The Cessna 120-140 Association Forum

Hello,
I have dual-power Venturies. My attitude indicator and directional gyro are a little off. I had the AI removed and checks fine on the bench. All hoses are new and the air filter looks new. Any suggestions? Any suggested reading material to understand how this old system works?
Thank you,
Mike

Air movement through the Venturi creates a vacuum. The vacuum draws air through the filter, connecting hoses, the gyro instrument itself turning the actual gyro (picture a waterwheel arrangement here) then through the next hose and the air exits the downstream end of the Venturi tube. The airflow through the Venturi when in flight creates sufficient vacuum to spin the instrument gyro at its design speed. On the ground, even in the prop wash there is not usually enough airflow to create sufficient vacuum. The old AN type gyro instruments require a slightly different vacuum than the modern gyros. Do you have a vacuum gauge in your system? Is there an adjustable vacuum regulator in your system? If the venturi's are not delivering the proper level of vacuum it may be due to a leak in the system plumbing, or it may mean the the venturi itself is plugged up with dirt or bugs. The setup is generally pretty simple so troubleshooting should be a process of elimination. Please let us know what you find for our own education.
Bill

The venturis produce their design vacuum at 100 mph. Below that, they produce less. There are different sized venturis available. You need to figure out what you have. It is a little tricky to differentiate between a 4" venturi and a 9" venturi (the number is the inches of vacuum produced at 100 mph.) Two 4" venturis would be marginal for two modern gyros.