CARB VENT LINES

Hello Steven

The vent tubes are atmospheric vent lines.  If they are fully obstructed the engine will die as soon as the pressure on top of the fuel in the float bowl is no longer maintained.  Too much pressure (such as ram air) the engine will run rich.  Too little air (such as the vent line run into a low pressure zone) the engine will run lean.  Fully blocked, it will stop very soon.  

Ideally the air in the float bowl must be the same as the altitude density you are flying at.  

Cheers

Im trying to solve some engine roughness that seems to happen only at altitude.  Pulling on the choke seems to cure it.  I found the vent lines, esp on the right to be pinched where they run between the bowl clip and the bowl.  I am going to fly this morning to see if that helps.

If activating the choke fixes it then for sure you are lean.  (the vent is not breathing correctly) 

Good luck, hope that fixes it for you.

Cheers

Those vent lines always look a little pinched when run through the bowl clip, but they would have to be pinched nearly all the way shut for it to become a real issue.  Where do your vent lines actually terminate?

Vent lines should go into your airbox if you have one.  The airbox then should also have a small drain at each side should any liquid accumulate.  I know of some people who run them into the air filter however this is not the best idea if a float should ever stick on you and cause some overflow from the vent.  What is important is to check that the area where they are breathing from is static air, not ram air or low pressure.  This is why the lines should not be extended long distances as they need to breathe the same as the carb.  Bing just puts them under the wire hold down for the float bowl and that is fine as long as the area there is the same pressure as your altitude.  (use a drip tray under the carbs as called out in the installation manual) Old lines can get soft sometimes and the ends begin to seal up by the vinyl line sticking together.  If they are exposed to a lot of UV light, as in an open engine installation, they get rock hard.  I always like to change them every 2 years for those reasons. 

Cheers