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Rebuilding The Bing Carburetor

With my engine and carburetors both hitting the 100 hour mark, it was time
for some fairly extensive maintenance.  The Rotax maintenance sheet
recommends cleaning the carbs, and checking them for wear, every fifty
hours, and rebuilding the carbs every one hundred hours.

Cleaning the carbs, and checking them for wear, is fairly simple.  You take
the carb apart and examine the metal parts for any signs of wear, and clean
out any gunk that may have accumulated in the jets.  I've previously
documented how to do this in an article on how to disassemble the Bing
carburetor.  This article can be found on the Powerchutes web site.

Rebuilding the carburetors is only a little more involved.  Basically you
give all the metal parts in the carburetor a REALLY GOOD cleaning by
dunking them in a bath of carb cleaner solvent.  You also replace the
gaskets and various parts that start to deteriorate, or wear, over time.
The idea is to return the carb to "zero time."  In other words, after the
carb is rebuilt it is supposed to be as good as the day you received it
from the factory.  The same process is done with the Rotax engine at the
three hundred mark, when it is given a complete overhaul and made "as good
as new."

The first thing you'll need to rebuild your carb is a carb rebuild kit.
You'll need one rebuild kit for each carb.  The carb rebuild kits can be
purchased from California Power Systems (and many other places).  The part
number is #13503.  Cost of the kit is $24.95.

Here is a list of the parts that are included in the carb rebuild kit:

"Top O Ring"
     (big O ring that goes under the lid of the carb)
"Air Screw O Ring"
     (tiny little O ring that seals around the idle air mix screw)
"Bowl Gasket"
     (big cork gasket that seals the float bowl to the body of the carb)
"Float Needle"
     (small, bullet-shaped valve with the world's tiniest spring clip)
"Hinge Pin"
     (straight pin, just over an inch long)
"Sieve Sleeve"
     (little plastic screen in the shape of a tube)
"Cable Grommet"
     (rubber boot over throttle cable on top of carb)

Basically these are all parts that can get worn over a period of time, and
need to be periodically replaced so that your carb doesn't fail while
you're flying over the Black Forest.

If you don't already have a copy of the tuning and parts manual for Bing
aircraft carburetors, call up the Bing Company at 1-800-309-2464 and order
a copy.  You MUST have this manual in order to know what you're doing.

You'll need a gallon jug of carburetor cleaner to give the metal parts of
the carb a good cleaning.  Basically you dunk the metal carb parts in a
bath of carb cleaner and leave them there for about ten minutes.  The carb
cleaner is a VERY powerful solvent, and will dissolve any gunk that may
have accumulated in the nooks and crannies of the carb.

WARNING!!!  Carb cleaner is a VERY powerful solvent.  Its active ingredient
is methylene chloride; the same stuff they use to strip paint, and you
don't want to fool around with this stuff.  IT IS NASTY!  Don't get it on
your skin, or in your eyes, or on your clothes.  It will give you bad
chemical burns in no time.

Rubber gloves and safety goggles are an absolute must when working with
carb cleaner!

Rubber gloves and safety goggles are an absolute must when working with
carb cleaner!

Did I mention that rubber gloves and safety goggles are an absolute must
when working with carb cleaner?

The fumes from carb cleaner are just as nasty as the liquid, so don't use
it in a room that doesn't have excellent ventilation.  Better still, don't
use it in a room at all; take it outside in the back yard.

Turn on the garden hose before you even open the can of carb cleaner.  Then
if you accidently spill some on yourself, or get it in your eyes, then you
can instantly flood yourself with water.  It would also be a good idea to
have somebody around, for safety, while working with carb cleaner, just in
case you have an accident with it.

THIS STUFF IS NASTY!

You want to be able to dunk the metal parts of your carb in the carb
cleaner, but you don't want to touch the carb cleaner solvent.  The easiest
way to do this is to pour the carb cleaner liquid into an aluminum pan, and
then use the drain basket from a deep fryer to lower the parts into the
carb cleaner bath, and to rinse them afterwards.  I also have a big pot
filled with fresh water to rinse the parts in, after the bath in carb
cleaner.  By using the drain basket, I can dunk the carb parts in the carb
cleaner, let them sit, lift them out, and then rinse them in clean water
without ever exposing my skin to the carb cleaner solvent.  But just to be
on the safe side, I still wear rubber gloves and safety goggles.

You can buy a gallon can of carb cleaner from most auto parts supply
stores.  I paid $18 for a gallon can of the stuff.

Okay, time to get started on the carb rebuild.  If you have two carbs, I
suggest cleaning and rebuilding them, one at a time.  This leaves one carb
assembled while you work on the other, and makes it easy to remember where
all the parts go.  If you only have one carb, be sure to take any necessary
notes while you disassemble it, so you don't have to guess about the
reassembly later.

First thing to do is remove the lid of the carb and pull out the piston
assembly.  Disconnect the choke line (if you have one).  Disconnect the
primer line (if you have one).  Then remove the body of the carb from the
engine.  If you have electric carb heaters, like I do, the carb heater must
stay attached to the body of the carb.  This is why I have ring terminals
on the ends of my carb heater wires for easy detachment from the rest of
the electrical system.

Remove the carb lid and piston assembly from the throttle cable.  Take the
piston assembly apart.  Separate the plastic parts from the metal parts.
Remember, only the metal parts go in the bath of carb cleaner!  Do not put
any plastic, rubber, or cork parts into the carb cleaner!  They will
dissolve!

Throw away the rubber boot around the throttle cable on top of the carb
lid.  Throw away the big O ring under the carb lid.  These parts get
replaced.  Set the white plastic cup aside.  Remove the jet needle O ring
from the jet needle and E clip assembly, and set it aside.  The carb lid,
spring, E clip, jet needle, and piston all go into the carb cleaner bath.

Before you unscrew the Idle Air Mix Screw, screw it all the way into the
carb body, and count the number of revolutions it makes so that you can
re-install it to its former position after the carb is reassembled.  Then
unscrew the Idle Air Mix Screw and remove the little O ring around the head
of the screw.  Throw the O ring away.  It will be replaced.

Next we want to remove the big Idle Speed Screw.  But before we do so,
measure the distance from the body of the carb to the bottom of the screw
head.  Then you easily return the screw to its former position after the
carb is reassembled.  Unscrew the Idle Speed Screw, and the spring that
holds it in place.  Both parts go into the carb cleaner bath.

Next to the Idle Speed Screw, on my carb, is a white plastic plug.  I don't
know what this plug does, but you don't want to put in the carb cleaner
bath.  Pop it out and set it aside.

My carb has a short length of plastic hose running from one side to the
other, with some drain holes at the bottom of the hose.  This is the
overflow hose.  Remove the hose and set it aside before the carb goes into
the carb cleaner bath.

If you any other plastic plugs or parts on the body of your carb, be sure
to remove them before putting the carb into the bath of carb cleaner.

Okay, that takes care of the piston assembly, and all the removable parts
on the outside of the carb.  Now remove the Float Bowl.  Remove the plastic
floats from the Float Bowl, and set them aside.  Throw away the Sieve
Sleeve.  It will be replaced. 

Unscrew the Main Jet and the Mixing Tube.  Remove the Needle Jet.  Unscrew
the Idle Jet.  All of these parts will go into the carb cleaner bath.

Throw away the cork gasket that seals the Float Bowl to the body of the
carb.  The gasket will be replaced.

Now we get to the only part of the process that might take a little
concentration; removing the Float Needle.  In order to remove the Float
Needle, we first need to remove the Float Arm that is attached to the Float
Needle by that incredibly tiny little spring clip.  But before we can
remove the Float Arm, we must remove the Hinge Pin that holds the Float Arm
in place.

The Hinge Pin is smooth at one end, and has a knurled end at the other.
The knurled end holds the pin in place.  Basically you need to drive the
knurled end of the pin out of the body of the carb, then the pin will slide
out easily.  The easiest way to do this is to use a punch with a very
narrow head.  Place the head of the punch against the smooth end of the
pin, and tap the punch lightly with a hammer.  A few taps should dislodge
the knurled end of the pin, and free it.  Be careful not to use the punch
on the knurled end of the pin, which will only drive the knurled end deeper
in the carb.

Once the Hinge Pin is freed, slide it out of the carb.  This will free up
the brass Float Arm, and allow you to slide the Float Arm out of the tiny
little spring clip that connects it to the Float Needle.  Once the Float
Arm is out of the way, you can lift the Float Needle out of the carb body.
Throw away the Hinge Pin and Float Needle.  They will be replaced.  Hang
onto the brass Float Arm.

Okay, now that you've got the carb completely disassembled, it's bath time!
All of the metal pieces of the carb should be in one pile, and all the
rubber or plastic pieces of the carb that haven't been thrown away, should
be in another pile.  Basically you should have the plastic carb floats, jet
needle O ring, white plastic cup (from the piston assembly), plastic
overflow hose, and white plastic plug in the pile that will NOT get dunked
in the carb cleaner bath.  Everything else (metal) goes into the bath.

The best way to bathe the metal parts is to separate them into two piles of
Big Metal Parts and Little Metal Parts.  The Big Metal Parts include the
main body of the carb, the Float Bowl, the Piston, Piston Spring, and the
Carb Lid.  All of these large parts can be placed in the drain basket and
lowered into the aluminum pan that has been filled with carb cleaner.  Let
the parts sit in the bath for about ten minutes, then raise the basket to
drain off the carb cleaner.  Rinse the basket and parts in the pot of fresh
water.  I like to rinse the parts, then dump out the rinse water and fill
the pot with fresh water and rinse the parts again.  I do this several
times until the carb cleaner liquid appears to have been completely rinsed
away.  I give the parts one final rinse under running water, for good
measure, then set the parts aside to dry.

If you have an electric carb heater installed on the body of your carb, the
heater cannot be removed for the bath in carb cleaner.  Although the heater
appears to be sealed up tightly, I would not care to risk destroying or
damaging the heater it by dunking it in carb cleaner.  So I only put enough
carb cleaner in the pan to just cover the body of the carb as it rests on
the bottom of the drain basket, but not enough to submerge the electric
carb heater.  I also take pains to keep the wiring of the carb heater out
of the carb cleaner.

Since the Little Metal Parts (jets, screws, etc.) might fall through the
openings in the drain basket, I put the Little Metal Parts in a glass
measuring cup and cover them with carb cleaner solvent.  Let the parts sit
in the carb cleaner for about ten minutes, then carefully pour most of the
carb cleaner back into the can.  Rinse the parts in the glass measuring cup
with fresh water until all the remaining carb cleaner is gone.  Set the
parts aside to dry.

Theoretically you can save the carb cleaner liquid and reuse it next time,
but I like to clean my carbs with only fresh, unused cleaner.

When the parts are dry, I like to blow air through all the orifices and
passageways to make sure all the water is gone.  Then it's time to
reassemble the carb.  Open up the carb rebuild kit and get out the new
Float Needle.  Drop the Float Needle into the appropriate hole in the carb
body, then carefully slide the tiny spring clip on the Float Needle back
onto the little flange between the arms of the brass Float Arm.  (If you
have a second carb, this is where it can come in real handy to be able to
look at this delicate assembly while reassembling the first carb.)

While holding the Float Arm in place, slide the Hinge Pin back through the
holes in the body of the carb, and through the hinge openings in the Float
Arm.  Use your punch and hammer to drive the knurled end of the Hinge Pin
back into the body of the carb, to seat the Hinge Pin securely in place.

Turn the carb upside down and make sure the twin arms of the Float Arm
assembly are perfectly parallel to the base of the carb.  Bend the center
tab, as necessary, to make them parallel.  More details on this are
available in
the Bing carburetor manual.

Now reinstall the Idler Jet, Needle Jet, Mixing Tube, and Main Jet.  Slide
the new Sieve Sleeve over the Mixing Tube.  Install the new cork gasket on
the body of the carb, and reinstall the twin Floats in the Bowl.  Put the
Bowl back on the main body of the carb.

Put the spring back on the big Idle Speed Screw, and screw it back into the
side of the carb until it's in as far as you previously measured it.  Put
the white plastic plug back into the body of the carb, next to the Idle
Speed Screw.  Take the tiny O ring in the rebuild kit and put it in the
slot just below the head of the Idle Air Mix Screw.  Then screw the Idle
Air Mix Screw back into the body of the carb, all the way, then unscrew it
the proper number of revolutions that you recorded earlier.

Reinstall the plastic overflow hose on the body of the carb.

Reassemble the jet needle, E clip, and tiny O ring that prevents the jet
needle from spinning inside the E clip.  Slide the new rubber Boot over the
throttle cable and put the new large O ring on the underside of the Carb
Lid.  Put the Piston assembly back together, put the carb body back on the
engine, slide the piston assembly back into the carb, and screw down the
lid.

All done!  You've cleaned and rebuilt your carb.  It should be as good as
new.  Now it should be good for at least another one hundred hours of safe,
enjoyable flying.

Have fun!

Chris Wolf
cwolf41@home.com