Function or purpose of a carburetor vent tube(s)

[Calif240]

I couldn't find this in a previous topic, and I'm seeing lots of different opinions on other sites... What is the purpose of the air vents on a carburetor (specifically my Holley 750 Double Pumper)?

In looking at the design and function of the overall system and reading several other books, it appears that the vents simply keep atmospheric pressure in the bowls so that the bowls don't get vapor lock (having negative pressure inside the bowl and restricting ability to feed fuel through circuits). The reason they are raised is so that during maneuvering, fuel doesn't slosh out of the bowls. Can someone tell me if I'm missing any other purpose?

That being said, my Holley is on a 1.25 drop base and the air vents end up hitting the top of the air cleaner. I was looking at modifying the air cleaner top so that the air vents actually go through the cleaner top with small grommets. If they are only "equalizing" pressure between fuel bowls and the atmosphere, then they aren't really sucking any air in. I've seen mods for off-road carbs where they connect the vents with an "arch" with holes in it, so I'm fairly sure that as long as you keep fuel from sloshing and allow the pressure to equalize, that it would function properly.

Can anyone lend any other perspective or knowledge?

Thanks,
Terry

[fy469rtse]

Good point terry, it's one I think Ron could answer, you that tight for room under the hood, I had to trim the ring with grinder that sits on top of carby, used a thin cutting blade, it got me another ten millimetres clearance, it helped get everything under the damn ugly scoop ,

[Cooter]

You pretty much have the purpose. Only thing you need now is a reason they are high as they are. The reason 4x4 and drag cars run a loop over is under hard excell. Fuel will sloshing out and drown engine.

[BSB67]

Can you post a pic of your carb?  Holley has used several different styles over the years.  Depending how much interference you have, you might be able to trim and angle the vent tube to get what you need without hurting anything.  Keep in mind that at some point your air cleaner lid will be too close to the top of the carb as well.  I think the minimum clearance is about 1 1/4" going from memory.

The vent tubes on the modified 850 DP (w/ black ring) in the pictures are the shortest I have and have had no issue.  However, this carb also has the plastic vent extension tube inside the fuel bowl as well.

[Calif240]

Thanks guys... Yes it's a stock 750 Double Pumper. My only worry with trimming them was them being too close to the cleaner lid after trimming. They would be less than 1/2" I think unless I ground them down all the way.
Terry

[Calif240]

I'll try to get a pic tonight too...

[firefighter3931]

Hi Terry, your description is correct....those are fuel bowl vent tubes designed to keep the fuel inside the bowls at atmospheric pressure. Inside the bowl there are plastic baffles that keep fuel from sloshing out of the tubes.

You can trim the tubes down as needed but trim just enough to clear the air cleaner lid...maybe 1/4 in below to make sure they're not blocked off. If your carb has a choke horn you will also want to consider using a flow thru filter top. Having the choke horn up tight against a solid filter lid will restrict airflow to the carb and you don't want that.


Ron

[BSB67]

Thanks guys... Yes it's a stock 750 Double Pumper. My only worry with trimming them was them being too close to the cleaner lid after trimming. They would be less than 1/2" I think unless I ground them down all the way.
Terry

A 1/4" is plenty.  If the angle of the vent tube angle is different than the lid, then they can touch.

[Calif240]

Awesome... I'll just do that then. The top I have is one I K&N extreme airflow tops.

Appreciate the advice guys!

Terry