Im looking for some Info on using flat slide motorbike carbs for my track car. Im using a twin cam 1600cc engine what sort of size should I be using for a quad carbie set up?

Depending on your setup & what rpm / HP you're running......

I'd start with some 42mm Mikuni Flatsides.
These can delivery enough fuel to keep a 1100cc sportsbike screaming @ 13000 rpm, so fuel delivery shouldn't be a problem.

However bear in mind, your standad header tune won't suit these too well - I'd suggest custom tuned length headers & or a re-jet to make the most of these.

They're a bitch to tune too - Make sure you cost some equalizing vacuum gauges into the overall cost.

Thanks a lot for your help. First sensible answer ive heard all week :P

No worries - Based on info I learned in a previous life as a motorbike nut