Fuel System. 

There is a flight limitation (placard) that restricts use of the right fuel tank to
level flight only when the quantity of fuel in the right tank is less than one third 
full.  The reason for this is due to the fuel line routing.  Each tank in the later
PA-22's has an outlet in the forward and aft portions of the fuel tank to have a pickup
when in climb and descent attitudes.  The lines from these pick ups travel down the window
posts into the fuselage.  On the left side the lines meet just prior to the fuel selector
valve (downhill) flow the entire time.  The right side fuel line routing is substantially 
different.  The aft pickup line comes down the aft door post behind the entry door and goes 
beneath the floor and then back UP the forward door post (hinge area) where it joins the 
forward pickup line that came down the forward door post from the wing.  At that juncture
the line "T"'s to travel horizontally across the bottom of the instrument panel area till
it reaches the left fuselage side wall and then goes down to the fuel selector valve. 

Due to the circuitous routing of the right tank fuel lines, it is possible to get an air 
"bubble" in that line under some flight attitudes/conditions which could cause loss of 
power (until the left tank is frantically selected or aircraft is returned to smooth
level flight). The routing of that aft line by going down to floor level and back up 
to cross over at the instrument panel and back down also has a little more restriction 
(friction to flow) so as to not flow quite the same rate as the left tank.

At the low point for the right tank, aft pickup line, there is a gascolator under the floor
under the right fwd seat.  This gascolator is to catch and trap water and needs to be drained 
before the first flight of the day.  This gascolator, unlike the gascolator on the firewall
has no screen and therefore does not act as a filter like the one on the firewall.

There have been some modifications accomplished to the right tank fuel lines (Miss Pearl for 
one) that re-route the line that carries the fuel across to the fuel valve by going under
the floor rather than up to the instrument panel, across and back down to the fuel valve.
While I cannot recall if this modification (FIELD APPROVAL REQUIRED FOR THIS MOD) removes 
the restriction of the one third right tank quantity, it does improve the fuel flow capacity
to the engine due to more direct routing, less bends in the fuel line etc.

The two commonly asked questions regards the fuel system in the PA-22 and PA-20 series 
seems to be the lack of a screen in the under floor gascolator and the use of the
right tank with restriction/limitation. Hopefully this note has explained that.