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  • September 23, 2021, 12:08:48 PM

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Author Topic: ST G21 on suction  (Read 1422 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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Offline 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

Offline dave siegler

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Re: ST G21 on suction
« Reply #2 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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Offline dave siegler

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Re: ST G21 on suction
« Reply #3 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
NE9N extra class
AMA 720731
EAA 1231299 UAS Certificate Number FA39HY9ML7  Member of the Milwaukee Circlemasters. A Gold Leader Club for over 25 years!  http://www.circlemasters.com/

Offline Paul Smith

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Re: ST G21 on suction
« Reply #4 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.
Paul Smith

Offline Paul Smith

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Re: ST G21 on suction
« Reply #5 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 #6 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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EAA 1231299 UAS Certificate Number FA39HY9ML7  Member of the Milwaukee Circlemasters. A Gold Leader Club for over 25 years!  http://www.circlemasters.com/

Offline phil c

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Re: ST G21 on suction
« Reply #7 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

Offline phil c

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Re: ST G21 on suction
« Reply #8 on: July 10, 2021, 07:10:07 PM »
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
The current engine builders almost all went to Schneurle porting and rounded corners
on the sleeve ports.  The G.21 had a square cornered exhaust port where most of the noise comes from.  A Fox 35 is even noisier.

I'd suggest an OS 3030 muffler the mounting holes match up. It's a bit heavy but works very well on the LA40-46 engines.
Otherwise almost any muffler will knock off some of the noise.  The little chip/restrictor mufflers don't offer much reduction over an open exhaust and a really effective one tends to heat up the engine.
phil Cartier

Offline Paul Smith

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Re: ST G21 on suction
« Reply #9 on: July 12, 2021, 08:32:29 PM »
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

Engines that were designed without mufflers don't much like to have a muffler strapped-on.
You might able to ease the pain a bit by jacking up the head to compensate for the inability of the exhaust to leave the engine.  If it works, you will get a substantial increase in run time due the reduced waste of unburned fuel.
Paul Smith


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