In terms of "bang for the buck", an aftermarket "Jet Kit" is hard to beat. From the factory, carburetors are generally set up to run on the lean side in order to meet EPA emissions requirements. Unfortunately, EPA mandated standards run counter to getting maximum power and efficiency from your motor. In effect, a jet kit "richens" the jetting in the carbs and alters the fuel delivery characteristics of those carbs, thereby unleashing some lost power and driveability. Ideally, using a higher flow air filter and exhaust system will alow you to get the greatest benefit from a re-jet, but even if your motor is bone stock, you will see a difference (better low end throttle response and quicker warm-up), just to a lesser extent.These kits are available from a number of companies. My own experience is with Dyno Jet and Factory Pro, but there are a few others.
Jet kits are marketed in terms of "stages", (ie Stage I, Stage II, etc.). These merely reflect the "level" of modification to a motor. For instance, Stage I would be for a bike with a stock exhaust system and with the stock air box, Stage II would be for a bike with an aftermarket exhaust and aftermarket air filter, and Stage III would be for a bike with more extensive modifications, like head porting and higher compression.
Messing with carbs was the "last frontier" for me. I would do oil changes, valve adjustment, and that sort of thing, but carbs? Hell no! Those things are complicated. In truth, they're not really complicated at all and, if you have basic skills, you can do this job. I will tell you that tuning can be somewhat time consuming, but I would much rather spend some of my time than pay a mechanic (who may or may not be more knowledgeable than me) $65 an hour to do this job. As you may have guessed, these pics are of the carbs from a 1990 Katana 750. Generally, these basics are the same for most Mikuni Constant Velocity (CV) carbs.
Well, I guess it's time to see what this involves.
First, the carbs need to be removed (this is probably the most time consuming part). The tank and the left side panel have to go, along with the throttle and choke cables. Getting the carbs clear of the airbox can be a PITA, but that's just one of the things you have to deal with. Doing it for the first time can be painful, though.
This picture shows you where you'll actually begin and the tools that are required. Not much, huh? Hell, you probably have these tools in your "junk drawer" in the kitchen. Some people prefer to use a set of calipers instead of a ruler. Suit yourself. That allen wrench is a 5mm size.
When you pull the carbs, there will be a small amount of gas in the bowls. Try to keep them level while you're removing them. This picture shows where the drain screws are. The outer carbs require a 5mm allen wrench, while the inner drain screws take a phillip's head screwdriver. You don't have to run the screws all of the way out. After a few turns, the gas will drain from a hole below the screw. Oh, I guess I forgot to mention that you might need a jelly jar or something to catch the gas . (Think of our environment for just a moment, you inconsiderate slob - that's a joke fer chrissake). Examine the gas that came out of the bowls. Are there small specks of dirt floating around in there?
This picture is a view of what you'll find when you flip the carbs over and remove the bowls. That is the float assembly and the main jet is labelled. You'll see the pilot jet in the next step. Main jets are included in the kit. Changing main jets (and needles) will affect upper mid-range (say 5K-7.5K RPM) operation and top end. The pilot jets affect idle and part-throttle operation and are located under the little rubber cap (just below and to the right of the main jet). You shouldn't have to change the pilot jets. Idle operation will be taken care of later.
I warn you that those bowl screws are often seized and it is common to chew them up trying to get them out. If you have a small impact driver, it might come in handy, here.
You may find dirt in the bowls and inside the carb. Be sure to spray everything down with carb cleaner (especially the pilot jet). Ideally, you don't want a speck of dirt inside the carb. One small piece of crud is enough to block up that pilot jet. Hold each of the jets up to a light and see if they're blocked.
Carefully lift the float assembly out of the carb and this is what you'll see. The pilot jet is recessed under that black cap. Check the O-rings (2 of them) on the float assembly to ensure that they're not swelled or damaged. Those O-rings can be the cause of a hard starting or flooding problems.
Unscrew the main jet and replace with the appropriate jet from the kit depending on the modification level of your bike. Be honest with yourself. If the bike is stock, don't try to use the largest jets in the kit or you will gain something called "experience" by doing this all over again.
Replace the rubber cap, the float assembly, and the bowl. Don't forget that cap. If you leave one out, you'll have major idle problems and if you lose one, you can't "make" a cap from something else. Trust me on this one.
At the bottom of this picture is the pilot adjustment screw. If your carbs have never been touched, there should be a metal cap on that screw (remember the EPA?). Your kit will include instructions to remove that cap. Properly adjusting that screw will make all of the difference in the world as to how your bike runs. Set it as recommended by the maker of the kit. 2 - 2.5 turns out from the bottom is the usual starting point. Turn the screw "in" (that's clockwise) until it bottoms (you don't have to wail on it), then back it out the desired number of turns. You will probably find that these screws are set at anywhere from .25 - 1.5 turns, from the factory.
Now, flip the carbs over (right side up) and remove the 2 screws that hold the black plastic cap. Beware, there is a tiny O-ring under that cap that seals the synch fitting. If you look at this picture, you may notice 3 holes. 2 are for the screws and the third, at about the 6:00 position of the opening, is where that O-ring belongs.
As you pull up on the slide, you'll see the needle. Replacement needles are included in your kit and development of those things is why your kit cost you $125. Just turn the slide upside down over your hand and the needle will drop right out. While you're at it, inspect that diaphragm to make sure there aren't any tears in it. When you put the slide back in, ensure that the rim of the diaphragm is "seated" in the carb body.
*note - Dyno Jet kits give you the option to drill the holes in the bottom of the slide and put an insert in there. I don't remember if Factory Pro does that.
Here's the needle and its associated washers and spacers. This is an aftermarket needle as it has a number of slots at the top. Varying the position of the E-clip in those slots will affect upper mid-range operation and gives you room to "tailor" your set-up. I guess now is a good time to mention that you'll need a set of needle-nose pliers, too. (Get it? Needles? Needle nose pliers? That's a joke, son.)
Again, be honest with yourself and follow the directions that came with the kit.

Well, that's just about it. Reinstall the carbs and fire it up. The instructions should include some tuning tips. Sorry, there will probably be some amount of tuning involved, but you should be very close at this point. If you need some more help in tuning, try the Factory Pro site or ask at the forum sites I've provided links to on my "Links" page.
In case you run into problems reconnecting the various hosess, I have posted the diagrams for the carb, tank, and emissions hoses (those are 3 seperate links). As with any similar job, take notes or pictures of things like this to help you get it all back together, again.
While you have everything apart, it might be a good idea to check the gas tank. Rust from your tank or sediment that has accumulated from fill-ups over the years can cause headaches. Just shine a flashlight in there. Can you see surface rust or stuff floating around in there? Just drain the tank, unbolt the petcock, and rinse the tank out with a little fresh gas. On top of the petcock is a screen that serves as your gas filter. For the most part, it works pretty well, but it'll still allow stuff to get by that will plug up a pilot jet. Use the carb cleaner that you have left over to clean the screen and the petcock. This may seem unnecessary, but a few extra minutes, now, can save you a couple of hours, a week or a month down the road.
What I've described here is the job on a pre-'98 750 Katana. If you are doing this job on a 98+ Katana, you can also check out Tre's page for some more info and specific recommendations on the final settings. His pictures are much better, too.

I guess I ought to mention float height. Normally, you would not alter this, but if you think that it's not set correctly or just want to check it, click here .

Oh, and by the way, now that you have done this and your bike runs like it should, you can never operate it on the highway. It is intended for "closed course operation" only.
Yeah, right...
WRITTEN BY DOUG