Pewter, I appreciate the thanks.
Mad explained it well. The main differences aside from the obvios size difference are
1. The FCR carbs are "flat slide" carbs and the Mikuni carbs are "round slide" , the flat slide design flows more air for the same size orafice than the round slide design.
2. The FCRs have an accelerator pump and the Mikuni does not, the accelerator pump sprays in a tiny stream of fuel when going from off-idle to hard acceleration so the off-idle throttle responce is much better for the FCRs
3. Mad already explained how the Mikuni operates off a vacuum signal from the motor but the FCRs are a pumper carb design.
4. The slides in the Mikuni are held open by vacuum so when you hit woops or a bumpy section of trail, the slides tend to bounce and that causes an unstable A/F mixture because even though the engine RPM does not change, when the slide bounces down, it puts the needle into the main jet and causes less fuel than is required to be delivered to the motor. The FCR slides are mechanicaly connected to the throttle plate and can not bounce in the rough stuff.