Wulf

My Feedback: (5)

I have a G21 .35 size with a pressure tap on oppostie side of venturi crankshaft housing-How is this tap used and can I run engine with tap plugged?
thanks for any answers

downunder

That's a pressure tapping for timed crankcase pressure which is much higher than what you get from the normal (untimed) pressure tap. It's only ever used in extreme cases like in C/L combat where no venturi is used for maximum possible airflow (and no suction whatsoever). If the hole is plugged then you can run the engine as normal with either a venturi or RC carb.

Wulf

My Feedback: (5)

OK ,I see,so the aluminum insert venturi is removable when the needle is removed---So all I have to do is plug tap and leave venturi as is to run regular C/L ,correct? And thankyou for the comeback-this engine was given to me by a friend whose uncle brought it back from Italy in mid 50`s .It has never been run-He was going to use it for combat to beat our McCoys,K&B 29Rs and Foxes-,at least that were his intentions long ago----
thanks Again--

dennis

My Feedback: (90)

They command a pretty good price on the bay and among collectors especially if yours is new or near new and not buggered in any way. The ST35 did break shafts when pushed so be careful as parts are really tough to come by.
Dennis

Wulf

My Feedback: (5)

Yea, shes new I think he may have mounted it once to bench run it -It did`nt run good because he did`nt know what the pressure tap was for back in 50`s--So he probably left it open and just stored engine after that attemp---Engine is a little dull from age but it`s new other then that-
Thanks for your help-I appreciate it--

MrMotor

The G 21 suction venturi has only about a 1/8 inch hole through it. If you can see the brass spray bar in the venturi and it has a large hole in it it will have to be run on some sort of pressure unless you find a suction venturi. When the engines were new they came with the pressure fitting in a bag with a screw plugging the hole in the case and with the suction venturi installed. Also in the plastic bag were the pressure venturi and the needle valve.

Wulf

My Feedback: (5)

I just checked the venturi size with a caliper and it is .285 in dia, so I guess it can`t be run without pressure.What type of fuel setup would you suggest to run this ole gal?

Wulf

My Feedback: (5)

Can I buy a non-pressure venturi anywhere or get the exact sizes to make one -anybody?

MrMotor

trascal makes a set of venturies to fit the G-21 series of C/L engines, but I don't have any contact information on him at this moment. Maybe someone else has his contact information.

Wulf

My Feedback: (5)

This engine has a very thin V on G21 side(inlet side)(size of pencil line)-I noticed the engines on E-bay(G21s) have a thick V

gcb

Wulf,

Supertigre G21-35's came with two or three inserts. Mine came with a screw (plug) in the pressure location and a pressure fitting and needle valve in a plastic bag.

Supertigre made many changes in a short time without changing designations. Mine has a screwed-on crank housing so you could switch exhaust to either side. The next generation of G21-35's had the crankshaft housing molded onto the crankcase.

Here is a pic of the G21-35 along with a G20-15. The 35 has a single rear BB and the 15 has two BB's.

George

Wulf

My Feedback: (5)

Yea,mine is like the one in front in photo--How is the pressure hooked up to tank?and can my engine be run with pressure fitting plugged without any other changes?

Wulf

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Her`se a picture of mine that I just took in front of me--

MrMotor

Your engine may run fine on suction without pressure with the spray bar running through the middle of it. I didn't realise your engine was that old being from before the late 60s. I have a new in box engine here from the late 60s that doesn't have the spray bar running through the center of the venturi. The needle valve is offset and is only visible when the pressure venturi is installed. The pressure venturi is about .394 inches with the needle valve only blocking a small portion of it on one side.

Wulf

My Feedback: (5)

The needle on mine runs right thru the center of venturi.TheID of venturi is .285 and you can see a space about .093 on either side of spraybar--I said earlier that engine was bought by my friends uncle ,actually I just found out that his Grandfater bought it in Italy in 1958 when he was visiting there and brought it back to the US after his visit--

downunder

As dennis said, you'd probably get a very good price for it on the bay especially seeing it's barely been run and it's a very early version. But if you want to use it then you'll need a fuel with at least 20% castor for that iron piston. From the size of your venturi and the spray bar size (about .1" ) I'd think it should work OK on straight suction but check that the hole in the spray bar is pointing slightly down from the straight across position. The area of the venturi (.285-spray bar area) is a bit on the large side for slowish revs so it'd probably work best with a prop around 9x6. For the pressure tap, just blank it off with a short length of fuel tube and a plug in the end of it unless you still have the original blanking plug.

When the pressure tap is used you just run a length of tubing from it to the tank the same as you do with muffler pressure. The trick here though is that now it acts like a fuel pump and even just turning the engine over slowly will pump huge amounts of fuel straight through the needle valve and it'll flood. It's called timed pressure because the crankshaft port now determines when the outlet gets uncovered which happens to be when the piston is coming down to compress the new mixture.

To start them with pressure you'd give the engine a prime with either the pressure or feed line pinched closed, start it up on the prime and as soon as it's running release whichever line you'd pinched closed. Then hope your needle setting was about right. The needle will be very sensitive and only open maybe a half turn because of all the pressure.

NM2K

My Feedback: (14)

Yes, in fact, unless you are hooking that extra tap up to pressurize your fuel tank, you MUST seal it off to the atmosphere. The fitting is for timed pressure.

You possess one of the sweetest engines ST ever made. I still miss the one that I had in the early eighties (R/C with no pressure tap). It was as powerful as the best forties available back then (sport engines) and it was light, with wonderful throttling characteristics. It could whip the apple sauce out of any plain bearing engine up to a .45 with ease and was super pleasant to operate.

Yours sounds like a control line engine. Perhaps an old combat engine. What say?

Wulf

My Feedback: (5)

Yes ,this engine is a C/L engine and was originally intended for C/L combat ,say on the old Quicker Combat ,one of which I have hanging on my PC room wall-

Wulf

My Feedback: (5)

I ran the G21-.35 on test stand this morning and used the pressure system--I started out at 1/4 turn needle and ended up at 1 1/4 turns to have engine run a little on rich side--Engine is a bear and has awesome power--It seems to lean as the tank drains is this normal?I ran about 6oz Fox fuel(28% castor ) thru the ole devil--This is the first time this engine has been run.It will probably set on collection shelf now ,unless my friend who gave it to me wants to use it and do some combat with me .In that case he sure deserves to put it back in service seeing he has`nt flown combat or anything since 1958---
--Thanks to all for your expert help and very informative advice-