Subject: RE: Why does the vented gas cap not work?
Author: Bruce Giller
Date: Feb 15, 2005, 7:04 AM
Post ID: 1718394643
Greg,
I'd think that a higher pressure in the gas tank would lead to fuel
being pressurized. Maybe to the extent that it "overpowers" the float and
you get an over rich mixture.
If you get a vacuum in the tank, then you get fuel starvation; this
condition seems just the opposite.
Bruce
'72 Eldo
-----Original Message-----
From: Greg Bender [mailto:gr-@thisoldtractor.com]
Sent: Tuesday, February 15, 2005 9:41 AM
To: Loopfram-@topica.com
Subject: Why does the vented gas cap not work?
OK, I'm sitting at my desk with two identical gas caps in front of me.
They are both from Dennis Kirk and are of the vented nature. Each has a
nifty one way check valve that allows air in, but does not allow gas or
air out. It is easy to test just by sucking and blowing to see how it
works.
So, what's the big deal?
Well, too much pressure inside the tank will cause the engine to faulter
until the pressure is released. The engine begins to act like it is
starved of fuel and I can hear the valves start pinging more. This has
happened to me many times while riding and the cure is always to loosen
the gas cap and allow the excess pressure to escape.
I am certain that the cause is too much pressure building up inside the
tank because I've stopped (more than once) and put my fingers under the
cap while unscrewing it...I can definitely feel air escaping. This
effect is also in line with the design of the gas cap, to let air in,
but neither gas nor air back out.
I believe that temperature changes (cold gas being warmed by the engine
and ambient temperature change increases) cause the gas (and the air
inside the tank) to expand more quickly than the vacuum caused by the
removal of gas is able to equilize the pressure.
Now, I'm going to drill some holes in one (as per John Ulrich's advice).
But, my question is this: Why does too much pressure inside the gas tank
cause the engine to faulter in the first place? Is it forcing something
to close off inside the carbs?
Regards,
Greg Bender
1971 Ambassador
2000 Quota