I think most folks agree that modern Japanese bikes are usually set up from the factory to run on the lean side of stoic. This helps them meet emissions specs without having to use complex fuel management schemes. The good old carburetor has been around for MANY years and tuning procedures are well documented and known. Richening up you mixture slightly makes your Nighthawk much happier in starting, warm-up, and running. The expensive way to do this would be to purchase and install a Jet Kit (K & N, DynoJet, Factory are several brands). A cheaper alternative would be to raise the needles out of the jets a little with spacers (washers). This method keeps you out of the bottom end of the carb body and can be done with the carbs still in the bike. Glenn Stephens has done this to his bike and was kind enough to document his procedure and I've posted it here. Then John Glickman also did the mod and took photos along the way for me to post here with Glenn's text. Credits also follow the procedure.

Easy Carb Washer Mod -- authored by Glenn Stephens:

PREFACE: Just got finished placing washers under my main jet needles. Now I'm embarrassed. When will I listen to you guys? I've been putting it off, thinking, "It can't be as easy as they say", or, "They must be better mechanics than I am". The job was embarrassingly easy. And, yes, it is easily done while the carbs are still on the bike. I just had to remove the gas tank, side panels, and the crankcase breather hose that was in the way. Without a doubt, my best friend during this operation was my Chapman Gunsmithing Screwdriver system. It has a bunch of bits and a little ratchet that worked perfectly. A standard screwdriver is too long to use under the bike's frame. If I hadn't had the Chapman set to make it easier to get to the ones with low overhead clearance under the frame, I'd have probably used the bits out of my cordless screwdriver, and held them with vice grips to loosen the screws. The scariest part of the job is trying not to drop the screws down into the motor. That's what I did. One went under the starter, and I caught hell getting it out. I was more careful with the rest of the screws. Also, when removing the screws, push down HARD with the screwdriver, so you don't strip out the screw head, and turn the phillips indentation into a circle.
REMOVAL: After removing the four carb cover screws, the cover came off the carbs easily.

There is a large spring under it that pulls out freely.

Then you will see the vacuum piston assembly, which looks like a funnel.

The top of it is rubber. Don't pull it out by the rubber; it may tear. Stick your finger down into the middle of the piston assembly and rotate it a little to free it up, and then pull it out. You will see the main jet needle dangling from the bottom of the piston as it all comes out of the carb.

Looking into the piston, you will see a star shaped piece of plastic. Take a Phillips screwdriver, push down on it, and turn it counter-clockwise 120 degrees. It comes right out. Don't knock off the spring that is attached to the bottom of the star. Invert the piston, and the needle falls out.

CARB MOD: It's easy to tell which washers are the #4's in the Radio Shack pack of washers (Catalog # 64-3022A). The pack contains 100 washers, 20 each of sizes # 2, 4, 6, 8, and 10. The #4's are the only ones that fit the needle. The two washers easily slide over the tip of the needle and on up to the head.

Then grab the head of the needle with needle nose pliers, turn the needle vertically so that the washers don't fall off, and gently guide the needle back into the hole in the bottom of the piston. You can forget using three washers. I tried, it, and the third one wouldn't fit. To replace the plastic star, open a bench vice a half inch, and set the piston on top of it, with the needle sticking down through the jaws of the vice. DO NOT let the jaws scratch the needle. Using long needle nosed pliers, grab the top part of the plastic star with the Phillips slot formed into it, and guide the star down through the piston and over top of the head of the needle, seating it in the bottom of the piston and compressing the star's spring. I did not have luck using the pliers to rotate the star 120 degrees clockwise to lock it back into place.

While holding the star to the bottom of the piston, I then inserted a Phillips screwdriver into the top of the star between the jaws of the pliers, pushed down with the screwdriver, and removed the pliers. Then I pushed down with the screwdriver, and the star easily turned clockwise and locked into place (well, I had to "wiggle" two of the stars to get them to rotate). The unit is now ready to put back into the bike.
NOTE: 16Feb09 - a guy named Dan sent me an email and said he was able to use an 8mm socket and extension to get the star to turn. I haven't tested this myself, but it sounds plausible.

REINSTALLATION: Looking down into the carb, you can see the main jet, into which the needle goes. Without touching the rubber, guide the piston back into the carb body. As it goes down, stop if you feel ANY RESISTANCE WHATSOEVER. That would mean the needle did not go into the hole. All four of mine swooshed right into the hole for me, but it pays to be careful not do damage the needle. Then stick your finger into the piston assembly, and rotate it until the rubber tab of the piston lines up with its matching slot in the carb body.

Replace the spring and cover. It's a good idea to put the four screws back in loosely, then snug them in rotation, and finally tighten them in rotation. Not too tight; they would probably strip easily.

RESULTS: I was absolutely amazed at the results. I was expecting NO improvement in my top end, but I got improvement EVERYWHERE. It was impossible not to notice. Not only does it take off faster, but it sounds A LOT better, like it has been "freed up". I took off from a stop, giving it all it had. When I shifted into second at the redline, the front tire came off the ground about an inch, and bounced back down like a basketball. I have driven this bike for 21,000 miles, and it has NEVER done that before. I stopped and tried it again, this time pulling back sharply on the handlebars as I shifted. The front tire came off the ground about four inches that time. It races to the redline without hesitation. It used to feel like it was going to run out of steam at about 8K, but now it races into the redline. When I got home, I did a few wheelies for the neighborhood kids. It doesn't seem to wheelie up any more forcefully now, but the rise and fall is a lot smoother and more controllable. It eases back down to the ground a lot less forcefully than before. I really don't think that most people will see results like I experienced today. This little experiment goes to prove that when I did the Tim Hodge mod on my bike, I definitely made the system VERY lean. My bike has been STARVING for fuel, obviously, and today it furiously sucked up the extra gas. I haven't heard of anyone on this list that has went as far with the Tim Hodge mod as I have; I've drilled ten 3/8" holes in my pipes (five on each side). In the future, I would advise anyone doing the Tim Hodge mod to do the washer/needle mod at the same time.

Text contributed by:
Glenn Stephens
Portsmouth, VA
2000 CB750 Nighthawk ("Little Wing") pics at...
http://users.erols.com/glenns1956/Index.html
(21.0K; and still going and going and going...)
Photos contributed by:
John Glickman
2000 Nighthawk 750

Back to the nighthawk750.com contents page...