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Author Topic: ST G21 on suction  (Read 872 times)

Offline dave siegler

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ST G21 on suction
« on: October 13, 2020, 05:42:17 AM »
Forgive me, I only had foxes as a kid.  I was gifted 2 St G21's ball bearing and want to run one of them on suction.  The Choke area is huge (3/8 ") and they don't draw fuel.  The location of the spray bar makes is difficult to choke it down with an insert.

How did you do it back in the day? It The back plate is tapped for pressure, but  I'd rather not go that route.
Dave Siegler
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Online Paul Smith

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Re: ST G21 on suction
« Reply #1 on: October 13, 2020, 07:19:58 AM »
Those G21's came with a suction venturi of about .125" diameter.  It had a sprinkler ring designed to work with rear venturi.  There were, maybe 6 or 8 little sprinkler holes around the circumference.  They worked very well.

I think your could make a nylon or aluminum venturi with just one hole going back to the spray hole and it would probably work. 
Paul Smith

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Re: ST G21 on suction
« Reply #2 on: October 13, 2020, 01:43:27 PM »
Are you going to fly a combat job on suction? Do either of the engines have MK2 on the mounting lug?


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Offline dave siegler

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Re: ST G21 on suction
« Reply #3 on: October 13, 2020, 07:38:43 PM »
Are you going to fly a combat job on suction? Do either of the engines have MK2 on the mounting lug?


Motorman 8)

Do not see MK2 on lugs.  Was given several old slow airplanes (mongoose 1, 2, and spiders) set up with hard tanks and drilled for ST 35's.  I want to use them up.

Paul, thanks  I make a reducer.  .25
Dave Siegler
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Re: ST G21 on suction
« Reply #4 on: October 13, 2020, 11:18:37 PM »
You'll have to go smaller than .250. If memory serves .1875" max and that's a nicely made perifrial jet venturi.

I might have what you need in my SuperTigre stuff. 


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Offline dave siegler

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Re: ST G21 on suction
« Reply #5 on: October 14, 2020, 05:17:18 AM »
You'll have to go smaller than .250. If memory serves .1875" max and that's a nicely made perifrial jet venturi.

I might have what you need in my SuperTigre stuff. 


Motorman 8)

let me know what you need to get for it. 
Dave Siegler
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EAA 1231299 UAS Certificate Number FA39HY9ML7  Member of the Milwaukee Circlemasters. A Gold Leader Club for over 25 years!  http://www.circlemasters.com/

Online Paul Smith

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Re: ST G21 on suction
« Reply #6 on: October 14, 2020, 03:56:19 PM »
I just now measured a couple of them.  The factory size is .157".  I bored one out and later had to plug it down to make in run.

In the early 1960's FAST was the only kind of combat and nobody had any need for suction.  Then our local rules forbad ball bearings and flat top pistons in Slow. So the use of G21's and C's in slow was not possible.  That was the day of the Greenhead and the Enya.
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Re: ST G21 on suction
« Reply #7 on: October 14, 2020, 06:30:26 PM »
Just found an OEM suction venturi for the G21 35 with original gasket. Choke size is .153" It needs a little cleaning up but is in good shape. $10 shipped PM for details

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Online Paul Smith

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Re: ST G21 on suction
« Reply #8 on: October 14, 2020, 06:54:11 PM »
I found a few, too.  I measured the G21 at .157" and the C at .117"

I have one that I drilled-out too big and it didn't work, so I downsized with a piece of tubing.  It worked OK then.
Paul Smith

Offline dave siegler

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Re: ST G21 on suction
« Reply #9 on: November 09, 2020, 07:14:38 AM »
I got a chance to fly it this weekend, works well, starts on 1 flip, shakes a lot and my club wants me to put a muffler on it. 

thank you
Dave Siegler
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Offline phil c

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Re: ST G21 on suction
« Reply #10 on: November 28, 2020, 09:21:59 AM »
I got a chance to fly it this weekend, works well, starts on 1 flip, shakes a lot and my club wants me to put a muffler on it. 

thank you

Since the stunt grunts mostly switched to electric they no longer like a roar.  They want just a slow, window fan amount of noise.
A muffler might help slightly with any non-flyers close by.  It's not a cure all, but any neighbor power mowing lawns might be satisified. The snobs won't be satisfied with anything louder than bird chirps!
phil Cartier


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