gravity feed fuel lines

[Koncretekid]

I noticed a strange thing while draining my 5 gallon BSA tank after cleaning (with soap and water).  There are two petcocks on opposite sides of the tank with original cleaned BSA "Ewerts" petcocks installed.  These have small passages of hardly more than 1/8".  I connected a new 1/4" fuel line to one side by about 18" long and a new 5/16" fuel line of the same length to the other and opened both petcocks.  The 1/4" line was flowing a full 1/4" diameter stream of water, while the 5/16" line was only trickling.  I timed the time to fill a 1 cup measuring cup and the 1/4" line did so in about 15 seconds, while the bigger line took twice as long.  I changed the hoses to opposite sides, and the same thing happened -- the 1/4" line flowed water twice as fast as the 5/16" line.  I checked to make sure both lines, which were the same brand and new, were clear, and when I blew thru them, the 5/16" line seemed to have much less resistance to my very scientific blow test.  So how is it that the 1/4" line flowed water at twice the rate ?  Think about this when you decide to put bigger fuel lines on a gravity feed system.

Tom

[Vishnuatepork]

I am just going to guess on this one, but the smaller diameter hose has a faster velocity than the larger.  If you know anything about shrink ported heads, I think its the same principal.  Its kind of like venturi effect.

http://en.wikipedia.org/wiki/Venturi_effect

cheers
alan

[Dean Los Angeles]

The 1/8" diameter is the major flow restriction. Draining from an open line there might have been enough flow to fill the 1/4" hose. The only friction is from the wall contact.
The 5/16" hose has more flow on the wall and could have caused the flow to be restricted, or caused turbulent flow, or . . .

That doesn't mean there would be a difference in flow when attached to the carburetor. I doubt that your main jet is an 1/8". Try it with the bowls off the carbs and see what you get.

[hotrod]

Try longer lines, it might be that the exposed surface area of the larger tube was almost enough to counter balance the pressure head of the line (like picking up water by plugging the end of a straw with your finger).


Larry

[Jon]

Try the same experiment with the line ends submerged & the lines full (air free)

Think the flow will be very similar then



jon

[charlie101]

The restriction is in the petcock. The 5/16 hose is allowing air rising in the tube, and those air bubbles restricts flow as they travel upwards, the petcock has a too small orifice to allow the flow the 5/16 hose permits. You get all the flow the petcock allow if you submerge the hose outlet in the fuel but it won't be the full volume the 5/16 can flow. Bigger hose than the 5/16 up in the air might flow at the petcock capacity as it allow space for both air and fuel, but in that case turbulence at the petcock outlet can still restrict flow a bit.

[Jack Gifford]

Try the same experiment with actual fuel (gasoline?) and post results.

[Koncretekid]

I repeated the experiment and submerged the ends of the tubes in two cups.  The 1/4" line still filled twice as fast.  Then I closed the petcock with the 5/16" line and held it up in the air.  I opened the cock and allowed the tube to fill slowly while lowering it to my bucket.  The line then flowed the same as the 1/4" line, although the water exiting the tube looked like a barber shop pole.  I submerged each line in a cup of water and they both filled at the same rate.  After this, I could stop and start the bigger line and it would flow well.  But if I picked up the line and blew back thru it to empty it, it would again flow slowly.  It seems as though the inside of the tube must be fully wetted before it will flow full.

I agree that either size line will work with my .062" main jet, but I'm only producing 42 hp.  With a single petcock with an 1/8" hole and 170 hp, you just might want to make sure you're fuel line is fully wetted before you start. Or get a larger petcock.  A friend of mine was having trouble with his 1100 cc Suzuki which would run fine in the pits, but would lean out on the track.  Turns out the petcock was partially blocked.  Disconnect the line at the carb and hold it up in the air to make sure it fills.  BTW, this is a good way to find out how much fuel you have left in a sealed fuel tank.

Tom