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jetting
I don't know the stock jet size of this bike. However, if that can be found out, changing the jet isn't a very difficult thing to do. On my 250 Ninja I had to drill out a little cover that sits over the jet. unscrew it and then insert the new jet.
As for the needle shim, all you have to do is buy #4 washers, I prefer brass, and place maybe 1-2 under the needle. The needle shim evens out only the mid-range. The idea is that lifting the needle allows a slightly higher amount of fuel into the combustion chamber.
Jets will flow more fuel over the entire range but it doesn't fine tune the "jetting" process. The jets are also responsible for evening out the top range of the power.
So, jets can have an overall affect of the powerband.
For example, a larger Jet allows more fuel at all times. A larger jet plus a needle shim may allow TOO MUCH fuel into the chanber.
This is why it is important to adjust one thing at a time until your entire power band and fuel flow is even. It requires tinkering, then going for a ride. Tinkering a little more, and then going for another ride and adjusting anything that needs to be taken care of.
The final step to make it perfect would be adjusting the idle mixture screw and making sure that all three adjustments make the bike idle and ride as close to stock as possible. The adjustments plus any modifications that you have made will deliver more power in certain areas while maintaining the same ride quality. (This assumes that you've done everything right.)
I hope this helps. I have some pretty detailed instructions on how to jet a Ninja 250 Carb, if anyone is interested. It's pretty much the same. The Ninja has 2 carbs and, of course, the GZ has only 1, which should make the whole process easier.
I would recommend installing a universal air filter on the bike. It opens up the mid range and makes the bike sound pretty mean while not bursting the ear drums of people near by. Yoou'll enjoy it.
Take care,
Jonathan
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