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Rejetting 91 nighthawk

7K views 9 replies 5 participants last post by  nighthawkguy 
#1 ·
Hello everyone, my name is Benjamin and I'm new to your forum. I don't own a rebel but I own a 91 nighthawk 250. I've ridden my friends rebel a bit and what a fun bike. I have a few simple questions about rejetting my bike. The mods I did are:

Removed airbox and replaced with a uni foam filter

Vented crankcase to its own air filter.

Rejetted the carb with a main jet to 115, pilot jet to 38, and 2 needle shims.

Valve adjustment to .003 inch

My questions are this:

1. What should the air/fuel mixing screw be set at for stock?

2. Due to a busy work life, what would be a good starting point for the air/fuel mixing screw to be at roughly?

If it helps I live in CT near Hartford, the bike has 10k miles, and new plugs. I've had the bike for 1k miles and I haven't had any problems. Thanks to all of you.


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#2 ·
Welcome to the forum. On a stock carb the Pilot screw (I believe that's what your asking) is lightly seated and then opened 2 and 3/4 turns. Since you re-jetted and played with the air induction, this setting may not apply any more.
 
#3 ·
Welcome to the forum! Rebel ownership not required.

Not sure when or if air/fuel changes were made on the Nighthawk, but as I recall, the first generation (1985-87) Rebels have an initial setting of 2 1/4 turns out from lightly seated while the second generation (1996-on) are set at 2 3/4.
 
#5 · (Edited)
... 91 nighthawk 250.
mods I did are:
Removed airbox and replaced with a uni foam filter
Rejetted the carb with a main jet to 115, pilot jet to 38, and 2 needle shims.
The stock setup for a 91 CB250 with its VE35A carburetor is a #110 main and #35 pilot. Airscrew is set 2.5 turns out. In stock mode it runs well except for some slight leanness off idle.

IMO, with the stock pipes your mods may be too rich. If the power seems soft and/or the exhaust stinks of raw gas, leave the #38 and go back to the #110 main, lift the needle with a single #4 washer, and start at 2.5 with the airscrew.
 
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