1985 Kawasaki ZX 900 A2 (GPZ) exhaust jetting

I own a 1985 Kawasaki ZX 900A, also known as GPZ. I bought this motorcycle 3-4 years back and it came with an aftermarket exhaust, yoshimura.It is a 4-1 exhaust. The person who sold it to me, just replaced the stock exhaust with this Yoshimura exhaust and did not take care of jetting. Due to this, whenever I take this motorcycle above 80-90 mph, it starts to struggle. It feels like it is asking for more fuel. So I bought a set of aftermarket jet kit and got the stock jets replaced with the bigger jets. Now it gets plenty of fuel, but runs out of enough air and stops on me after driving for a while. According to the manual for the aftermarket jets, I should use individual air filters for each carb and remove the stock air filter box. Now here are the questions that I have,

Is there anyone who has some experience with aftermarket jetting and have any advice for me ?

This bike still has the stock air filter box, I removed the cover from the air filter box to increase the air supply and it runs better, but I can't run it like that all the time since I might end up getting moisture in carbs as well as it will be a trouble if I am out during the rain. I am thinking about removing the cover permanently and replacing it with a piece of moisture and air filter. Any recommendations about selecting the filter ? Is there any other way out ?

Thank you,

Deepak

Hello, Deepak, and WELCOME to KZR!
We're glad you're here!

Am thinking possible fuel starvation due to clogged vent in fuel cap.

Could test run with fuel cap ajar to determine whether there's a tank venting issue.

Might also be clogged petcock screen or clogged in-line fuel filter.

Could be wrong, but am very skeptical of needing to run pods. Would keep stock airbox.

Would also read plugs after high speed throttle chop to determine whether main jet size is okay.

And assure proper fuel level in floatbowls, using clear tube test.

Good Luck!

Pod filters are not required. Which jet kit did you buy?

This should help sort out the problem and also some history on those motors and carbs.

Most kits have 2 stages. One for the airbox in place and one with pod filters. There's a base setting for each setup.

Main jet size will be different between the stages.

DJ Kits have their own jet number sizes and come with a good assortment of jets.

In general, a 110 DJ Main Jet will flow slightly more fuel than the OEM jet of equal size.

If you have the stock main jets for the carbs, find the main jets in the kit that are a size or two larger than the stock jets and try them.

If it has mains that are 4 or 5 sizes over stock, then it's setup for pods and will be too rich for the box.

Pull the tops and see what the needle clip position is as well. Start with clip on 3rd groove from top. Also look to see if the slide bellows are twisted. That was a problem on early Ninjas. The A2 should have 3 steak marks on the slides where the bellows attach. They look like someone took a chisel and tapped it with a hammer. The staking prevents the bellows from twisting up. There's no mistaking it if they are twisted. You'll spot that right away.

Also, a simple and sometimes found problem on those bikes was if the engine was over filled with oil, the excess got pumped into the airbox and saturated the filter.

Stock air filter was an oiled arrangement type so it can simple be cleaned and reused it that happend. If you oil level is high, correct that as well.

As I recall, I ran like a 125 -130 main [keihin main, not DJ] with stock carbs, pods and Murray Pipe.

Pods work pretty well on a 900 Ninja. The design of the side covers effectively blocks cross winds from affecting the carburetion.

Uncork the intake side and she'll run pretty strong for you.

When you get a chance, do a compression check. If it's not up around 180lbs or more, holler back.

Larry C