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Vented gas cap
Posted by: Andrew ()
Date: August 09, 2011 11:16AM

I replaced the gas cap on my van with a new one from a Corvair vendor. It's vented with a plastic check valve on the underside. I've noticed that When I remove the cap I can feel and hear a surge of air going into the tank like the tank was under vacuum. The cap fits very tightly and is hard to turn.

Is this normal?

This doesn't seem right to me. I've had some vaporlock problems recently and I'm afraid that if the pump pulls a vacuum on the tank, it that much harder for the fuel pump to do its job properly.

I still have the old cap. I only replaced it because sometimes fuel sloshes out when the tank is full and you make a hard right turn. In fact the old cap is actually nice shiny chrome, not the cheap cad/zinc plating like the new one sad smiley

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Re: Vented gas cap
Posted by: MattNall ()
Date: August 09, 2011 11:18AM

That is not good!

Looks like the valve is backwards...

Should let air IN.. stop liquid from getting out..

You should be able to modify it..

MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
...............110-PG.................................................Webered-Turbo

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Re: Vented gas cap
Posted by: lgoodwin ()
Date: August 09, 2011 11:30AM

I've been thinking the locking cap on my 62 is not venting. When I remove it, air rushes in, just like on yours.

BTW, I accidentally squashed my gas cap a couple days ago. Set it in the drain channel for the hood while getting out the MMO, then tried to shut the hood. Still works, the ears just got bent over. Happily, the hood is ok!

<table align='center' width='0'>
<tr>
<td><img src='[www.automation-scs.com]; alt='1962 Corvair 900 sedan' width='190' /></td>
<td><img src='[corvaircenter.com]; alt='1967 Corvair Monza 140/4-speed' width='190' /></td>
<td><img src='[corvaircenter.com]; alt='1968 Corvair 500 coupe' width='190' /></td>
<td><img src='[www.automation-scs.com]; alt='1983 Mexican Mustang 5.0' width='190' /></td>
<td><a href='[www.automation-scs.com&]; target='_blank'></a></td>
</tr>
<tr>
<td align='center'><font size='1'>1962 900 sedan needs a home</font></td>
<td align='center'><font size='1'>1967 Monza 140/4-spd</font></td>
<td align='center'><font size='1'>1968 500 coupe (now my son's)</font></td>
<td align='center'><font size='1'>1983 Mexican Mustang 5.0</font></td>
</tr>
</table>

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Re: Vented gas cap
Posted by: Andrew ()
Date: August 09, 2011 11:33AM

Here's a picture of it. It's a Motorad part. There are the two tabs that lock the cap into the filler neck, then 90* apart are 2 tabs that lock down a washer that traps the check valve in the cap. I'm sure I can bend those back to take the valve out, but I'm sure that would void any warranty that I would have on it. Besides, I'm tired of getting the wrong parts from that vendor that has "perfect reproductions" of parts that are "always missing" or "always broken." I thought part of the reason we paid a premium for replacement parts from this vendor was because they already vetted the parts for us.

The rubber washer seal is already flipped up from being being in contact with gas too.

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Re: Vented gas cap
Posted by: corventure Dave ()
Date: August 09, 2011 11:43AM

Gas cap must be vented, locking style or not.
Story...
My father, when he was still working in the late 1960's or so, had to make supply pickup's at the L.A. Airport. After vacation and returning to work, he made the 120 mile trip with the company van.. a Dodge. The tank was left full as was company procedure. He made it down there and was amazed that the gas gauge was still reading near full. Normally it used half a tank. He topped it off anyway and drove back to work... another 120 miles but it only took a couple gallons. He figured the gauge or sender had gone bad because it stilll read almost full when he went to top it off. Removing the cap, there was a "lot" of suction. Topping off the 20+ gallon tank took only another 2 gallons.
Well.... that fuel pump had sucked the gas tank flat because someone had lost the gas cap and put a sealed cap on that Dodge Van!!! That's one heck of a good fuel pump they had there!

Corventure Dave

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Re: Vented gas cap
Posted by: 66cv8 ()
Date: August 09, 2011 01:58PM

I'm gonna go out on a limb here and state that most of you guys that have vaporlock and/or fuel pump issues are using the wrong caps or have clogged vent holes.

If your cap ain't letting atmosphere in you ain't gonna get fuel to the carbs.

Joe

66 500 Crown 350/4 on road
66 Corsa Convertible 140/4 rough
69 Monza Convertible 140/4 in progress (original 95/3)
Nu Joisey

" A LITTLE KNOWLEDGE IS A DANGEROUS THING"

"STUPID is the NEW SMART"

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Re: Vented gas cap
Posted by: Richard M ()
Date: August 09, 2011 06:46PM

That Andrew is a pressure/vacuum style fuel cap. The spring valve works both ways. It will vent, but only above or below a certain amount of pressure or vacuum. Not free flowing.

Richard, Gulf Coast Auto Parts
Sarasota, FL

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Re: Vented gas cap
Posted by: stitch ()
Date: August 09, 2011 06:55PM

I wasn't sure if my after-market locking cap was venting or not. Took a 1/64 bit and drilled right through the key hole to the other side. Didn't effect the tumblers or anything. Worked out good. (Took cap OFF first! ggg)

"If you can't fix it with a Hammer, you have an Electrical problem."
Stitch...
Schertz, Texas.
(Smallish town/burg 17 mi. NE of San Antonio)

'Ms Penny'
!967.. 4th body
8th off the line

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Re: Vented gas cap
Posted by: stitch ()
Date: November 23, 2012 06:40AM

Well, now I'm wondrin' if I shouldn't have drilled the 'make-shift' vent. If I go a week or longer, takes a while to fire. (runs great after that!) Methinks that because it is vented BOTH ways, into tank and release from tank, that after sitting for awhile, the gas starts crawling from fuel pump back to tank. Could this be a problem for the delayed starting? Or is this just a normal fact? (BTW, not interested in electric fuel pump. Won't be converted!)

"If you can't fix it with a Hammer, you have an Electrical problem."
Stitch...
Schertz, Texas.
(Smallish town/burg 17 mi. NE of San Antonio)

'Ms Penny'
!967.. 4th body
8th off the line

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Re: Vented gas cap
Posted by: acardon ()
Date: November 23, 2012 01:25PM

Quote

I've noticed that When I remove the cap I can feel and hear a surge of air going into the tank like the tank was under vacuum.

What you are hearing could be air coming OUT of the tank and not a vacuum because of heat expansion in August. The one way valve should work fine as designed.
There should be plenty of fuel in the carberator bowl after a week, to start the car, no matter how much ethonol.
If you have a concern if your cap is vented, turn it upside down and put some gase in it. It will run through a vented cap. Drilling a hole straight through a cap will usually result in gas sloshing out on turns. A vented cap should have baffles to slow down sloshing out. This isn't a Corvair cap but it shows the baffleing.


Don
Southlake, Texas
The Trunk Belongs in Front, Ask Any Elephant
Caution: Images in This Mirror May be Waterpumpers

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