This is from the MRA (Colorado) rule book. Which makes a low cost class/s and requires Box stock from air box intake to exhaust port. For the most part.
SMRI rules are a little more loose. In that they allow removal of the air box.
Over the past years I have tried to keep my ninjette as close to MRA race specs. because I once had plans to do some racing up north. (may still do that). Also if I remember right CVMA ninjette class is almost the same as MRA and I would love to race that track also. Maybe someday if I live long enough.
2.10 Production Cup Classes
This is an amateur class designed to provide lower cost, highly competitive “spec racing”.
Production Cup racing is separated into three displacement categories: 250cc, 400cc,
and 500cc. All Production Cup motorcycles are required to be four-stroke single or twin
cylinder, produced and sold for street use in the United States.
Only the following modifications from showroom stock are permitted:
A. Bike must comply with safety standards listed in section 5.2 EXCEPT that the
Production Cup Class does not require a steering damper. Steering dampers may,
however, be added at the riders discretion.
B. No engine modifications of any kind are permitted. Engine configuration must
remain as delivered from the factory. Rebuilding or refreshing motor with
Standard Specification Stock Production OEM parts is allowed.
C. The stock airbox, air box cover, air filter and intake snorkel must remain in
place and connected as they came from the factory. The air box drains may be
sealed for fluid retention. No other modifications of the airbox, air filter or intake
snorkel is permitted.
D. Bodywork, windscreen, fairing stay and bodywork mounting brackets may be
replaced with parts of unlimited origin. Relieving bodywork for clearance of
hand and foot controls, exhaust and frame sliders is permitted.
E. Exhaust systems, including exhaust bracket assembly may be replaced with parts
of unlimited origin.
F. Carburetor jetting consisting of main jet, pilot jet, needle and slide hole size may
be changed. No other modifications to carburetor are allowed.
G. Fuel injected motorcycles may utilize and aftermarket tuning device for the
purpose of fuel management only.
H. The PAIR valve may be blocked or removed with parts of unlimited origin.
I. Suspension may be modified with different springs, valves and oil. The stock
rear shock body and stock fork tubes must be retained and unmodified. Preload
adjusters may be added to the forks. The stock damping rod must be retained,
but may be modified. Rear ride height may be altered or adjusted, including use
of aftermarket dog-bones.
J. Brake pads may be replaced with parts of unlimited origin, however brake
calipers and rotors must remain stock.
K. Brake lines may be replaced with parts of unlimited origin.
L. Hand and foot controls, rearsets, handlebars and levers may be replaced with parts
of unlimited origin. Brake master cylinders and throttle control must remain stock.
M. Gearing may be changed by replacement of chain and sprocket only. These parts
may be of unlimited origin.
N. Tires must be DOT rated tires only.
O. Stock charging system must be fully connected and functional at all times.
P. Instrument cluster may be removed and gauges may be replaced with parts of
Q. Only DOT approved street legal pump gas may be used. NO race gas or
performance fuel additives are allowed. At the discretion of the MRA, fuel may
be supplied to competitors at prevailing market price.
R. Parts which are not critical to bodywork or integral support may be removed,
but they must be removed at the stock mounting or bolting point. Cutting of
materials or frame is prohibited, with the exception of the kick-stand bracket
which may be removed for ground clearance. The countershaft sprocket cover
may also be modified or removed.
S. For Kawasaki EX250’s manufactured prior to 2008, the following modifications
beyond stock are permitted:
a. Wheels may be replaced ONLY with OEM 2008 and up 17” Ninja 250 wheels
or any year EX500 wheels.
b. Rear shock may be replaced ONLY with OEM 2008 and up Ninja 250 shock
or any year EX500 shock.
c. Front end including upper and lower triple, forks and brakes may be replaced
ONLY with OEM 2008 and up Ninja 250 front end or any year EX500 front end.