Could well be either the choke sticking open or throttle cable not allowing the carbs to fully shut.
Latter is easy to check, just look at the butterflies inside the carbs, do they close fully when the throttle is closed?
Choke cable, well it could be stuck holding the choke open...likewise the wee bits inside the carbs that perform the choke action (little plungers) could also be stuck.
Suggest having a look on fleebay for an xj900 manual on CD...be worth it's weight in gold...sell the haynes on there to some unsuspecting muppet at the same time
