This article is from the Ducatis Motorcycles FAQ, by Duke Robillard email@example.com with numerous contributions by others.
The Ducati standard jetting in the US is somewhat lean to accommodate
emissions requirements, and the airbox is restrictive for noise control.
The Dynojet kit, in conjunction with a K & N air filter, seems to be THE
standard modification for carburetted Ducatis that improves the tune of the
motor all the way around. Less restrictive exhaust cannisters at this point
would be a tasty and useful touch as well. After all, you bought the Duke
for that nice Ducati music, right?
Contact Dynojet (406-388-4993, FAX 406-388-4721) before ordering a kit to
make sure the kit you order or have installed is appropriate for the
altitudes you ride in. It does make a difference. You get several sizes of
jets to experiment with, and weaker diaphragm springs so the slides rise
faster, quickening throttle response. Be prepared to play with the setting
for a time, following their guidelines.
It used to be that you had to do some serious cutting into your airbox if
you decide to add the K & N kit, but nowadays, K & N makes a filter
specifically for our airboxes.
While you're at it, many owners spring for reduced gearing with aftermarket
sprockets. The general consensus seems to be 39 teeth for the 900ss and
derivatives (including the 750ss), and 42 teeth for 907ie's. This is also
to restore the European performance levels, as only the Americans get
unusually elevated gearing for EPA reasons.
Julian Bond has another suggestion:
If you're happy with the standard levels of noise, at least make sure
you're 900ss has the standard European jets. Replacement exhausts, Dynojet
or Factory kit, K&N and open airbox certainly work but can take some
fiddling and dyno time to get them to work properly. The airbox gets
seriously loud at some RPMs which you may not like. The Euro jetting is:-
Carb Mikuni 38-B67
Needle Valve Jet Y-2
Main Jet 140
Starting Jet 70
Idling Jet 42.5
Needle Jet 5C19, 4 pos from top
Air screws 4 turns out