My 1996 Suzuki GS500 idles fine but when opening the throttle they die before picking up then it ticks over at 4000RPM,, PLEASE HELP?

The Suzuki GS500 may just need new spark plugs. This is an older bike. If the plugs check out you may want to look around. I don't know how much you know or how far you want to go but. A loose timing chain or sprocket, bad spark plugs, leaking intake manifold, old gas or dirty/faulty carb can cause this symptom.

Are the carbs set up for the new motor? Have they been balanced? Is the idle set to 1400rpm?

What is the fuel pump pressure? When you put the throttle down, there is not enough fuel to match the air inlet and it dies.

To much air not enough air or the fuel

Had that problem last year and it ended up being a loose ground on the engine side... try cleaning up your contacts on the battery and on the motor... every time the sifter cable moved on my engine it would bump the ground.

most common ly it sounds like a crankshaft position sensor, or crankshaft position sensor.... is it throwing a check engine light? if so scan the codes...

Sounds like this bike has been standing for a good while & the carbs need cleaning & resetting (or there is water in fuel in carbs)
Ride Safe Bike-Doc

idle air controll motor is bad