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Author Topic: ST G90  (Read 2335 times)

Offline John Rist

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ST G90
« on: June 14, 2009, 04:51:39 PM »
I am trying to set up a RC ST G90 for a scale aircraft.  The problem is that The ST G90 runs rich in the mid range to the point that at some throttle settings it will load up and quit.  You have a HS needle and a idle screw but the mid range is fixed.  I am running the top and bottom needle as lean as I can and the mid range is almost OK but still rich.  I guess the question is how do head shims and nitro content affect these settings.  in otherwords if you change either and do not change the needle setting does the mixture change and if so which way?

 HB~>  HB~>  HB~>   ???  ???
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Offline Ralph Wenzel (d)

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Re: ST G90
« Reply #1 on: June 14, 2009, 05:25:51 PM »
You know, it might be as simple as changing props to let the engine unload a bit more.

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Ralph Wenzel
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Offline John Rist

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Re: ST G90
« Reply #2 on: June 14, 2009, 06:03:12 PM »
Quote from: Ralph Wenzel on June 14, 2009, 05:25:51 PM
You know, it might be as simple as changing props to let the engine unload a bit more.



It's worth a try.  I need to run a 3 bladed prop.  I will look at the next lower pitch.
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Offline dennis lipsett

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Re: ST G90
« Reply #3 on: June 14, 2009, 06:08:15 PM »
Quote from: John Rist on June 14, 2009, 04:51:39 PM
I am trying to set up a RC ST G90 for a scale aircraft.  The problem is that The ST G90 runs rich in the mid range to the point that at some throttle settings it will load up and quit.  You have a HS needle and a idle screw but the mid range is fixed.  I am running the top and bottom needle as lean as I can and the mid range is almost OK but still rich.  I guess the question is how do head shims and nitro content affect these settings.  in otherwords if you change either and do not change the needle setting does the mixture change and if so which way?

 HB~>  HB~>  HB~>   ???  ???

you might want to try this.
 The side with the needle valve and fuel nipple is held on with 2 screws. Loose them and just like a perry carb move it slightly counter clockwise, This will move the opening in the spraybar towards the rear and somewhat diffuse the flow of fuel in the idle mode. This cure has been used for years. The only thing to remember is it will not correct gross misadjustments so don't move it too far.
dennis
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Offline Brian Hampton

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Re: ST G90
« Reply #4 on: June 14, 2009, 10:55:55 PM »
Rotating the spraybar to adjust mid range is mentioned in the ST instruction leaflet and the factory default setting is with the fuel nipple pointing towards the rear mount hole. Some Enya carbs do exactly the same thing but they've very conveniently added a ratchet lever so you can adjust it while the engine is running plus they have a graduated +/- scale to show rich/lean.
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Offline John Rist

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Re: ST G90
« Reply #5 on: June 14, 2009, 11:27:53 PM »
Quote from: dennis lipsett on June 14, 2009, 06:08:15 PM
you might want to try this.
 The side with the needle valve and fuel nipple is held on with 2 screws. Loose them and just like a perry carb move it slightly counter clockwise, This will move the opening in the spraybar towards the rear and somewhat diffuse the flow of fuel in the idle mode. This cure has been used for years. The only thing to remember is it will not correct gross misadjustments so don't move it too far.
dennis

Dennis if this works I owe you big time. It makes sense - I will give it a try. Thanks!!!!   #^
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Offline John Rist

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Re: ST G90
« Reply #6 on: June 19, 2009, 09:43:01 PM »
Quote from: John Rist on June 14, 2009, 11:27:53 PM
Dennis if this works I owe you big time. It makes sense - I will give it a try. Thanks!!!!   #^e
bigger th

I tried rotating the spray bar and it did improve things. However it still ain't right. Still runs rich in the mid range and loads up at some throttle settings.   I have an OS Max 65 FP.  As it turns out The OS carbs almost fits.  The OD of the OS carb is .040 larger than the ST carb.  I can turn this down easly and make it fit.  I will let you know how it works.
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Offline Jim Oliver

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Re: ST G90
« Reply #7 on: June 20, 2009, 05:44:35 AM »
Hey John,

Two words: Perry Carb. H^^

Cheers,
Jim
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Offline Glen Wearden

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Re: ST G90
« Reply #8 on: June 20, 2009, 06:56:01 AM »
Another two words might be Kavan carb.  I've got one on my Merco 61, and it's great.  It's an air bleed type for the lower end.  You can get them from MECOA.  Just my $.02.

Glen Wearden
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Offline dennis lipsett

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Re: ST G90
« Reply #9 on: June 20, 2009, 01:38:19 PM »
Quote from: John Rist on June 19, 2009, 09:43:01 PM
bigger th

I tried rotating the spray bar and it did improve things. However it still ain't right. Still runs rich in the mid range and loads up at some throttle settings.   I have an OS Max 65 FP.  As it turns out The OS carbs almost fits.  The OD of the OS carb is .040 larger than the ST carb.  I can turn this down easly and make it fit.  I will let you know how it works.

One other simple aolution. Reduce the diameter of your fuel tubing. Suprisingly many use too large an ID on their fuel line and oversupply the engines need. Seems strange but it does work.
Dennis
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Offline John Rist

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Re: ST G90
« Reply #10 on: June 20, 2009, 10:42:08 PM »
I looked in Tower and did not see a ST 90 size Perry carb.  The small fuel tubing is worth a try.  But like I say the OS Max 65 FP carb I have in hand is the next try.  It also is an air bleed style carb.  At this point in time I am running out of time if I want to make the Scale NATS.  Right now I am in Texas at a SOCCER match (Grand Son).  When I get back to Huntsville Alabama. I can try the OS max carb with a couple of howrs work.  I would be suprised if it dosn't work.
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Offline Jim Oliver

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Re: ST G90
« Reply #11 on: June 21, 2009, 07:10:54 AM »
John,
I suspect that the real problem is that most all the 2 stroke "carbs" have a rich mid range.  This is/was something that we battled during my RC Pattern years.  Sometimes it was necessary to set the idle mix very lean to get the mid-range usable.

The fix it to change the contour/angle of the slot that causes the carb barrel to move in/out as the throttle opens and closes--------can require replacment of the carb barrel.

The Perry carbs were last made by Conley, as I remember-----might try a google on that??

Jim
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Offline john e. holliday

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Re: ST G90
« Reply #12 on: June 21, 2009, 07:56:43 AM »
If I remember right, back in the day when I was playing with R/C we would file a small notch in the barrel of the carb.  If done right the engine would not go rich at mid range.  If you went too far it would go lean too soon.  Can't remember who would do the mod for me tho.  Having fun,  DOC Holliday
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Offline John Rist

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Re: ST G90
« Reply #13 on: June 25, 2009, 09:14:19 PM »
Quote from: Jim Oliver on June 20, 2009, 05:44:35 AM
Hey John,

Two words: Perry Carb. H^^

Cheers,
Jim

Well a carb off of a OS Max 65 FP seems to run better than the ST carb. Plenty of power and good mid range. It was to small but I was able to wrap it with copper shim tape till it fit.  Hower, the remote needle make it hard to start.  With the cawling on you can't choke it to get the fuel line between the NVA and the carb primed.  Anyway Jim, I had stated that I couldn't find a Perry carb but I had not looked on the Big Board.  The Perry carb will fix this beast.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270413063982&viewitem=&sspagename=STRK%3AMEWAX%3AIT&salenotsupported#ShippingPayment.

The question is whitch one should I get?  The ST 90 carb measures .590 x .350.  I could stand less power so I am looking at the #1501  (.590 x .312) or the 4500 (.590 x .340).  By the way the .350 I measured is the diameter at the spray bar.  I am thinking the smaller carb may be what I am looking for.  What say you all?   ???  H^^
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Offline Ralph Wenzel (d)

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Re: ST G90
« Reply #14 on: June 25, 2009, 09:45:50 PM »
As far as choking the beast, John, if you're running with muffler pressure, just cover the muffler outlet with your thumb and turn the engine over a few times. You'll still have to experiment a bit to determine just how many revolutions.

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Ralph Wenzel
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Offline John Rist

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Re: ST G90
« Reply #15 on: June 26, 2009, 12:14:24 PM »
The Perry carb is on the way.  Will post a full report when I get it installed.
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John Rist
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Offline John Rist

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Re: ST G90
« Reply #16 on: June 29, 2009, 07:34:42 PM »
Quote from: Jim Oliver on June 20, 2009, 05:44:35 AM
Hey John,

Two words: Perry Carb. H^^

Cheers,
Jim
Jim,
You are spot on my good man.  Perry Carb came in today from the first run things were better. I now have it dialed in and ST 90 runs great!  Thakks!!!! #^  #^  #^  H^^
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Offline Dominic Bayani

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Re: ST G90
« Reply #17 on: September 04, 2009, 11:52:12 AM »
Quote from: John Rist on June 29, 2009, 07:34:42 PM
Jim,
You are spot on my good man.  Perry Carb came in today from the first run things were better. I now have it dialed in and ST 90 runs great!  Thakks!!!! #^  #^  #^  H^^


Which Perry Carb did you choose?

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