Help! Overheating/Lean ST G90

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

KojakDave

Posts: 243
Joined: 3/15/2004
From: Lakeland, FL, USA
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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

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

w8ye

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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.

< Message edited by w8ye -- 7/10/2006 10:34:57 PM >

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

bla bla

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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!

< Message edited by bla bla -- 7/10/2006 10:39:51 PM >

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

KojakDave

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From: Lakeland, FL, USA
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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

< Message edited by db4962 -- 7/11/2006 2:43:36 AM >

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

w8ye

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

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

KojakDave

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From: Lakeland, FL, USA
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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

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RE: Help! Overheating/Lean ST G90 - 7/11/2006 3:36:30 AM

Cyclic Hardover

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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

< Message edited by Cyclic Hardover -- 7/11/2006 3:39:57 AM >

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

KojakDave

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From: Lakeland, FL, USA
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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.

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RE: Help! Overheating/Lean ST G90 - 7/11/2006 5:31:39 AM

Daryl Martel

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From: WinnipegManitoba, CANADA
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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.

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

wcmorrison

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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

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RE: Help! Overheating/Lean ST G90 - 7/11/2006 2:08:09 PM

D Bronk

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

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

w8ye

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

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

KojakDave

Posts: 243
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From: Lakeland, FL, USA
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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

< Message edited by db4962 -- 7/11/2006 2:46:30 PM >

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

Hemikiller

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From: Killingworth, CT, USA
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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......

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