I recently installed a K&N air filter in the stock air box and changed the main jets from 115 to 118 (one size larger) on my 1984 650 Nighthawk with 6,400 miles on it. The bike ran better than stock with the new K&N, but I decided to rejet the carbs to see what both mods would do for the bike. The bike no longer stumbles off idle, has gobs more top end and seems to rev quicker. The only drawback is that I REALLY have to keep an eye on the tach because the revs build quicker and it gets to 10 grand much faster now!

The K&N cost me about $40 and the new jets were $7.50 each from the dealer here in San Diego. Aftermarket jets may run a little less than Honda’s, but I bought the Honda jets anyway. I tried to do this with the carbs still hooked up, but I couldn’t get enough clearance to work with a short screwdriver or "z" offset screwdriver. Make sure the battery has a good charge or put it on a charger. It will need a full charge to refill the float bowls afterwards.

The following is the procedure I used to change the jets without disconnecting the choke or throttle cables:

• Remove side covers, seat and gas tank.
• Drain float bowls.
• Loosen clamps on front and rear intake runners.
• Place a shop rag on the alternator and starter to prevent any scratches.
• Remove the rear intake runners from the carb assembly and air box.
• Slide the carb assembly back until the front is clear of the intake runner. They fit snug, so you may have to twist or use a little up and down movement to remove them.
• Move the assembly towards the left side of the bike and rotate it so the #1 carb (left) is on the top and the #4 carb (right) is sitting on the alternator cover.
• Remove the float bowls. Discard and replace the O-ring gaskets on the float bowls to save yourself a possible headache in the future.
• Remove the main jet holder with a pair of pliers, and then remove the main jet from the holder with a slotted screwdriver that fits exactly into the main jet.
• Remove the idle jet with a small slotted screwdriver.
• Let the idle jets and main jet holders soak in carb cleaner for about 20-30 minutes. An easy way to do this is to spray some carb cleaner in the cap that came with it and drop them in.
• Making sure you have a good grip on the parts, blast some compressed air through the jet orifice to clear out any residue.
• Reinstall the idle jets.
• Reinstall the main jet holders.
• Install the new main jets.
• Reinstall the float bowls.
• Rotate the assembly back to original position.
• Put a light coat of Armor-All inside the rubber intake runners to make it easier to reinstall them.
• Slide the carb assembly into the front intake runners.
• There are two methods of reinstalling the rear runners. You can either attach them to the carbs first or to the air box first. Either way, it is a pain in the butt and can take longer than you think to finish. Just make sure not to use a sharp screwdriver to fit them into the air box.
• Tighten the front and rear intake runner clamps. Make sure you check the underside of the runner while you are tightening it. They tend to slide back a little while being tightened.
• Take a minute to remove the bottom of the fuel cutoff valve and clean the strainer inside it. Remember to reinstall the O-ring inside it. My bike quit on me a couple of times last year and cleaning the strainer fixed the problem. Old gas tanks have a lot of nasty stuff inside waiting to get to your carbs!
• Consider installing an inline fuel filter to catch what the strainer doesn’t.
• Reinstall the gas tank, seat and side covers.
• Fire it up and check for leaks. Make sure the drain screws on the float bowls are tight, not just snug.
• Now, kick some butt on the street!

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