I had a similar problem with my VF500F, but it was carb'd. I haven't offered advice mainly because it was still a problem when I crashed it. Things I got advice to try (and they didn't resolve it) included changing the boots between the carbs and engine, changing the tubing from the engine vacuum to the fuel tank, resynching, cleaning the carbs, lubing choke, and others that escape me right now.
Dragging knee is for the track, and dragging tail is for the lot. --Kane Friesen
When you're in a car, you're watching a movie; when you're on a bike, you're in the movie. --Robert Pirsig
Identity theft is not Fun