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Permalink Reply by John Austin 13 hours ago

NOTE!  I just edited this post because I had backwards the way the Cessna cap works!  Sorry - it was a long time ago and I "misremembered"  !!!

Steve - Are the Cessna caps vented to allow air into the tank AND allow vapor pressure out?   The reason I ask is years ago, I had the original Zenith caps on my STOL 750 and they leaked.  I put initially Cessna caps on and they sealed great!  HOWEVER, the problem was Cessna's have separately vented fuel tanks and my STOL 750 did not!  (The Cessna caps I had have a flapper valve that keeps fuel (and fuel vapor pressure) from sloshing out.  In the STOL 750, with no separate vent, that means if the fuel expands or vapor expands with summer heat, etc., there's nowhere for the pressure to go and it pressurizes the fuel tank.  I could hear a big "whoosh" sometimes when I opened the tank. Don't know if that's enough pressure to rupture the tank, but probably could play havoc with fuel flow! That's when I went to the modified Zenith caps with a snorkel and 45 degree opening facing forward into the airstream - in theory, relatively "neutral" venting, allowing tank pressure out and outside air in as necessary.

I "think" the Cruzers' have independent vents, but don't know about the 701.  Also don't know if there are Cessna caps that are vented to allow air in and vapor pressure out?  So, just be sure of your venting status.

John

N750A

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Permalink Reply by Michael radtke 12 hours ago

Hi john,

     great reply!   Sure wouldnt want steve to stop his fuel flow. i bought some aftermarket caps (vented)  and made

some snorkles for them. havent tried them yet, but was hoping for a few psi and even fuel flow between the tanks.

also i dont know of caps sold by zenith w snorkles.

that is homebrew aftermarket.

michael.

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Permalink Reply by John Austin 11 hours ago

Thanks Michael, but I had it all wrong and have edited the reply!  How embarrassing! Anyway, the same principle applies in that one has to be sure they have tank venting either way to accommodate both positive and negative tank pressures!

In any event, here's how I modified the Zenith fuel caps -  Modified "Snorkel" Vent Cap

I did a further modification by drilling 2 offset small holes in the lower end of the snorkel that open in the space between the exterior cap and the interior cap.  I sealed off with JB Weld  the bottom end of the snorkel and opened the original vents in the periphery of the cap so as to allow pressure flow (either way!) into the space between the exterior and interior cap.

Clear as mud, rigiht?

John

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