Say you have an empty filter, empty because you drained the system all the way (or you are running a vacsucker and it just did you dirty again).

Now you turn on the gas by hitting the reserve (reserve on a Raptor, or hitting Prime on the stock vacsucker) and the gas flows down and starts to fill up the float bowl.  It takes more time for the gas to flow through the needle valve than the fuel line, so the gas flow to the needle valve is more than enough and the gas begins to back up above the needle valve.

At this point, the hose becomes a two way street through much of its length, passing displaced air up and gas flowing down.  By definition, the volumes must be equal because of displacement, equal gas in and equal air out.

So, during "fill up hose" phase the gas flow down is halved at each of the restriction points in the system.

The filter cavity isn't a restriction as it is large compared to everything else, but the ID of the nipples is a restriction as the passage is smaller than the hose.  Each bend is a possible restriction if it was allowed to pinch the hose down.

The dual "both ways" flow issue ends when the hose is filled up, but the hose won't be filled totally until the bowl, long line, filter and short line are filled.

How long does this take?

On the stock vac petock, it takes a bloody long time with you fumbling around on finding Prime and all.   Times of 60 - 100  seconds was typical on my machine after I got the gas going again in Prime (I could see it in the clear filter when it finally started to trickle and I could see when the filter filled up).    And if you left the vacsucker in the ON position on your vac petcock it was really forever as the gas never flowed.  My Prime worked, after a fashion -- but nowhere as well or as fast as the Reserve on the Raptor worked.

On a Raptor, good gas flow was immediate and the system filled itself back up in 15-20 seconds, float bowl, filter and all.   This was with a 1/4" hose and nipples.

From a dead empty system, the bike is able to start running again when the float bowl gets full.  Generally you can keep it running from this point, but with a vacsucker the bike seems to not like to run at full power until the fuel system fills up all the way.  

Back in my vacsucker failure days, I hated cranking on the starter, so I generally waited until the vacsucker supplied enough fuel to the point the fuel filter got half full before even trying to start it up -- it worked better that way.


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The Math Involved

area of a circle  =  pi times (the radius squared)

1/4" hose   = .250" hose ID    A = 0.0491 in²
5/16" hose = .315" hose ID    A = 0.0779 in²
3/8" hose   = .375" hose ID    A = 0.1104 in²

So, if this initial flow disruption bothers you, opt for a larger hose size and fit the filter nipples accordingly.  3/8" filter nipples are available as is the 3/8" hose.  5/16" might be a better compromise as they fit the nipples on the petcock and the carburetor better than the 1/4" or the 3/8" hoses do.

Going from my current 1/4" hose ID up to 3/8" hose ID can double my gas flow on fill up, which only offers an advantage really when I am filling the system up after a stall out as the needle valve can be fed happily by either hose size once the gas stacks up over the needle valve.

And, since I Raptored, I don't have those pesky fuel out situations any more like I used to.   My float bowl is full all the time now days.

But for what it is worth, a 3/8" hose and filter can flow gas through the whole system at a rate that during the two way fill up period is about equal to the rate a 1/4" hose and filter can flow when completely filled.  Because the hose volume is doubled though, you lose some time for hose fill accordingly, but your "fuel down" rate to the point of the needle valve is excellent all the time.

The area inside the 3/8" hose is slightly more than doubled compared to the area inside the 1/4" hose.   Expect to have more fun making the bends on the 3/8" hose as the wall thickness and extra diameter work against you on them bend zones.

    5/16" hose is the best compromise, I think

Charon Posted :
Reply #44 - Yesterday at 17:23:12      
The main conclusion I reached, which will probably require further experimentation, is that none of the filters can be counted on to prime from dry unless the inlet is lower than the outlet, giving the air in the filter a chance to be pushed along the fuel line to the carburetor. This might also be dependent on the diameter of the fuel line, but I only used 1/4" ID lines. I think the air in the fuel line above the filter (introduced when the line is cut and the little bit of gas drains out) combined with the air in the filter cuts off the fuel flow at the very low pressures involved.

Once the flow got started, I think any of the filters would flow enough to satisfy the demands of the S40/Savage, which only requires about 3 gph at full power. My worry would come if the fuel system were run empty (forgot it was in Reserve or Prime). From dry I would worry the filter might not restart flow.

I found the Durlast FF7144DL (I had - made in China) has about 2.4 PSI of restriction across the filter media and with stock petcock you have to hold the outlet (open to atmosphere) higher than the inlet to get it to fill, so the air had some place to go.  That is the same thing that Charon found with his testing.  The restriction across the media and the air bubble in the filter are enough to cause the limited fuel flow issue that I was seeing on my bike.  In operation the filter would flow once full of fuel  (JOG told me that in one of his posts), but I was concerned about the media restriction if I would run out of fuel at some point.

I found the Fram G2 (Oldfeller’s recommendation) has almost no restriction (less than .1 PSI) across the filter media.  I installed a Fram G2 (made in Israel) on the bike and it was relatively fast to fill with fuel and has been working well on the bike for two tanks full of fuel so far.
 
I did not intent to start a sh*t storm on the site, but I now have a better understanding of how some of these filters can impact the normal operation of the engine with the stock petcock in place.  The Raptor may have a little bit larger I.D. diameter coming through the petcock and be able to deal with the air bubble better than the stock petcock.  Thanks everyone with your input on my filter question and helping me getting the issue resolved.

 
Grayowl, this was not a shite storm, it was quite polite actually.  In reality, this discussion really didn't have much to do with you at all.

You simply stumbled upon one of our standard discussion/war topics which got discussed some more and some experiments were done and some more progress was made.

Progress made:  

Charon did gas flow testing and discovered new things that he didn't expect (and neither did anyone else).  Experiments are always such fun that way -- we loves them, we do.

We calculated a simple filter flow test based on the usage of the Savage engine at 1 cup per minute as being ample.

We found all filters can easily flow this amount once filled -- but it points out another issue with the stock vac petcock running your carb out of fuel, the air flow back up the plumbing causes a low flow condition until the line is filled with backed up gasoline.

GyroBob posted a new filter and a new installation method that has some real promise.  

This information was all added to the tech post and the 1/4" hose size was unrecommended with preference going to 5/16 and 3/8" hose sizes.

I just want to comment about the "air lock' conditon.  This is a completely valid concept that is common in the design of water distribution systems.  When designing water lines we "must" include a fire hydrant or air release valve into the system at any high point in the system in order to bleed out trapped air in a system when filling it with water.  What can happen is that the air that is trapped at the top of the system forces the water to flow under the air.....and it can reduce the capacity of the water system dramatically.  If the slopes are steep - it is sometimes impossible for water flow alone to completely push the bubble out - and in an 8" diameter water line you could end up with only 1-2" of water flow under the bubble.  I had always read about this and one day I actaully got to experience it.  One of the small towns I work for had a water main break and after it was repaired - the Mayor called me as they could no longer get adequate water supply for the town.  The system had worked fine for years.....but suddenly they could not get enough water into town.  I came out and looked at where the break had occured and it was at the bottom of a hill.  I found a fire hydrant at the top of the next hill and asked the maintenance department to open the hydrant and check for air.......and after flowing air and water out.....the flow into town was back to normal!

I can see how the tiny inlet/outlet openings in some of the filters could cause a flow problem....especially when air is trapped in the filter housing.  

I found a filter that is available at almost any automotive parts place at a very reasonable cost ($2.50).  It has no media restriction to speak of and pretty desent filtration at 12 microns. It fits up under the frame without any issues and flows good on the stock petcock.  Ran the bike at high speed for 70 miles this afternoon without any fuel starvation issues.

WIX - Fuel Filter
Part Number: 33011
Line: WIX
•      1 year limited warranty
•      UPC: 765809330115
Detailed Description
WIX offers high quality, high efficiency fuel filters for automotive, medium and heavy-duty trucks, as well as farm, construction, mining and other equipment applications.
Some features include:
•  Low initial restriction providing for proper fuel flow and long service life.
•  For fuel water separators, self venting drains allow for efficient and easy draining of water.
•  Materials, designs and construction that ensure WIX fuel filters provide excellent performance under all types of operating conditions.
Style: Fuel (Complete In-Line) Filter
Service: Fuel
Height: 4.125 in.
Outside Diameter (Top): 1.4 in.
Micron Rating: 12
Inlet: ¼ - 5/16
Outlet: 1/4- 5/16

I have even added some restriction with my “u-turn” for the fuel line and it was fine.  That was made out of 5/16 fuel line fittings.