RV-12 Fuel Cap Vent

[JPalese]

Does anyone have the drawings of how to vent the fuel cap?

My recollection is it used to be in KAI 37 but I can't find it there any longer.

[rgmwa]

The cap itself is no longer vented. There is a separate vent line from the tank as per KAI 37.

[scottmillhouse]

Drill the vent hole anyway in the cap. You need it with a full tank on takeoff. Obviously fuel blocks the new vent and fuel pressure goes down which is kind of scary on a take off. It will stabilize when you level out and burn fuel. There is no problem with the cap vented too.

[tomkk]

Originally Posted by scottmillhouse

Drill the vent hole anyway in the cap. You need it with a full tank on takeoff. Obviously fuel blocks the new vent and fuel pressure goes down which is kind of scary on a take off. It will stabilize when you level out and burn fuel. There is no problem with the cap vented too.

First time I've heard of this - I've never seen ithappen in my -12 ...

[MMiller]

John, you remember correctly, but the step was removed with the 12/27/2013 revision that added a fuel tank vent line.

Here is screenshot of the relevant instructions from a 2011 version of section 37.

[Mich48041]

Even if the fuel tank is so full that gasoline fills the vent line, the fuel pressure at the bottom of the tank will be higher than when the tank is not quite so full. That statement can be proven with a manometer.

[DBRuth]

I drilled the vent hole slightly smaller than the plans called for, tap drill size for a 4-40 tap. Then I tapped both holes. If you don't like the results of the new vented cap, you can put a 4-40 screw in one of the holes. YMMV

[JPalese]

Originally Posted by MMiller

John, you remember correctly, but the step was removed with the 12/27/2013 revision that added a fuel tank vent line.

Here is screenshot of the relevant instructions from a 2011 version of section 37.

Thanks to all for your replies.

[scottmillhouse]

On drilling the vent anyway. Based upon my experience, ever time I filled my tank to the filler neck and fuel came out the vent, the next flight I would have low fuel pressure (1-3 psi) on climb out. A year and a half and about 170 hours since I drilled the hole in the cap, I have not had a low fuel pressure problem. I see no down side on having the cap also vented unless your plane is out in the rain without a canopy cover. Also it is a back up in case the dirt dabblers like your fuselage vent.

[RFSchaller]

The only time I?ve had insect problems was on another home built I had. Something took up residence in my pitot tube. Became pretty obvious on takeoff roll when the airspeed never cam alive. It was a pain to disassemble the hose fitting and blow out he tube because I had a hard time getting to fitting tomreseal.