Feedback Score: 0 reviews
Join Date: Jan 2011
Location: Smack in the middle of Illinois
Thanked 287 Times in 282 Posts
Yes - you need to run it with the jetting that's in the carb
Changing the exhaust usually only requires increasing the size of the main jet by about 2 sizes larger than what is currently installed - it's changes to the intake that affect jetting most noticeably. Boring an engine does not affect the jetting. The carb is jetted for the flow characteristics of the carb, the fuel being used and the restrictions of the intake filtration system, not the size of the engine. Altitude is not a critical consideration unless above 6000 feet, then the jet sizes need to be decreased to compensate for the 'thin air'.
If installing a UNI filter in the stock air box, no jet changes may be necessary - if running with the lid off the air box off, the idle and top end jetting are usually affected the greatest. Normally the idle can be compensated for by simply adjusting the fuel screw. If the intake restriction is reduced great enough, then go up one size on the pilot jet and adjust the fuel screw for the smoothest idle. Run the vehicle following the parts suppliers break-in recommendations until the 'break-in' procedure is completed.
Now you can work on the high speed jetting. Warm the engine up completely - with fresh 91 to 94 octane non ethanol gasoline (or whatever fuel you intend to use - Honda recommends 93 or higher octane) Alcohol blended fuel should be 2 octane points higher than the manufacturer's recommendation, so you should run a minimum 95 octane with 10% ethanol.
With the engine fully warm (15 to 20 minutes of run time) get where you can run a straight line until the engine reaches maximum safe operating RPM - if the engine does not reach red line, it is probably lean and needs jetted up. First increase should be 4 or 5 sizes up from what is currently in the carb. Increase in steps of 2 (i.e. 145 to 155, then 155 to 165) until the engine blubbers like the choke is on. When you get to the point the engine blubbers at top end (too rich) then back down on the main jet two sizes at a time until optimum performance is achieved. With the engine hot, make any necessary final adjustment to the fuel screw for the smoothest idle.
Finally operate the vehicle at a steady appx. 1/2 throttle - if the engine surges, the slide needle needs to be raised one notch - if it blubbers, the needle needs to be lowered one notch (raising the needle means moving the clip down and lowering the needle means raising the clip)
There is no formula or chart that specifies what size jet to use for the modifications you have made. In fact, two vehicles with identical modifications may not have the same jetting due to minor differences in air flow characteristics between intakes and exhausts of each machine.
NOTE: sometimes the initial increase of 4 to 5 sizes of the main jet may be too much - it's just a starting point and adjustments have to be made from there.