Fat Boy 4 Complete Exhaust System-Honda 300EX-250X

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INSTALLATION DUNCAN RACING
INSTRUCTIONS INTERNATIONAL, INC.
FAT BOY 4 COMPLETE EXHAUST SYSTEM

MODEL: Honda 300EX/250X


1.Remove stock exhaust system; Retain stock head nuts.

2.Before starting your Fat Boy 4 installation. You must make sure that the cylinder head exhaust gasket is intact and in good condition (usually they stay in the cylinder head). Under normal conditions it is OK to reuse them. If they are missing or in need of replacement they may be purchased from your Honda Dealer Part # 18291-MV9-000. DO NOT RUN MACHINE WITHOUT EXHAUST GASKETS. It also a good idea to make sure that the exhaust studs mounted in cylinder head are in good shape. If there is any doubt of their condition, replace with new oem studs.

3.Install Fat Boy 4 header pipe using stock oem head nuts. Do not secure header at this time. Leave nuts a few turns loose.

4.You are now ready to install the Fat Boy 4 muffler/tail section. The tail section is secured to the Fat Boy 4 muffler at the factory before shipping. If these (4) 6mm stainless button head allen screws are ever loosened or removed they must be reinstalled with some form of medium strength thread lock (recommended Loctite 242 Blue).
Before sliding tail section onto header it is recommended to apply a thin coat of hi-temp silicone to the inside of the tail section (female side) approximately covering about ½” of the initial part of the tail pipe. This will keep exhaust from leaking at the slip-fit joint and make removing pipe in future easier.

5.Install the (1) bolt 10x35mm flange type (supplied by DRI) in the muffler mount going into sub-frame and tighten securely.

6.Tighten header pipe. Make sure to tighten header nuts evenly in a criss-cross pattern.

7.AIR INDUCTION: For maximum performance DRI recommends installing K&N Filter with air box lid removed.

8.Rear master cylinder reservoir bottle must be relocated. There is a relocation bracket included in this instruction packet that allows bottle to be moved clear of tail pipe(Figure 1 & 2).

9.Carburetor jetting must be changed, see below for additional recommendations

10.Additional Performance and Jetting Recommendations;

CDI modification: On all 300EX models the Rev Limiter engages at a very early RPM. To receive maximum performance from your new Fat Boy 4 exhaust system a modification is recommended to your CDI to remove Rev Limiter.
DRI offers this modification for a reasonable fee and quick turn around time.
Send your CDI to DRI (address below) modification cost is $50.00 plus shipping cost. Turn around is 1-3 business days.

Gearing: It is recommended to increase c/s sprocket by (1) tooth over stock for general riding conditions

Jetting Recommendations: (call for 250X models)
The following guidelines are for Honda 300EX’s with the following type set-ups.

Sea Level Temperature between 60° and 90°

Fat Boy 4 Complete Exhaust System

Stock Engine K&N Air Filter Air Box Lid Removed.

Pilot stock
Needle stock
Main Jet: # 132

*DRI has included, at no charge, the necessary jets for the above jetting changes with an extra main jet each way.

The above specs should be an excellent starting point for most installations.
Jetting should be checked after install.

Be careful not to run machine too lean-engine damage may result. Too rich power will be lost.

Jetting Notes:

Colder temperature requires richer settings.
Higher elevations cause engine to be low on compression and generally require leaner settings.
Air box lid on requires leaner settings than with lid off.

For more information contact:

Duncan Racing International, Inc.
10734 Kenney Street Suite A
Santee, CA 92071
USA
(619) 258-6306

Check our Website for additional Honda 300EX Performance items. www.duncanracing.com