Venturi System not producing enough vacuum

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Venturi System not producing enough vacuum

Postby pbsroth » Thu Jul 11, 2019 9:38 pm

1952 Ces 170 B. Does anyone know who works on venturi vacuum systems in Colorado. I'm only getting 2". No Vert gyro compass or Art. Horizon. Seem that today's mechanics know very little about these systems. How do you static test the system? Has anyone replaced the venturi with a vacuum pump, and if so what system works well. Any help anyone can give me would be appreciated. Email would be better, but Forum is OK also.
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Re: Venturi System not producing enough vacuum

Postby N2625U » Thu Jul 11, 2019 10:35 pm

Check the inside of the venturi. Mine had a lot of rust in it disturbing the airflow going through it. When I smoothed it out with a round wire brush it raised my vacuum 2 inches. Make sure the small hole inside the venturi is clear. I had checked all hoses and connections and they were good. The rust in the venturi was the problem for me.
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Re: Venturi System not producing enough vacuum

Postby n2582d » Thu Jul 11, 2019 10:53 pm

Sounds to me like nothing is wrong with your system. If I read you right all that the venturi is powering is a vacuum turn and bank. These require a vacuum of 1.8" - 2.2". Normally, on systems that require a higher vacuum for the artificial horizon and directional gyro, there is a restrictor valve to reduce the vacuum to 2.0" for the turn and bank. It appears your vacuum relief valve (fig. 76-19) is correctly set to 2.0" for the T&B.
Restrictor Valve
pipe_switch.jpg (15.3 KiB) Viewed 2411 times
If the only thing the venturi is powering is the T&B you might want to consider going back to the original style electric T&B and remove the entire vacuum system.
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Re: Venturi System not producing enough vacuum

Postby lowNslow » Thu Jul 11, 2019 11:16 pm

What are your venturis rated? There are generally either 2 in.hg., 4" in.hg, or 9"in.hg. These are very simple systems and I would think any capable mechanic could figure it out. I would not go with a vacuum pump especially if you are only powering a T&B, besides the fact that venturis are more reliable then a pump.
'53 170B N3158B SN:25400
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Re: Venturi System not producing enough vacuum

Postby cessna170bdriver » Fri Jul 12, 2019 5:34 pm

I agree with Gary; if all you have is a vacuum turn and bank, then 2 inHg is all you need. To check for proper operation, just establish a standard rate turn according to your turn and bank, then time a 180 degree turn. It should be close to 1 minute.
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Re: Venturi System not producing enough vacuum

Postby hilltop170 » Fri Jul 12, 2019 8:03 pm

How long has it been since the vacuum instrument in question has been overhauled? They tend to be neglected until they don’t work anymore.
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1951 170A, N1715D, s/n 20158, O-300D
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