BASIC RACE PREPPING OF A BRIGGS & STRATTON CARBURETOR by Mark Bergfelt
Many karting enthusiasts find themselves in
a dilemma. They have an intense desire to win, but more often
than
not, they don't have the financial resources to come up with the items
that they feel that they need to get to the front of the pack.
Quite
often the novice will spend many dollars on accessories that may not
get
them the desired result. The toughest part for the new karter,
especially
the one with limited resources, is knowing just what parts and services
will make him faster on the track. He needs to know how to get
the
most bang for the least buck.
One way that karters can save money and get
good results is by doing their own carburetor preparation. This
article
will illustrate the most basic preparation techniques to turn your
stock
gas carburetor into a WKA legal alcohol and air mixer. We will
not
dazzle you with flow bench information. If you are reading this
in
search of help for your racing effort, you probably don't have access
to
one anyway. That does not mean you can't make your carburetor do a
satisfactory
job. It's true that all Briggs carburetor castings are not
created
equal, and a flow bench is helpful for sorting them out, but these
instructions
will help you to do the best that you can with what you have.
Getting
the most out of what you do have is most important.
To accomplish our mission, we will be
completely
disassembling a carburetor. Before you start, get a container to
put all of your small parts and screws into to insure that they don't
get
lost. I have found that an empty waterless hand cleaner container
is perfect for this.
Removal of the carburetor from the engine is pretty
straight
forward. The first step is to drop the fuel tank from the bottom
of the carburetor. Remove the support bracket screw down near the
bottom of the engine, first. Usually a 3/8 socket wrench gets the job
done.
Next remove the three screws that fasten the tank to the
carburetor.
A Torx T-20 screwdriver will be needed to remove the screws currently
being
installed by the factory. Earlier designs needed flat
scredrivers
and even earlier ones, phillips type screwdrivers, for removal.
They
all got the job done, but my preference are the ones that the factory
is
currently using. You can replace them with 10-32 x 5/8"
socket
head (Allen) cap screws if you like. For those you will need a
5/32"
hex key (allen wrench).
Removal of the carburetor from the cylinder
comes next. Current production engines require the use of Torx
T-30
screwdriver. Briggs & Stratton supplies a special tool for
those
screws. It is part number 19391 and allows the screws to be
reached
from the side. Earlier models of the five horse Briggs engine
used
a screw with a 3/8 hex along with two straight, crossed slots.
You
could use a flat screwdriver on them and a 3/8 open end wrench,
sometimes.
Briggs makes a special screwdriver for those screws. It carries
part
number 19305. It also allows access from the side. The
screws
used with that tool are my favorite method of securing a Briggs
carb.
Some racers prefer to use 1/4-20 x 3/4" socket head cap screw.
They're
OK and allowed by the rules. For those you will need a 3/16" hex
key (allen wrench). Once you have the two mounting screws out,
the
carburetor will probably come off with them. If not, the only
thing
holding it on will be the adhesion of the gasket(s). Simply tug
on
the carb. and it will come off.
It's no fun at all to work on dirty parts,
so it is strongly suggested that the carburetor get a good cleaning at
this point. Some good parts solvent will get the job done.
There are many available, so take your pick and be sure to follow the
safety
instructions. Most are flammable. If you have compressed
air,
blow dry the carburetor. Otherwise, wipe it off with a rag or
shop
towel. The carburetor that we are prepping in this article is
brand
new so we'll skip that step.
At this point there are a lot of directions
that you could go and they would all be ok. I'll suggest that you
take out the needle valve assembly first. A 1/2" box or open end
wrench is what you will need. Simply crack it loose and unscrew
it.
Next, remove the brass jet. Briggs calls it a "seat-needle
valve".
Use a flat blade screwdriver to unscrew it. Put the jet and your
needle valve in to your small parts container.
Removal of the bell crank that operates the
throttle is a good next step. Use a 5/16" nut driver to remove
the
special shoulder screw that holds it on. With the bell crank
loose,
carefully unhook the throttle link and throttle link spring. You can
disgard
the bellcrank, since you will be replacing it, but don't loose the
shoulder
screw, throttle link and throttle link spring. Some people
question
the purpose of the throttle link spring. It is there to keep the
throttle from "floating" or fluctuating, causing the engine to run at
an
unsteady speed, when a governor is used.
Removal of the diaphragm cover is really
easy.
All that you need is a thin blade flat tip screwdriver. Take out
the four screws and put them into your container. The diaphragm
cover
might be stuck in place by the diaphragm. If it is, a sharp tap
with
the screwdriver handle will get it off. Be careful not to damage
anything. Next, peel off the diaphragm. At this point the
diaphragm
spring and cap will probably fall out. Put them in your parts
container.
A big screwdriver can be used to pry out the
welch plug that seals off the end of the carburetor. Place the
screwdriver
into the airhorn of the carburetor with the end of the screwdriver
against
the plug. Pry hard and it will pop out. Watch that you
don't
poke yourself and pry away from your face and other people. If
you
are going to reuse that plug, place it over a 3/4" nut and tap it back
into a dome shape with a ball been hammer. Put the plug aside.
Unless the fuel pipes are damaged, the only
thing left to remove from the carburetor is the throttle shaft and
shutter
assembly. You will need a Torx T-10 screwdriver to remove the
screw
that holds the throttle shutter. Older carburetors used a slotted
screw and a thin flat tip screwdriver with a long blade got that screw
out. I prefer a magnetic type, but you can only appreciate the
reason
when you go to put the screw back in. Once the screw is out, the
throttle shutter will fall out. At that point you can pull out
the
throttle shaft from the top of the carburetor. When you remove
the
throttle shaft, be sure not to loose the two small seals that are there
to keep dirt from getting into the airstream through the throttle shaft
bushing. It is illegal to race with out the seals and not too
smart
either since any dirt that gets in between the throttle shaft and
bushing
will wear them out quickly resulting in a loose and wobbly throttle
shaft.
Inspect the screens on the two fuel pipes
that stick out of the bottom of the carburetor. If they are
clean,
leave them alone. If they look even slightly clogged replace the
plastic fuel pipes, To get them off, twist each one with a box
end
wrench and pull hard at the same time until you muscle them off.
For the long fuel pipe you will need a 9/16" box wrench and for the
short
one a 3/8" box wrench.
At this point you now have a pile of
miscellaneous
pieces that used to be a carburetor and we hope will be a real killer
when
you are done with this project.
The main difference between a gasoline
carburetor
and one for alcohol is the size of the passage ways through which the
fuel
travels . You need approximately twice as much alcohol as
gasoline,
to get an engine to run correctly. For that to happen, there are
three holes that we need to enlarge. One of these holes is the
one
through the center of the needle valve seat which will we will refer to
as the "jet" from now on. The other two holes are located on the
throttle body and were uncovered when you unscrewed the brass
jet.
One of the holes is larger than the other. The large hole, known
as the main metering hole, must be smaller than .062". The
smaller
hole, called the Idle hole, must remain under .028". To enlarge
the
main metering hole you will need a number 53 twist drill (drill
bit).
A number 53 drill measures .059. You might want to risk the
possibility
of getting disqualified and use a 1/16 drill bit. A 1/16" bit is
.0625" and is knats hair over the limit. Let your conscience be
your
guide. What ever you decide, use a pinvise to twist the drill
through
the hole by hand. Under no circumstances should you use an
electric
drill motor!
A number 71 drill bit measures .026" and a
Number 70 is .028". Again let your conscience be your guide on
choosing
which drill to use to enlarge the idle hole. Be sure to use the
pinvise.
The size of the hole in the brass jet is a
non-tech item so you can and should experiment with the size of that
hole.
A serious racer has an assortment of jets to tune his engine to
changing
weather conditions. The carburetor that we are preparing here
will
be equipped with a .052" jet. Please note, this carburetor will
be
used on an unrestricted engine. Generally, with the Briggs
carburetor,
the smaller the restrictor, the bigger the jet that is needed. At
first, this defys logic, but that's a whole different article and we're
not going there right now. To drill jets, I like to put the bit
into
a bench vise with the bit sticking straight up. Then set the jet
onto the bit. To make a .052" hole we will be using a #55
drill.
Using a large screwdriver, screw the jet onto the drill bit until you
completely
drill through the jet. At this point it is advisable to mark the
jet with the size or at least some type of mark so that you know what
it
is. This is important because you will be changing jets often if
you want to have a chance of running consistently. I prefer to
use
1/16" number stamps. It would probably be most cost effective for
the average karter to by a set of drilled and marked jets that come in
some type of case.
If that were all we did we could put the
carburetor
back together and it would run an engine on alcohol. Who wants to do
that
when there are a whole bunch of improvements that you can make at
home and still be legal?
According to the rule book, "Maximum
carburetor
throat inside dimension is .695"." A stock carburetor has a bore
that is relatively smaller than that. If the carburetor is going
onto an unrestricted engine it can be reamed or honed to just under the
maximum size. I use an adjustable reamer that is set to .693" to
enlarge the carburetor throat. Place the square portion of the
reamer
shank into a vise, with the reamer sticking straight up. Place
the
carburetor over the reamer so that the reamer is inserted into the
carburetor
bore. Use an adjustable open end (Crescent) wrench to turn the
carburetor
on the reamer. A slight push on the opposite end of the
carburetor
with your free hand helps move things along. Also, a good shot of
WD-40 on the reamer and carburetor helps produce a smoother job.
After you are finished reaming the carburetor, clean it up throughly
with
solvent and blow it dry with compressed air. I recommend skipping
the reaming step if the carburetor is to be used on a restrictor
equipped
engine.
The throttle shaft is the next item that the
rules allow you to monkey with. The area on the throttle shaft
where
the throttle attaches is flat but it is not parallel to the opposite
side
of the shaft. The back edge can be as small as .086" and the
leading
edge .040". You can use a small file to file one or both of the
edges
until you reach those dimensions. Be sure to maintain the
original
shape. Rounding off the edges or changing the contour will get
you
disqualified in a GOOD carburetor tech. inspection.
The rules allow you to surface the diaphragm
cover. To do that, get a piece of 600 grit wet or dry sand paper
and lay it down on a piece of window glass, (plate glass is best, a
surface
plate is even better, but if you are reading this looking for answers
you
probably don't have one) with the paper side down. Spray some
WD-40
on the sandpaper and lay the diaphragm cover plate on the sandpaper
with
the gasket surface down. Rub the plate on the sand paper in a
figure
eight motion until the entire gasket surface shows evidence of being
sanded.
At that point you are done. Clean up the diaphragm cover plate
with
your solvent and dry it off.
The rules allow you to crimp a coin or washer
onto the mixture needle and you probably should. This makes the
the
small amount of adjustment that you can do with the needle valve a
little
easier. While holding a penny in the screwdriver slot, smash the
screw head slightly in a vise until the penny is crimped in place.
At this point you can begin reassembling your
carburetor. Begin by installing the throttle shaft and
shutter.
Make sure that your throttle shaft seals are in place when installing
the
shaft. Next place the throttle shutter into the carburetor bore
and
position it onto the throttle shaft. A pair of surgeon's
hemostats
are helpful here. A thin pair of long nose pliers may also get
the
job done. Use a long thin magnetic screwdriver to install the
throttle
shutter screw. Before tightening it all the way, wiggle the
throttle
assembly open and closed a few times to position the shutter.
then
tighten the screw.
Install a new bellcrank that is supplied with
the stock carburetor linkage kits that are available at any kart
shop.
Reinstall the throttle link and spring and special shoulder screw in
their
original positions.
One of the most effective things that can
be done to improve the performance of an "out of the box" carburetor is
to insure that the throttle is opening all of the way at full
throttle.
This is best done using a flow bench but significant improvements to
performance
can be made without one. To do this, while looking down the
carburetor
bore, bend the stop tab on the throttle shaft with needle nose pliers
so
that the throttle shutter is straight or parallel to the sides of the
bore.
You can also use a die grinder to grind the stop to achieve the same
thing.
Both methods work and are legal.
Place a new welch plug into the end of the
carburetor bore. Before doing this, though, apply a very small
amount
of grey epoxy cement in the recess where the plug snaps in. A sharp rap
with the ball end of a ball peen hammer will lock it in place.
Wipe
off any epoxy that oozes out.
Screw in the enlarged brass jet. Tighten
it with a screwdriver. Install the needle valve assembly
next.
Make sure the gasket washer is in place, screw it in with your
fingers.
Finish the job with a 1/2" wrench. Screw the needle valve
screw
in all of the way then back it out 1 1/2 turns.
If you removed your plastic fuel pipes, put
your new ones on now. Twist and shove them on using the same box
wrenches that you used to help remove them.
Place the diaphragm spring into it's cavity
on the side of the carburetor body. Lay the spring cap onto the
spring.
Position a new diaphram into position over the spring and cap.
Lay
the diaphragm cover over the diaphragm and install the four retaining
screws.
Get them all slightly snug before finish tightning any of them.
If you are going to use an air filter, and
you should, even if you race on paved tracks, install your air filter
adapter
now. Apply some silicone gasket sealer around the intake flange
of
the carburetor before installing the adaptor. The sealer will
insure
that all possible entry points for dirt are eliminated. It also
helps
retain the adaptor in the event that the retaining screws should loosen
up. Install the adaptor and tighten up the screws. A little
locktite on the screws is advisable.
At this point you should have a completed,
race ready, Briggs & Stratton alcohol carburetor. The only
thing
left to do is install the carburetor and fuel tank in the reverse order
that you removed those parts. It is wise to use brand new gaskets
and add a drop of locktite to the threads of each screw since
carburetor
flange, fuel tank flange and fuel tank bracket screws all have a nasty
habit of coming loose.
It's always satisfying to do well with equipment that
you prepared yourself. It's even better when you are on a shoe
string
budget and can beat the guys with the thicker wallet. Good luck
with
your new carburetor.
You will need the following items to accomplish this task;
1. small parts container
2. 3/8 socket wrench and accompanying ratchet.
3. Torx T-20 screwdriver, or a flat
or phillips screwdriver (see text)
4. Torx T-30 screwdriver, or B & S tool part number
19391, or a flat
blade screwdriver, or a B & S tool part
number 19305 (see text)
5. parts cleaning solvent and materials
6. 1/2" box or open end wrench
7. screwdriver, long blade with thin flat tip.
8. Torx T-10 screwdriver
9. pin vise
10. number 71 twist drill (drill bit)
11. number 53 twist drill (drill bit)
12. number 55 twist drill (drill bit)
13. 1/16" number stamp set (optional)
14. vise
15. open end adjustable (Crescent) wrench
16. adjustable reamer
17. small flat file
18. piece of plate glass
19. 600 wet or dry sandpaper
20. WD-40
21. dial calipers
22. long thin needle nose pliers or (hemostat)
23. stock throttle linkage kit
24. air filter adapter
25. silicone gasket maker
26. locktite or other thread locking compound
27. small ball peen hammer
28. 5/16 nutdriver or socket wrench
To comment or ask questions contact the author by e-mail; bergfelt@verizon.net