Vent lines


Premier Member
IBA Member
I'm surprised there isn't more discussion here about fuel cell vent lines. I have one of those aluminum RCI fuel cells with the ball-style tipover valve in the vent. I've routed my vent line from the tipover valve down and aft, through a pigtail loop, and then exiting behind the license plate. See the attached photos.

My question is this: Since the tipover valve is already present in the vent, do I really need to pigtail the vent line? I'm worried that if some liquid gets trapped in the pigtail loop it will slow down or block proper venting. I'm thinking I can eliminate the loop and just have the vent line dump behind the license plate.


1. Safety
2. Passing Butt Lite tech inspection
3. Seamless function



Premier Member
IBR Finisher
My experience with vent lines on aux tanks, with and w/o check valves and pig tails has been that most venting issues are caused by a deviation in the vent line or fluid trapped in the line. I once had a check valve on a fuel cell that, if I slightly over filled the aux tank, then hit the twisties, it would slosh some fuel on top of the check valve. Then it wouldn't vent AND fuel wouldn't flow either. AAAAAAHHHGG! Do you know how sexy it looks when you're stopped by the side of the road blowing into your aux tank's fill neck while you plug other lines with your fingers? Not very.

I suggest you ditch the pig tail. I've had really good results with the modern roll over valves that are part of the vent line. Even under 'bike nap' conditions. They don't trap fuel, but do block it when they should, and continue to vent afterwards when you get the bike upright again. The pig tail is just asking for fuel to get trapped after a nap and then no venting will occur thru that line, (just as you're suspecting).

I would ask the Butt Lite Tech people, just to be sure with their rules. For the IBR we were not looking for pigtails, but we were looking for vent lines to exit clearly past or well to the side of the tire. And also for vented cells, not vented caps!

The most common things I caught while doing tech were no vent line, improper vent line routing, (dumping on tire or too close to tire), and cells not mounted solidly.


Premier Member
IBR Finisher
... Do you know how sexy it looks when you're stopped by the side of the road blowing into your aux tank's fill neck while you plug other lines with your fingers? Not very.
Been there, done that.

I've had several bikes with a rollover/check valve fitted to the upper corner of the tank and attached to the vent line which I pigtail in the vertical plane. My undertstanding is that when the bike is upright the ball bearing sits at the bottom of the valve minding its own business and if my bike has a lie down the ball bearing rolls to the 'top' of the valve and shuts the flow.

I have suffered the stupidity, once, to fill my bike's two tanks and then park in the sun. I know, I, know. On that day the fuel expanded, bypassed the rollover valve, meandered up and circled around in my pig tail and dripped away merrily onto the ground behind my bike. The pig tail would have been 'full' or at least as full as a drainage 's'bend gets, depending on graviy, surface tension and the pressure gradient in the vent line allowed.

There isn't much volume in the pig tail itself, but I have always presumed that when riding away from a fuel stop fully topped up that accelerating causes a bit of flow backward out of the vent line. Some gets trapped in the pigtail and maybe some squirts out, but the fuel in the pigtail eventually gets sucked back up the line and into the aux tank. Some may evaporate, I suppose, just thinking about it now...

I reckon Eric's inline vent sounds better, but I would keep the pigtail for a tank-affixed rollover valve because they don't really do much unless your bike falls over.
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Premier Member
IBA Member
Thank you for the responses. I had a feeling the pigtail is unnecessary, but I don't always think of everything.