I got a used 99 F4 and it has a D&D slip on. I don't know if the previous owner rejetted and was wondering if this could be determined by looking at the color of the needle (On my dirt bike the stock needle was steel and the aftermarket ones are brass so I wondered if this were also true with streetbikes). I ask this because the bike has a tendency to stumble at low (off idle) rpms.

 

Unfortunately you have to take the carb off and look at the needle itself. It may just need to be cleaned if its only doing that at low RPM's.