Everyone seems to be looking for the fcr 39mm or larger... I am wondering what size the factory mikuni carb is.

36mm

stock carb is a bsr36.

Wow, I would have thought smaller... Just because of all the claims of huge gains with a 39mm. 3mm makes a big difference huh?

Oh yeah, TRUST ME!!!! Its well worth it. But before you do the mod, educate yourself with the FCR39 carb. Plenty of info on here for that and got alot from TT.

Biggest reason is because the stock carb is a vaccum setup. The FCR's have an accelerator pump which squirts fuel into the engine for better throttle respose

No, the biggest reason is because you're going from a vacuum-operated carb, that works from engine vacuum to open the slide, to a real, no chit cable-operated slide. The FCR 39 operates the slide when you push the throttle - the stock BSR36 operates the slide when you push the throttle, and the engince vacuum catches up, and pressure differences make the slide open.

You whack a BSR36 open, and the slide will open once the engine vacuum makes it open. It doesn't take long, but there IS a delay. That's inherent in the design of a CV (Constant Velocity) carb.

You whack a FCR39 open, and it's OPEN. As soon as you hit it, it's OPEN. It's directly connected to your throttle cable. There's a direct connection between moving your throttle lever, and opening the slide. It's truly cable operated, and directly connected from thumb to carb.

That's the reason the FCR39 has an accelerator pump, too - when you OPEN it, it's on. You need more fuel when you go from Full/Off to Full/On, and the accelerator pump provides that extra, raw fuel, directly into the intake throat on the carb.