Its not the jet kit that makes the real difference, per say, but whenever you make adjustments to the way the engine breathes, you need to make adjustments to the fuel/air ratio at the same time.
Lets say you take a perfectly stock motor, and the jetting is set up perfectly for the airflow. If you make a change, say K&N filter pods, and a full exhaust replacement (old was 4-2, new is 4-1), and both are capable of flowing a considerable amount of air than the stock setup, the engine is now running on the lean side. More air plus same fuel = higher air/fuel ratio.
If the air/fuel ratio is to high, first off, the power just won't be there. Second, since there is more air in the combustion chamber, the temps will rise, and the possibility pre-detonation might occur increases. If there is alot of excessive pre-detonation, damage in the combustion chamber can occur. If you're lucky, its just a hole burnt into a piston.
So what you would do to bring the air/fuel ratio back to an acceptable level would be to increase the size of the main jets in the carbs. How many sizes it needs to be increased can only be determined by changing and testing, over and over. I always recommend going one size at a time, and riding the bike normally for a set period of time. That way you can notice the difference under your normal riding conditions.
This is the long way. The quick method is to put the bike on a dyno, make a base run, change the jets, make another dyno run, change the jets again, and keep the process going over and over until the bike makes max power under controlled conditions.
Only problem with this method is you are at the mercy of the dealer. Unless you really know the guys who will be working on your bike, how will you know that the bike is perfectly set up? Do you really think they changed the jets 8 times to find the perfect combination? Did the dyno sheet show runs for each jet size that came in the jet kit? What happened to all those other size jets that came in the kit? There should be left over parts...
But the main reason to rejet a bike after making changes to the intake/exhaust systems is to bring the bike back to a proper air/fuel ratio, to produce the maximum possible power, and to also prevent engine damage. You don't need to buy a jet kit to do so. You can always go to the dealer and buy several different sizes of jets, and things can be just as good as the jet kit.
Also, if you make a mistake with installing jet sizes, its usually not as destructive as if you drill out the wrong hole because the instructions were not perfectly clear (or if you just suck at following directions).
First, you can rejet you bike without getting a jet kit. Main jets cost about $5 apiece at ronayers.com.
A lot depends on where you live. The factory jets the bikes for average elevation . . . I really believe that that they jet them so that they will not run too lean a sea level which could cause damage. As you increase in altitude you need a leaner mixture because of less oxygen per volume of air. An airplane has a carb that has a mixture control to lean out above 5,000 feet. So even on a stock bike you can sometimes get a little extra buy playing with the jets (usually by going smaller).
The jet kit include some new parts to include the needle that fits down the middle of the main jet. This controls the mid-range. The stock on only has a clip at the top. You can raise it with shims buy you cannot lower it. The replacement has a clip with several notches so that it can be raised and lowered.
The bottom line is you probably won't get much out of a jet kit unless you are way above sea level or have added a full exhaust. You might check out Factory Pro. They have a better kit than Dyno-Jet.
They are lean in the mid-range. Bikes that are jetted lean on the top end soon self distruct causing costly warranty problems. Lean mid-range can be cured by a couple of nickle #4 washers under the needles.
I am damn near on the coast. It has been my experience that I can get a very small hp increase by going to a very slightly larger main jet. Most of those I talk to actually get a hp increase by decreasing the main jet size and shimming the needles because they are at higher altitude.
You don't actually "need" one, but if you were to pick up a full set of jets one size larger and another set one size smaller, and run both sets of jets for a week each, you can find out if there is an improvement with a slight jet change.
As long as you didn't damage anything while installing the jets (which is always a possibility), it wouldn't hurt to try, at the very least.
But you never "have" to do something, if you feel the bike is running perfectly fine.
You can get a set richer and a set leaner by buying only four jets because of the way the ZX-6R is jetted. The two middle cylinders have bigger jets 157.5 and the outside have 155. Buy two 160 and 2 152.5 and you have three sets, rich, normal and lean.
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