Vacuum/Venturi System

[cwienk]

I've looked through past posts and still have some questions. Some background, I recently purchased a 1951 170A. It was completely rebuilt over 10 years ago but hasn't flown much because the builder passed away shortly after it was completed. I've been working through a few squawks. One in the vacuum system. It is set up with two venturis, one on each side. The Directional Gyro and Artificial Horizon are connected to the system. The Turn Coordinator is an electric gyro. The suction gage only indicates about 3.5" in cruise. I've tried some adjustments on the regulator, but it hasn't seemed to make much difference yet. I pulled the regulator out and visibly could see the spring moving when turning the adjustment screw. After reading earlier posts, is there any way to confirm what "size" venturis are installed? It sounds like the 4" and 8/9" are pretty close to the same size and I haven't found any specific measurements for either. My understanding is if the venturis are 4", I might not be able to get much more vacuum pressure in the system. Any other suggestions on how to troubleshoot the system?

Thanks in advance,

Cody Wienk
N5720C

[cessna170bdriver]

I know the factory position for Venturis on B-models is both on the right side of the airplane, and I can’t imagine any reason the A-model should be any different. My B came to me with a 4” Venturi on each side, and they didn’t operate any better than yours do (and that was with no regulator at all). Switching to 8” “super” Venturis helped very little. My hypothesis is that the left side Venturi is not as well lined up with the prop wash, and hinders more than helps the one on the right.

Gary Friesen (aka N2582D) did some research and found very specific measurements for locating dual Venturis. When I had my airplane rebuilt, I had the forward side skins replaced and the Venturis relocated to the factory-specified location on the right side. Now I get 3-3.5” just on run up, and 4.5-5” soon after takeoff (with a regulator).