Later GPZ900R power are reduced compared to 84/85 (A1-A2) due to emission requirements and perhaps as marketing strategy to sell their newer (but slower) bikes. The full power was 115hp and the reduced versions were 108hp.
I bought used carbs off 85 model (A2) from Ebay. From the outside it looks the same as my original ones, but it's got the following jets:
Main: 135
Pilot: 35
Needle: N27B
There are comprehensive information regarding the jettings between models here:
Tips: the throttle cable actuators are different. Later model use the push-pull wire, so if installing an A1/A2 carb you need to also buy the correct throttle cable. Or in my case I just moved the cable holders between the 2 carbs.
The next difference is airbox intake size. It's quite self-explanatory below:
And lastly, the original exhaust has different baffle length and diameter. Original exhausts are pretty much unobtainable so an approximate can be had by cutting the baffle like below.
The baffle can be taken out by removing the rivets and pulling it using some home-made tool such as long rod with a catch welded at the end. It took some hammering/persuasions as that narrow taper tightly pressed into an opening inside. But thankfully not welded.
After riding it I noticed much of the difference is at the top RPM, let's say 7500RPM and above. The engine feels to be spinning much more 'freely' and 'willingly' compared to before. I like it.
