S&S Cycle Performance Replacement Cylinder Head Kit for 1984-99 Big Twin Engines and Harley-Davidson Sportster Models - Manual (Page 6)

Carburetion
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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.
● 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 rpms. This dilution will cause engine roughness or misfiring when engine is held in this range.
Exhaust Systems
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Drag pipes - While drag pipes can be used with good results to establish performance guidelines on the pre-1984 engines, they
are generally not recommended for 1984-up big twin motors 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 for big twin engines consists of the stock header
pipes with the crossover tube and a set of low restriction mufflers. S&S offers a series of mufflers that can be used with stock
header pipes that work very well. This combination will typically produce more horsepower and torque 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
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Gearing depends on the total weight of the machine and rider/s, the size of the engine, cam/s, 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 the formula is as follows:
Engine Revolutions Per One Revolution of Rear Wheel =
(Clutch Sprocket*) x (Rear Wheel Sprocket*)
(Motor Sprocket*) x (Transmission Sprocket*)
*Number of teeth on each sprocket
Compression
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Generally speaking, big twin 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 11 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.
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