Ric

My Feedback: (4)

I've heard that some people are soldering a brass tube to the atmospheric vent on walbro carbs, then routing the tube opening to the mouth of the carb (3W?). This is supposed to eliminate or equalize any high or low preessure zones within a cowl that would cause tuning/ mixture problems at various airpeeds. Has anyone else heard of this? What do you think?
Thanks,
Ric

ZM2000

My Feedback: (1)

Yes that is true except where to position the open end of the brass tube.

Some suggest that it should be near the air suction of the carb so that the same pressure is applied to the diaphram as entering the carb while some suggest that it should end up inside the aeroplane behind the firewall where the natural pressure should apply without any disturbance.

-ZM

Antique

My Feedback: (4)

If the carb is facing directly forward into the airstream the tube in the venturi thing works..If the mouth of the carb is not directly facing forward the tube should be vented into some still air so that the diaphragm gets only atmospheric pressure..There are very few carbs facing directly forward..The drone engines we use in our race planes were originally pushers with the carbs facing the front of the plane..The carbs had small tubes at a 45 degree angle in the carb bodies and into the cover over the diaphragm..There were no other vents in the cover, so the same pressure going into the venturi was on the diaphragm..It worked well...When we re mounted the carbs with venturis straight up and down on the cylinders the tubes had to be removed and a vent put into the cover....Due to the offset of the cylinders one carb was completely enclosed the the other stuck slightly out of the cowl. Worked fine, the plane went 234 mph straight and level....