gen ones are getting old, and like any old carbed bike, its realistic to expect to up both preventative and emergency maintenance mate. I wouldn't fit the Ivans until youve solved the problem, otherwise its just more blind alleyways looking for the answer.
If you take your carbs out and thoroughly clean everything (including every tiny hole in emulsion tubes etc) not only may you solve it, but it will make it far easier the second time around to get the carbs in and out for the Ivans, as you will have a better idea of the steps involved.
Im slowly working on my xjr13; have replaced the stock inlets for 36y unrestricted jobbies, cleaned carbs along with checking float heights and balance, 4degree ignition advancer and 17t gearbox sprocket (stock is 18), but do everything one step at a time then try the bike for a few days before moving on to the next job(in my case its the AIS).
The good thing about the fazer is its pretty much engineering from the ark, so you will be able to sort it mate
mine starts better with the Ivans but I have to let the pump prime fully, and im running slightly richer than ivans instructions.