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Venturi Location (inst vacuum, not carb)

Gordon Misch

MEMBER
Toledo, Wa (KTDO)
I put my venturi about mid-height at the trailing edge of the boot cowl on my -12, and it never has generated a vacuum except rarely and sporadically. The only instrument it's connected to is a turn and bank that was overhauled by Keystone immediately prior to installation. I believe locating the venturi where I did was poor judgment because I think it's likely blanked by the cowl cheek.

My question is where is the best location for it? I'm thinking up higher, near the bottom of the windshield. I'll appreciate all the advice I can get! Thanks - -

Venturi Location.jpg
 

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Thank you! The drawing doesn't specify whether that's the upper or lower engine mount bracket but it's hard to imagine it being the upper? Could someone please confirm that?
 
Aha! Thanks again! TJ's pic shows it up at the top. I miss his posts.

Yes, I did put a suction gage on, and it shows zero suction in flight. But blowing air through the venturi definitely generates a vacuum.
 
And used vice grips to hold the nuts...excellent! Tie a string to the tool so it doesn't submarine if it slips off the nut. Or with one gripped if removing the fastener.

Gary
 
Thank you! The drawing doesn't specify whether that's the upper or lower engine mount bracket but it's hard to imagine it being the upper? Could someone please confirm that?

  1. the picture in drawing obviously shows lower engine mount
  2. did you clean it before installation with air to remove any blockages/mud?? in slot..

been decades since Ive reinstalled one of those...
 
Use float pump out plugs to keep bugs from nesting and muck filling the venturi. In a good wind it can turn the gyro's wheel and fill with rain or snow when parked...plug.

Gary
 
  1. the picture in drawing obviously shows lower engine mount
  2. did you clean it before installation with air to remove any blockages/mud?? in slot..

been decades since Ive reinstalled one of those...

Yep, it's clean and flows correctly thru the suction port.

That's what I thought about the drawing also, but that's still slightly behind the cowl cheek. Seems others I've seen might have been up high? But not sure. I also need to look to see if I can find any pics of it in the same configuration as when I bought it. But that was about 1975 so pics might be elusive. It was pretty much (maybe entirely?) stock at that time.
 
Googling PA-12 images, I'm not finding many that include the venturi. However those that I do find are split evenly between up high and down low, each more or less in accordance with the PA-18 drawing that Musket posted - depending on which engine mount, upper or lower.
 
Experiment. Tape some yarn to the boot cowl in a vertical line and go fly. Look out for ... well you know flow. Use GoPro cam if you have one.

Gary
 
By chance is there a closed needle valve plumbed in the vacuum line? I've seen the venturi on an unmodified 12 up top just below the windshield. It runs a T&B. jrh
 
I don't trust them in the cold. Any cockpit moisture gets sucked out and frozen in the venturi. Dynon D3 or similar next for me.

Gary
 
Experiment. Tape some yarn to the boot cowl in a vertical line and go fly. Look out for ... well you know flow. Use GoPro cam if you have one.

Gary
Yep was thinking about that. Also thinking about fastening the venturi to a stick so I could hold it thru the window.
 
By chance is there a closed needle valve plumbed in the vacuum line? I've seen the venturi on an unmodified 12 up top just below the windshield. It runs a T&B. jrh
Nope, there is no needle valve, though it looks like the print Musket posted includes one. Thanks for the position report!
 
Buy one of these: https://www.aircraftspruce.com/pages/in/slipindicators/bank_indicat.php

And toss the Venturi and the t and b.

Save some weight, and simplified your life.

MTV
 
My advice is sell all that crap and use the money to buy a cup of coffee. Then get some of the new digital stuff!
DENNY
 
I don't trust them in the cold. Any cockpit moisture gets sucked out and frozen in the venturi. Dynon D3 or similar next for me.

Gary
I hadn't thought about that issue, though I don't fly in the cold weather anymore. I already have a certified electronic horizon and a Dynon D1. But of course they're both electric, and furthermore if they disagree which one wins? That's why I am wanting one vacuum instrument.
 
I guess I'd want a panel vacuum gauge to make sure the suction was still there and that's why the needle was still straight up.

Gary
 
Then throw that other crap out!! Stick in one of the ball /bent tube things (mount it high) and be done with it. How many backups do you need?
DENNY
 
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