What is the name and purpose of this carburetor part?

[jfman]

 

What is the purpose of the little diaphragm on the side of the carb body? What is its correct name (Yamaha parts fiche calls it a diaphragm set)

 

Also if it fails or if the holes leading to it are plugged, what will be the symptoms?

 

Has this part been known to fail on ventures.

[BlueSky]

Yes, they somtimes fail. Some have used stuff like Yamabond to repair holes in the diaphragms. Vacuum against this diaphragm moves a fuel metering rod in and out of the jet that feeds the engine above idle. If there is a hole in the diaphragm or the metering rod is stuck, the engine will not run right above idle speed. #not a mechanic

[Prairiehammer]

https://www.venturerider.org/forum/attachment.php?attachmentid=118151

 

What is the purpose of the little diaphragm on the side of the carb body? What is its correct name (Yamaha parts fiche calls it a diaphragm set)

 

Also if it fails or if the holes leading to it are plugged, what will be the symptoms?

 

Has this part been known to fail on ventures.

 

It is variously called the "coasting enrichment valve" or "air cut-off valve" or "transient enrichment valve".

When the throttle is closed, there would a transient lean condition, causing popping upon deceleration (coasting). This device enrichens the circuit to prevent popping.

The diaphragm can be damaged by aggressive carb cleaners and the diaphragm can also develop holes. The result will be a vacuum leak and popping.

Air Cut-off how it works.docx

[jfman]

Yes, they somtimes fail. Some have used stuff like Yamabond to repair holes in the diaphragms. Vacuum against this diaphragm moves a fuel metering rod in and out of the jet that feeds the engine above idle. If there is a hole in the diaphragm or the metering rod is stuck, the engine will not run right above idle speed. #not a mechanic

 

I think you have it mixed up with the diaphragm from the main slide/needle.

[WIZ]

If you pull the carbs apart to clean them, the only way to get cleaner and air through the passages leading to the air cutoff circuit is to depress the small ball that the rod on the diaphragm presses on. I just used the old cutoff diaphragm that I was replacing to depress it when I cleaned mine.

 

Ron