Help.............

I recently bought a KZ400D from the USA. The carbs were missing so I fitted a set of Mikuni VM 30's from Speed Motoco. Fitted them and it starts and runs at idle very well with a nice steady tick-over. Problem is, it wont run at anything above 1/8th throttle. its runs fine on pilot system but cuts out when I open the throttle. It doesn't pop or bang or falter, it just cuts right out like I have switched the ignition off so I'm thinking its probably not a carb tuning issue, could be bad coil??
Any suggestion would be welcome.

Check your ignition timing. The advancer might be stuck. If it isn't check your coils, condenser and points if you have them.

Check and adjust as needed valve lash. If any are way out, the carbs cannot be properly set.
Check to be sure that the ignition advancer is clean, lubed and turning freely.
After doing these things you can dive into the carbs. Bench synch, low speed mixture screws both set the same, idle adjuster in the middle of its range, fuel level correct. Start and warm up the engine. Vacuum synchronize and fine tune low speed screws.

If you are not using the stock air box, you are in for much quality time with your carbs.