Walt a fellow RV-12 owner showed me this great upgrade.
Bing 64 carburetors used on my RV-12 with the 912ULS engines use atmospheric pressure sensing via the vent lines to control a diaphragm that moves a needle within a jet to adjust for rich or lean. These vent line/tubes need to sense the same atmospheric pressure that is at the air cleaner intake of the carburetor. Rotax designed an air box that connects both carburetors and has ports for the vent tubes. Since we don’t use the airbox on the RV-12 the vent lines are recommended to be placed under the float bail wires and terminated.
“WALT” wrote – With all the weird air flows possible inside the cowl I suspected that these vent tubes were sensing everything but stable air pressure. I put pressure sensors at the ends of both vent tubes and went flying. I was not surprised to see the pressures at both tubes varying and almost always different from each other. This meant the carburetors were being adjusted rich and lean throughout the flight.
THE FIX: I used longer Tygon F-4040-A tubes and put them through the back of their respective air cleaners. I inserted a piece of 1/8″ stainless tubing bent to 90 degrees for the turn at the back of the air cleaner and a solid area so I could add a zip tie without crushing the tubing.. The zip tie is used to prevent the vent tube from going to far into the air cleaner. I melted the end of the tube and put 4 side holes with a hot wire for venting.
I did basically the same as Walt did above, but I found drilling the small holes in the tube was easier and cleaner. I did melt the tube end together, I guess Walt did this to maybe stop the stainless steel tube from being sucked through into the carburetor. He didn’t say, but I did it, I also used black silicone around where the tube enters the air filter.
I routed my Tygon tubes through a small cut piece of aluminum tube, so the Tygon tubes had a place I could direct the route and tie down without crushing.
DID THIS HELP – Yes, I’ve never had my engine run this smooth and it seems easier to sync the carburetors or it could be I also rebuilt both carburetors during this new vent line install.
I think balancing the vent sensing tubes is a plus!
