If it has run the same with this hardware for some time as you say, then yes, something has changed. You can mount the tank the same as for your four stroke, and a quick test would be to try your four stroke tank, even though it may not be big enough. If it make the run better, just copy the plumbing on a tank of larger capacity that the K&B would need. Your current tank is not a unilow. For the K&B I would suggest a uniflow set up. The four stroke is a different animal, and I don't use a uniflow on my Saito powered Score because it works better. If you don't currently run pressure, try that also. Have you checked the engine back cover screws? Venturi and spray bar sealed well?
Type at you later,
Dan McEntee