In-line filters

Several years ago, I had a Suzuki 550 and twice had to clean the carbs
because of small particles. I ended up putting an in-line filter on
it. Should that be something that I should do for my son's 1987 Yamaha
YZF he just acquired? If so, what is a good one?

 

The problem with CV carbs is evaporation of the gasoline when the
motorbike sits unused. No filter can do anything about the gum and
varnish caused by fuel evaportaion. I recommend running about 3 ounces
of a carburetor cleaner like Berrymans B-12 Chemtool through a full
tank of gasoline whenever you hear the exhaust start to go
piffle-piffle-SNAP when you roll off the throttle, or any time the
engine is hard to start or stalls in traffic...

You could get a small plastic inline filter from any auto parts store
or from Walmart. It would back up the small filter that's already in
the gas tank. If there is a lot of rust and crud built up in the gas
tank already, a second filter won't keep the junk from tearing up the
seal in your petcock if you're in the habit of running the gas tank
down until the engine quits and then switching to reserve...

Going on reserve is a bad practice, unless you know your gas tank is
absolutely clean inside. When you go on reserve, any water or rust
that's sitting on the bottom of the tank will go into the
carburetors...

So, if you're the type of rider that uses reserve, an extra filter
might not be such a bad idea...

 

Doesn't the YZF have a fuel pump anyway? Or am I thinking of something
else? I think that a lot of the tank sits below the carb level so it
needs a pump, but ICBW.

If it has a pump, it'll have a filter. Otherwise, just about any filter
is a good idea, yes.

 

The problem my KLR has is that it sits unridden for long periods of
time and the gasoline evaporates, blocking the idle jets and
passages...

Any original equipment fuel system *should* work just fine, without the
addition of aftermarket filters. Many riders think they are doing their
motorcycle good by installing clear plastic fuel hoses and inline
filters and then wonder why there are huge bubbles and air pockets in
those hoses and the filter. They'd never had that problem to worry
about with the original equipment black rubber hoses...

If you had a column of *water* one foot high, it would produce a static
pressure of 0.433 pounds per square inch at the bottom of the fuel
hose. Since the specific gravity of gasoline is about 0.5, the static
head of gasoline would be 0.216 pounds per square inch...

And, the distance between the bottom of the gas tank and the carburetor
of
a motorbike with a gravity-fed fuel system might be only 6 inches...

So the float valve only has to hold back about 0.11 psi of gasoline.

Ain't statics wonderful?

Gravity flow fuel systems have been in use for hundreds of years,
without the user needing to be aware of the existence of such a concept
as static head though. And most of us don't live in a Third World
country where we have to filter all gasoline through a chamois skin
before filling our tank with it. Besides the acumulation of rust and
crud that occurs over the years we still do run into fuel feed
problems.

Then we need to be aware of:

1. Gas cap vents that don't vent;

2. Fuel lines that go up before they go down, forming a vapor trap (1);

3. Fuel lines that are bent too sharply, pinching off the fuel supply;

4. Fuel lines that vapor lock because they are in very hot areas or
actually lay on hot surfaces;

5. Float bowl vent hoses that get get plugged up. There are brass stand
pipes in the bottom of some float bowls that might be associated with
float bowl venting;

6. Air pressure that blows up the float bowl vent hoses as the
motorcycle is ridden in side winds. This blocks fuel flow to the float
bowl under some conditions, like the wind that came from a 45 degree
angle and upset the carburetor of the original 650cc Honda Hawk GT;

7. Plugged up in-tank filters;

8. Plugged up petcocks;

9. Plugged up filters inside the carburetor itself, just above the
brass float valve seat.

(1) What's the boiling point and vapor pressure of gasoline, anyway? Is
it more than about 1/10th of a pound per square inch? Can't liquid
gasoline just *get along* with evaporating gasoline and flow down to
that gawd-damned float bowl? Enquiring minds have a lot to think about
;-)