Cleaning Your
Carb’s Inlet Screen
BY DICK KOEHLER, EAA 161427
ONE OF THE TASKS we all face when doing an annual condition
inspection is checking all the screens in the fuel system for trapped
debris. The final filter in the fuel system is in the main fuel-metering
device. We will look here at the common Marvel-Schebler carburetors on our planes, but the Bendix fuel-injection system has a similar
inlet finger strainer on the main servo unit.
The two most common carbs on our planes are the Marvel-
Schebler MA- 3 and MA- 4-5. Although these carbs were originally
manufactured as Marvel-Schebler, their manufacturing rights have
passed through many hands, including Borg Warner, Facet, and
Tempest, so you may find parts references from many sources,
including after-market manufacturers such
as McFarlane. The MA- 3 series carbs are
used on four-cylinder Continentals and the
O-235 through O-320 Lycomings. The
MA- 4-5 series are common on the
Continental O-470 and Lycoming O-360
through O-540. Note that many sub versions
of these basic carbs are used on various spe-
cific engine/airframe combinations with
different settings for jets and economizer,
but the fuel inlet systems are the same.
■ Let’s look at the MA- 3 first. The fuel strainer is held in place by the
same nut that holds the fuel inlet fitting, so the fuel line must first be
disconnected at the carb (after turning off the fuel). The fuel inlet is on
the side of the carb opposite the throttle lever. Using the appropriate size
wrench or socket, remove the fuel inlet and strainer assembly nut. Note
that there is a soft metal gasket between the nut and the carb body.
It will look like a metal washer, but is actually the gasket. The service
manual says it should be replaced with a new one each time, but I have
found that if you are careful, you can often reuse the gasket. In any
case, it is wise to have a replacement available. The original part number
is 16-A36. These run about $3 to $5 each and may have various letters
added to the basic part number by the various suppliers. For instance,
McFarlane calls it an MC16-A36.
■ Examining the finger screen, note that the fuel flows from the inside
out, so any debris will be on the inside. Flush the screen from the
outside in with mineral spirits, toluene, MEK, or similar solvent while
gently tapping the unit to help dislodge any contamination. I like to
do this over a glass or white plastic bowl so that I can examine the
debris for troubleshooting purposes. Once the screen is clean, blow it
dry with low-pressure compressed air and recheck that all debris has
been removed.
If the screen is slightly wrinkled or crushed, you can often salvage
it by rolling it gently between your fingers or on the edge of a table. If
the screen has only a small broken area, you may be able to repair it
with solder, but the manufacturer’s position is that it must be replaced.
Replacement cost for these screens is about $70, so it pays to treat them
with care.
Carefully reinstall the screen and gasket on the carb body and torque
the nut to 10 to 12 foot-pounds. Reinstall the fuel line and turn on the
fuel to check for leaks.
