S&S Worldwide Motorcycle Accessories - Manual (Page 15)

Carburetion
●
All S&S ® test engines are run using S&S carburetors. S&S
Super E and G carburetors are recommended for most
applications with the Super G being used more often on
larger displacement, freer breathing engines with higher
compression ratios. Typically, engines equipped with S&S
heads require the same or slightly leaner jetting than those
engines fitted with stock heads. Consult the carburetor
jetting instructions for specific jetting recommendations.
● If another type carburetor is used, it must be made to run
rich enough to operate properly and to prevent engine
damage. If you have a problem with another carburetor,
S&S cannot help you and recommends you call the carb
manufacturer with any questions you may have.
● If the motorcycle is used exclusively on a drag strip where
engine temperatures vary, slightly richer jets may be
necessary for best performance. Larger jets/richer mixtures
will enable one to run a colder engine which is sometimes
desirable. This is best determined by experimentation.
● Carburetor jetting and spark plug color - While spark plug
color may be used to help determine carburetor jetting, S&S
recommends that our instructions be used as primary jetting
guide and that plug color indications be used only as
secondary aid. We have found that different brands of
gasoline, gasoline additives, engine heat (due to ignition
timing), and brands of plugs and heat range used distort
plug color drastically making plug reading difficult for the
average tuner. Also, new plugs usually require a road test of
10 miles or more to properly develop the color which means
that quarter mile tests may not be long enough and hence,
not always a good indication of carb jetting. It is best to use
proven spark plug combinations and to consult the spark
plug manufacturer if you have questions. If one desires to
become more proficient at plug reading, Champion Racing
Division has a very informative booklet which may be
helpful. For details, write:
Champion Spark Plug Co.
PO Box 910
Toledo, OH 43601
● Cams and exhaust systems can make some engines difficult
to carburate. We have found that certain cams and exhaust
systems will cause poor performance at a specific rpm, and
attempts to correct jetting for that specific level usually
destroys carburetion over the balance of the range. A
combination of cam overlap, reversion, and back pressure, or
even lack of back pressure can cause mixture dilution at
certain engine rpm’s. This dilution will cause engine
roughness or misfiring when engine is held in this range.
Exhaust Systems
●
Drag pipes - While drag pipes can be used with good results
to establish performance guidelines on pre-1984 engines,
they are generally not recommended for 1984 and up big
twin engines in street applications. Big twin engines are
easier to carburate with muffled systems.
● Muffler systems - Most stock and many aftermarket
exhaust systems are too restrictive and made exclusively
for looks with little consideration given to performance.
A very good, economical street system consists of the
stock header pipes with the crossover tube and a set of
low restriction mufflers. S&S offers a line of slip-on
mufflers that create performance economically. This
combination will typically produce 10 hp more than drag
pipes in the midrange. Since the midrange is where the
vast majority of normal driving occurs, it makes this
system ideal for the street.
Gearing
●
Gearing depends on the total weight of the machine and
rider's, the size of the engine, cams, exhaust system and type
of riding to be done. Most high performance engines, and
particularly those with larger displacements, are capable of
pulling more gear. We suggest you break the engine in with
stock gearing to minimize the load on the engine. After the
engine is broken in you will have a better feel of its potential
and can change gearing accordingly.
● For those who wish to determine their final drive gear ratio,
refer to the formula shown below:
Compression
●
Generally speaking, 1984 and later engines with the proper
camshaft selection can operate using higher compression
ratios with fewer problems than their earlier counterparts.
Keep in mind, though, that while engines with higher
compression ratios make more horsepower and perform
better, they also tend to lose that performance edge faster,
require more maintenance and start harder. As a rule, we
prefer to limit the compression ratio to no greater than
about 10 to 1 for engine combinations used in normal street
operation. A word of caution is in order. Before building an
engine you may regret later on, carefully consider your
riding needs, riding style and overall performance objectives.
15