Tank Vent follow-up

Jeffrey Walker walkerjl at charter.net
Sun Jul 8 15:40:21 PDT 2007

I'm not convinced that boring out the inlet and outlet of the filter you are
using will significantly reduce the pressure drop you are experiencing
across the filter.  I believe that it is the filter media itself that is the

On a gravity drain system like the fuel system, there typically isn't
significant flow rates through the line to cause such a pressure drop unless
you see restricting orifices in there.

The filter media though... well, they design media to have a very very fine
mesh or porosity.  In order to allow more flow, they try to increase the
surface area by folding pleats back and forth... the smaller the filter, the
harder it is to do.

Anyway, I bet if you switched the filter to a different model or even just a
new one you'll have better flow.

My personal test for whether or not a filter is plugged is to take it off
and try and blow through it.  Your vodka bottle test is innovative, what I
call the bucket and stopwatch method for determining flow rate.  (And I'm a
pro, I test leak rates of primary containment isolation valves at nuclear
power plants, in accordance with 10 CFR 50 Appendix J. see

However, I would in no way expect nearly the same flow rate of the
unfiltered line with the filter installed with gravity drain.  In fact, if
your GPz drank that heavily, you'd be lucky to get 20 miles from a tank of

Like I said, I have this internally calibrated pressure gauge in my mouth,
and I try to blow through the filter... well, I can at least tell if they
are plugged or not.

So you have identified one probable cause:  the filter may not flow enough.
Have you looked at other contributing causes?  I have had the vacuum line to
the petcock get a kink in it (actually still happens all the time, I need to
replace that hose).  With a kink in it, it doesn't get enough vacuum to open
the petcock fully.

I have also had a vacuum actuated petcock that had the internal filter get
gummed up on the regular setting, but not on Prime or Reserve.  This was
actually on a Suzuki.  I only noticed the fuel starvation on flat out WOT
runs.  That took me a while to figure out.  At first I suspected the tank
vent, and that my tank bag interfered with the venting.  (What I think now
is that when the tank bag was on, the fuel starvation was more apparent
because the aerodynamic drag was greater since I couldn't tuck as tight). In
the end I had to take that petcock apart and clean it out.  Your original
suspicion, the tank vent, I wouldn't assume unless you ride in really dusty
conditions, or you live in very hot, humid conditions and the fuel venting
off your tank is gumming it up.  I did experience this on the labyrinth seal
of the vent on my KLR tank cap, but then again, I'm talking about really
dusty and muddy conditions.

To test if your petcock is the problem, you could either get a piece of fuel
line to install without the filter and go for a ride, or you could take off
the vacuum hose from the carb, apply a vacuum to it with your mouth while
using the stopwatch and vodka bottle and the petcock at the normal position,

Personally, I don't like the glass body filter that you are using, as I've
had one break on me on my KLR once (not sure if it was dumb bad luck, or the
vibrations and jolting got it).  The filter I have currently on my GPz has a
metal body, and is fairly compact.  It's worked very well over the last 4
years, but now that I'm thinking about it, I should probably replace it.

If you do bore out the inlet and outlet of your filter, be careful about how
much clamping force you apply to it with the hose clamps afterwards. (Or are
you still using the OEM spring clamps?)  It probably won't hurt, and who
knows, it may free up just enough flow to alleviate the fuel starvation
symptoms, but I'm skeptical.

Good luck,
Jeff in Washington

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