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It took a while but I have found a way to �repair� a failed Briggs �F� series coil 7/12/2007

Repairing a faulty Briggs and Stratton F series coil
by incorporating a commonly available moped coil.

Introduction

It took a while but I have found a way to replace a faulty winding on an antique Briggs
and Stratton F series coil. This magneto coil is used on antique Briggs and Stratton F
series engines as well as several other models. I believe this general technique should
work with the A series coils also. These coils are used on the antique Briggs and
Stratton Model Y and other models.

Ill also add an additional note in this preface. Ive seen pictures of several scooter and
dirt bike coils that appear to be good candidates for this procedure. It became apparent to
me, as I proceeded through this venture, that the challenge was to locate a coil with the
proper physical dimensions that would fit in the available space. The moped coil I use in
this narrative is the 1
st
coil I found that would fit in the available space, and when adapted
to this engine it worked. I havent tried any of the others inferred above.

When I started this venture, I was hoping that this procedure would not be as intrusive as
it turned out to be, however the procedure only requires the use of common hand tools
(an electric grinder, if you have one, will prove to be very handy); other wise a few files
will suffice. You will also find marking blue indispensable.

In this process you will irrevocably alter the geometry of the laminated steel core of the
Briggs and Stratton magneto coil so once you perform this process, the steel laminate
can no longer be used with an original Briggs and Stratton winding. This is not a
procedure you would use if you were striving for a historically accurate engine, but
rather a procedure to follow if you want your engine to run, you have a failed coil, you
want your engine to look original, and you do not wish to afford the wait / expense of a
coil rewind. Since this modification is entirely contained under the flywheel (as the
original Briggs and Stratton magneto coil is) this alteration is invisible to the outward
appearance of the engine, and therefore the engine appears un-modified. This is
significantly different than, for example, if you adapted a low-tension coil and battery
circuit, which is an entirely different route this discussion does not pursue.


Summary

The replacement high-tension coil I use is a Bosch moped coil that can be purchased for
about $20 at several online stores. Below are two sources. I think Ive even seen this
coil on eBay once or twice.
http://www.1977mopeds.com/catalog/index.php?main_page=product_info&cPath=29&p
roducts_id=290

http://www.mopedshop.com/Internal-High-Tension-Moped-Coil-p/puc3644505122.htm

The coil looks like this


Ive had this modified magneto coil assembly on a Briggs and Stratton Model FH for
several months now, and the engine has several hours of run time on it with this magneto
coil alteration. I have had no problems starting the engine with the petal start and have
had no problems with it running once it starts. I believe the moped coil winding produces
at least as hot a spark as an original Briggs and Stratton coil winding, however that is my
opinion. I have not measured this in any way to substantiate this claim. I have found that
the moped coil works with the same condenser as the original Briggs coil.

This procedure results in a magneto coil assembly that fits within the flywheel space and
functions in an identical manner to the original coil. When I made this prototype, I did
not document the procedure with pictures as well as I now wish I had. My excuse is that
I was leery of this replacement working. I built the coil, installed it on the engine,
replaced the flywheel and was surprised when I spun the flywheel and got a nice hot
spark on the first spin. Ive been looking to locate another broken F coil so that I can go
through the procedure again and thoroughly document it with pictures but I havent found
a broken Briggs F coil cheap enough to suit my fancy, so this article has limited pictures
for you to follow. I have however, augmented the photos with hand made drawings.

The altered Briggs and Stratton magneto coil assembly with the Bosch moped coil
substitution, mounted on the magneto plate and near ready to have the flywheel re-
installed is pictured below:




In summary, the procedure consists of
un-wiring and removing the Briggs coil assembly from the magneto plate
disassembling the Briggs iron laminated core
removing and discarding the original Briggs primary and secondary windings
re-working the Briggs iron laminated core to accept the new moped coil
re-assembling the Briggs iron laminated core and installing the moped coil within
re-installing the altered coil assembly to the Briggs magneto plate and re-wiring

This involves a fair amount of work, but is nothing that cant be done in an afternoon.
What I chose to do is re-work the original Briggs iron laminated core so that the new coil
will bolt onto this modified iron core without modification. My thoughts are that if the
moped coil fails, I can merely replace it by unbolting the old moped coil, and bolting a
replacement in its place. At $20 for a new moped coil, I can even afford to have one or
two in reserve and this would be about a one-hour job. Ill outline the procedure I
followed below.


Procedure

Disconnect the hi-tension lead on the failed Briggs coil from the spark plug wire and
disconnect the low voltage primary wire from the points and condenser circuit. You can then remove the four screws holding the coil in place and remove the coil assembly from
the magneto plate.

It is very handy to have a spare Briggs and Stratton magneto plate available, because you
need to trial fit the new assembly onto the magneto plate several times. There is no
reason however, other than convenience, that you cannot use the magneto plate on the
engine, and there is no reason to remove it from the engine if you do not have a spare.

Once the iron core coil is removed from the engine, drill out the rivets or cold chisel them
off so that you can disassemble the laminated core.



You dont need to be careful with the original wire windings because you will be
discarding that assembly.


You should end-up with some parts that look like the photo below:
Part 1-A and Part 1-B are the pistol shaped laminated core members that pass
through the original Briggs and Stratton electrical windings. There are many of
these members and that may not be apparent because in the picture they are
stacked one on top of the other. They are identical and can be interchanged.
Parts 2-A and 2-B are fillers from the laminated core top and core bottom.
These interleave between the Part 1 components to fill the empty spaces. There
are also many of these members and that also may not be apparent because in the
picture they are also stacked one on top of each other. They are also
interchangeable, and as you disassemble the iron core, if it is not already apparent
what Im describing, it will be. Place all these parts aside for now. They can be
combined together and I will refer to them as Part 2 for the remainder of this
article. Parts 3-A, 3-B, 3-C, and 3-D are used to hold the original Briggs electrical
windings in place and protect them. There is one each of these members. They
serve no useful function in this coil re-build, other than possibly adding mass to
the laminate and possibly strengthening the magnetic field. I re-used them when I
re-assembled the prototype.





You may be tempted to re-assemble the iron core assembly without using all the Part 1
and Part 2 members. I would resist this impulse because I believe that the denser the
laminated iron core, the stronger the magnetic field will be as it passes through the
electrical coil windings of the moped coil. A denser magnetic field should result in a
stronger ignition spark being developed. Having said that though, I will admit that the
prototype has been re-assembled with three of Part 1 and three of Part 2 omitted from the
assembly with apparently no ill effects.

In the following procedure you will be altering Part 1-A and Part 1-B. You do not need
to alter any of the Part 2 or Part 3 components. Set them aside for now.

Combine all of Part 1-A and Part 1-B into one group, here after referred to as Part 1
and then divide that single grou