I recently installed a VAPE conversion on my DT400. Timing should be spot on and it has plenty of power on acceleration. However on deceleration it bucks almost violently. As soon As I accelerate it seems great. I’m wondering if it caused by exhaust back pressure or what else I should consider. Timing is set at spec 2.9mm btdc. It also has died at a stop occasionally. 

The VAPE is firing at all throttle positions and since its clean at accel, probably not spark or exhaust. That said, a good BBQ of the exhaust has many values.

In your case, guessing clogged fuel port on carb or valve, maybe floats too low, possibly idle jet needs cleaning.   

“However on deceleration it bucks almost violently.”

All four of my big bore bikes have that bucking issue on deceleration. I assumed it was just how they are. It’s most noticeable when I’m riding on the road. So I simply pull in the clutch and use the brakes slow down and stop as opposed to downshifting or engine braking to a stop. It isn’t that noticeable when riding off-road. Maybe I’ll pick the worst one and try the suggestions made by pabdt.