 90 degree bend to get to the carb, its pinched, thus causing a lean run and getting hot. May not look pinched but its enough to mess it up



KojakDave -> RE: Help! Overheating/Lean ST G90 (7/11/2006 4:18:38 AM)

It's the 65" P-47.. Here's a pic of the plane and the engine/fuel tubing. If you see anything that strikes you as odd, let me know. I know that the engine orientation may seem a little weird, but this is actually the recommended mounting position from H9... in fact, the holes for the engine mount are already drilled so that the engine will be positioned this way.



Daryl Martel -> RE: Help! Overheating/Lean ST G90 (7/11/2006 5:31:39 AM)

Nice looking plane. Engine installation looks fine. What size prop? A 14 X 6 should be the right match. Maybe you're not getting enough fuel flow to be able to richen it up? Do you think muffler pressure is adequate? All the .60 size and over S.T. engines have small diameter fuel nipples (as compared to other engine brands). I don't think this helps the ability to flow fuel through the carb. I unscrew my nipples out of the carb and run a 1/16" drill through them - still leaves plenty of "meat". Opening it up has got to improve flow a little. Also, running a Perry/Varsane diaghragm pump really improves reliability, and getting enough fuel flow will never be a problem. I''ve just fitted a S.T. .90 with a similar muffler (Bisson though) to a bipe - am curious how it'll run.



wcmorrison -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:02:32 PM)

Is it running hot without the cowl? Could be that there is not enough cooling air, need a big vent, at least as large as the front intake to allow air to cool engine. Otherwise, lean runs indicate air leaks. Check carb seating and back plate for tightness. And make sure your fuel has some castor oil in it to protect against those lean/hot runs.

Good luck,

Cheers,

Chip



D Bronk -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:08:09 PM)

The most likely culprit.. If you don`t put that back in ,you could lose the entire CARB.



w8ye -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:17:46 PM)

The bolt only has threads on one end and the threads and the nut are on the other side.



KojakDave -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:33:59 PM)

quote:

ORIGINAL: wcmorrison

Is it running hot without the cowl? Could be that there is not enough cooling air, need a big vent, at least as large as the front intake to allow air to cool engine. Otherwise, lean runs indicate air leaks. Check carb seating and back plate for tightness. And make sure your fuel has some castor oil in it to protect against those lean/hot runs.

Good luck,

Cheers,

Chip



The maiden was flown with the cowl on and i could put my fingers on the engine at the end of the flight...very warm, but not hot. The recent overheating has occurred with no cowl because I've had it off looking for air bubbles, air leaks, etc. Within just a minute of starting it gets too hot to touch. The oil content is 18% with a castor/synth mix.


quote:

ORIGINAL: D Bronk

The most likely culprit.. If you don`t put that back in ,you could lose the entire CARB.


Also, the carb retaining bolt is in the engine and the threaded end with brass nut is on the opposite side from where the photo was taken.

quote:

ORIGINAL: Daryl Martel

I unscrew my nipples out of the carb and run a 1/16" drill through them - still leaves plenty of "meat". Opening it up has got to improve flow a little.


I don't think I'm ready to start drilling anything on the engine just yet...It's only a couple of weeks old and I don't want to void the warranty. I'll just box it up and send it to Hobby Services soon if I can't find the problem.

I just thought of something that I should look at. On the second flight I had a hard stop on landing. I wonder if the clunk could have gotten stuck with a sharp bend in the line, allowing enough fuel at idle but enough of a kink to lean out at open throttle....or does that sound improbable?

Thanks for all the input folks, please keep the ideas coming.

Dave



Hemikiller -> RE: Help! Overheating/Lean ST G90 (7/11/2006 2:41:30 PM)

I had a similar problem with an ST90 in my GP Super Skybolt bipe. It would run great on the ground and in flight for about two minutes, then would go lean and the engine would stall. Went through the same situations; resealed everything possible, redid the entire fuel system, etc.

The culprit turned out to be the CB Tatone Pitts style muffler. It has two 3/8" outlets, which apparently is too much for the ST90 and was causing a loss of fuel pressure. I temporarily plugged the outlet closest to the engine and it ran flawlessly. I could run it full throttle, nose straight up for an entire tank of fuel. As soon as the second outlet was unplugged (my cap popped off) it went lean and stalled. I drilled and tapped the casting for a screw in plug and it's been running beautifully ever since.

You might want to reset your needles and cap one of the pipes on the Slimline muffler, just to see what happens......



KojakDave -> RE: Help! Overheating/Lean ST G90 (7/11/2006 3:11:17 PM)

Hemikiller,

It seems like an easy thing to test...I'll give it a try when I get home,

Thanks,

Dave



Ernie Misner -> RE: Help! Overheating/Lean ST G90 (7/11/2006 8:18:03 PM)

What did you find with the clunk? That would sure do it. I assume the backplate screws are tight also. Have you tried running on the ground with the cowl removed and checking for any air bubbles in the fuel line?

Ernie



pilotpete2 -> RE: Help! Overheating/Lean ST G90 (7/11/2006 8:18:11 PM)

Dave,
I am setting up a G90 with a Slimline Pitts, I have been warned by more than one person about the need to plug one of the muffler ports, in my case the exaust needs about 6" of the extension tubing to clear the cowling, so I think maybe that will increase the back pressure in my situation.
Good luck,
Pete



BillS -> RE: Help! Overheating/Lean ST G90 (7/11/2006 10:17:28 PM)

The engines are certainly not rocket science but they can be a pain in the behind.

Consider testing the engine on the bench on a test stand. My experience is when you can see and get to the engine to observe, make changes, control tank height and other issues the problems are quickly found. All too often something about the airplane set up will mask the problem. In addition the temptation to make too many changes at once is great with the engine in the airframe. If it is not reliable on the bench it will never be reliable in the air.

Bill



Cyclic Hardover -> RE: Help! Overheating/Lean ST G90 (7/11/2006 10:20:39 PM)

Nice looking plane.-------------Okay , I may be wrong but I see absolutely no opening on the bottom of your cowl. If this is so, well there you have it. You ought to cut out at least a 2x 3 inch vent on the bottom to get some air flowing. Now when you say it's overheating, how do you know this? All engines are hot to touch after a while but does not mean its overheated and there are no temperature specs around to go by. The only sure fire way I have every been able to tell if an engine overheated is that you will lose power in flight and will be similar to the engine quiting. I have had this happen several times brought on by running to lean. By the way , fuel lines look okay



D Bronk -> RE: Help! Overheating/Lean ST G90 (7/11/2006 11:09:49 PM)




quote:

ORIGINAL: D Bronk

The most likely culprit.. If you don`t put that back in ,you could lose the entire CARB.


"Also, the carb retaining bolt is in the engine and the threaded end with brass nut is on the opposite side from where the photo was taken."

Sorry about that ..It looked like the parts were missing..[8|]



KojakDave -> RE: Help! Overheating/Lean ST G90 (7/12/2006 12:16:03 AM)

Well, I think that we've figured out the problem. I used a temporary plug in one of the exhaust tubes and was able to get the engine to throw some smoke and pass the pinch test. It's running well at about 4 to 4 1/2 turns out on the high speed, and I can live with that. I went ahead and crimped the tube that I had the plug in and the engine is still running well.

The engine is still hot to the touch, but like you said, I don't know what the normal operating temp is for this engine. I also fired up my OS 61 fx and ran it for a little while to get some sort of comparison and it seems to run at about the same temp...oh well. As far as overheating in the cowl there are cooling holes cut in the cowl, but they're intentionally camouflaged....I drilled the black squares on the bottom of the cowl out so they're harder to spot as cooling holes when it's flying. i have the equivalent of 6 sq. inches (2" x 3" ) by cutting out six 1" squares.

Thanks again,

Dave



doubledee -> RE: Help! Overheating/Lean ST G90 (7/12/2006 12:45:35 AM)

db4962,
The ST90 is a superb engine, they are a little finicky on the tuning especially when new. They take a while to break in but as it continues to run in it will run better and better, especially on the low end and at transition.
The low end will have an effect on the high end so you must rock back and forth between the two. Don't worry about the low end too much until it is well broken in.

A good start point on the low end is to stick a toothpick in the carb throat and close the barrel down on it. Put a length of fuel line on the inlet nipple that is long enough for you ot blow into. Blow into the fuel line whilst opening the the low end need until you hear air hissing. Once you here the hiss of air you are at a good starting point for the low end. Then hook everything back up and try to adjust the high end for the right amount of richness using the pinch method. Don't concern yourself too much with the low end until the engine is well broken in. How soon that is will depend on how you fly your airplane and how rich you run it.
Of course, all this is on the assumption you don't have any fuel/air leaks and have proper pressure to the fuel tank.
Hope this helps.
Regards,
doubledee



Cyclic Hardover -> RE: Help! Overheating/Lean ST G90 (7/12/2006 4:10:34 AM)

Like he said the G90 is also my #1 all around sport engine of choice. I also own Jett and other top of the liners but the G90 can cover alot of area.



Flyboy Dave -> RE: Help! Overheating/Lean ST G90 (7/12/2006 5:42:28 AM)

I see you do not have a fuel filter on the fuel line to the carb. That's a no no. [:o]
You might have a bit of lint or something in the carb. Pull the main needle
out and make sure the nozzle is absolutely clean....

....and put a fuel filter on there. [;)]

FBD. [:D]



KojakDave -> RE: Help! Overheating/Lean ST G90 (7/12/2006 5:45:16 AM)

LOL...it just so happens I bought one today FBD...already put it on tonight. Good eye but too late.

Thanks,

Dave



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