The Murphy Builders Site

Perhaps this is a statement of the obvious but there must be much more vent air
flowing than fuel flow for this problem to occur which means that ram air is
flowing into the J tube and out of the fuel cap vent. Anything that reduces the
flow has to reduce the error. I put separate guage and vent ports in my tanks
but I still don't like the idea of flowing air through the tank as it tends to
increase the likelihood of having a combustible fuel air ratio in the tank.
Ken

Bob Patterson wrote:

Hi Brian !

Part of the problem might be because the vent tube is pointing
forward into the slipstream. Since it is meant to be just a vent, you
could try a straight vertical tube with a couple of saw cuts on the
back side, or bend the front of the tube down into an inverted "J",
so it doesn't get pressurized by ram air.

As Wayne suggests, you might want to check to see that you are
getting enough air INTO the tanks from the caps. I drilled 1/8" holes
at the front AND back of the filler neck, above the bottom of the
cap, so rain wouldn't get in, but lots of air could ! Lack of sufficient
air through the caps is the reason for the cross tube & vent in the
first place .... (At least enough air must enter to replace the volume
of fuel used.)

.......bobp

Wayne, is there a way to redo the vent. Mine is cross vented and if I blow
into the hole the fuel gets blown out of the sight guages, so when I get low
it shows no fuel in the tanks. I have already put a restrictor in the BOTTOM
el of the sight guages and was wondering if there was something else I could
do to stop the pressure on the guage...Thanks....Brian

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You're right, Ken ! That's what I was trying to say - too many
of the vent tubes are inverted "L"'s, forcing air into the line, tank,
and guage. My suggestion was to bend them down into a "J", so there isn't
as much ram effect, or just use a straight tube with rear saw cuts to allow
some air entry. I agree - you don't want a stream of air flowing through
the tank - just enough air getting in so you don't end up with a
vacuum when fuel leaves....

......bobp

-------------------------------orig.----------------------------------------
At 05:12 PM 9/2/01 -0400, you wrote:

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Perhaps this is a statement of the obvious but there must be much more vent air
flowing than fuel flow for this problem to occur which means that ram air is
flowing into the J tube and out of the fuel cap vent. Anything that reduces the
flow has to reduce the error. I put separate guage and vent ports in my tanks
but I still don't like the idea of flowing air through the tank as it tends to
increase the likelihood of having a combustible fuel air ratio in the tank.
Ken

Bob Patterson wrote:

Hi Brian !

Part of the problem might be because the vent tube is pointing
forward into the slipstream. Since it is meant to be just a vent, you
could try a straight vertical tube with a couple of saw cuts on the
back side, or bend the front of the tube down into an inverted "J",
so it doesn't get pressurized by ram air.

As Wayne suggests, you might want to check to see that you are
getting enough air INTO the tanks from the caps. I drilled 1/8" holes
at the front AND back of the filler neck, above the bottom of the
cap, so rain wouldn't get in, but lots of air could ! Lack of sufficient
air through the caps is the reason for the cross tube & vent in the
first place .... (At least enough air must enter to replace the volume
of fuel used.)

.......bobp

Wayne, is there a way to redo the vent. Mine is cross vented and if I blow
into the hole the fuel gets blown out of the sight guages, so when I get low
it shows no fuel in the tanks. I have already put a restrictor in the BOTTOM
el of the sight guages and was wondering if there was something else I could
do to stop the pressure on the guage...Thanks....Brian

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HI Bob

I think what Ken is trying to say that if you have any kind of flow allowed
thru the tank, then the ratio of air to gas will increase which could get it
into the combustible area. If you just have ram air effect via the inverted
'L', then you will not have flow, but just the ability to replace the volume
of fuel flowing to the engine.

For example, if you have an 'L' or 'J' tube on your fuel cap plus other vent
holes drilled into the neck of the tank, this will allow continual flow thru
the tank. This lowers the saturation level of the vapour in the tank & more
fuel will volatize to replace the vapour vented thru the 'streaming vent'.
This also wastes fuel. As well, it will reduce positive or head pressure in
the tank which of course moves fuel from the tank to the engine. Neither of
these things are good.

Hope I have not misjudged the intent of your point Ken. Apologies if so!

Brian #328R

-----Original Message-----
From: murphy-rebel@dcsol.com [mailto:murphy-rebel@dcsol.com]On Behalf Of
Bob Patterson
Sent: Sunday, September 02, 2001 8:41 PM
To: murphy-rebel@dcsol.com
Subject: Re: Sight guages


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You're right, Ken ! That's what I was trying to say - too many
of the vent tubes are inverted "L"'s, forcing air into the line, tank,
and guage. My suggestion was to bend them down into a "J", so there isn't
as much ram effect, or just use a straight tube with rear saw cuts to allow
some air entry. I agree - you don't want a stream of air flowing through
the tank - just enough air getting in so you don't end up with a
vacuum when fuel leaves....

......bobp

-------------------------------orig.----------------------------------------
At 05:12 PM 9/2/01 -0400, you wrote:

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Perhaps this is a statement of the obvious but there must be much more vent

air

flowing than fuel flow for this problem to occur which means that ram air

is

flowing into the J tube and out of the fuel cap vent. Anything that reduces

the

flow has to reduce the error. I put separate guage and vent ports in my

tanks

but I still don't like the idea of flowing air through the tank as it tends

to

increase the likelihood of having a combustible fuel air ratio in the tank.
Ken

Bob Patterson wrote:

Hi Brian !

Part of the problem might be because the vent tube is pointing
forward into the slipstream. Since it is meant to be just a vent, you
could try a straight vertical tube with a couple of saw cuts on the
back side, or bend the front of the tube down into an inverted "J",
so it doesn't get pressurized by ram air.

As Wayne suggests, you might want to check to see that you are
getting enough air INTO the tanks from the caps. I drilled 1/8" holes
at the front AND back of the filler neck, above the bottom of the
cap, so rain wouldn't get in, but lots of air could ! Lack of sufficient
air through the caps is the reason for the cross tube & vent in the
first place .... (At least enough air must enter to replace the volume
of fuel used.)

.......bobp

Wayne, is there a way to redo the vent. Mine is cross vented and if I

blow

into the hole the fuel gets blown out of the sight guages, so when I get

low

it shows no fuel in the tanks. I have already put a restrictor in the

BOTTOM

el of the sight guages and was wondering if there was something else I

could

do to stop the pressure on the guage...Thanks....Brian

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Brian
Yes I think you and Bob both understood my intent. If you have a ram roof vent,
then also having large vents in the fuel filler necks will tend to increase
sight guage errors and airflow through the tank if connected according to the
manual. I think this would explain the sight guage errors that Wayne and others
reported.

So far I haven't drilled any vent holes in my fuel filler necks. I haven't flown
yet though and don't want to rely on one vent for both tanks so I will
eventually drill a small hole in the filler necks or use a J tube on both fuel
caps as well as on the tank cross vent. Or perhaps just stuffing aluminum wool
into the roof J vent as a filter and flow restrictor and following the builders
manual for the filler neck vent is a sufficient solution? Too much venting
definately beats the heck out of too little....
Ken

Ralph
Thanks for the tip on McMaster Carr, I will check them out for teflon tubing.

Ken

"Brian H. Cross" wrote:

snip
Hope I have not misjudged the intent of your point Ken. Apologies if so!

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