goes on first, as in the first thing you slide on the fuel screw.
Then the washer
Then the o-ring
What was the last thing anyone touched on the carb before this problem arose ? Don't rule out anything. You mentioned 'had to re-jet'. Well, with the exception of drastic temp and elevation changes and without major intake/exhaust changes there's no need to re-jet a bike that had been running fine.
Usually something like a partially plugged pilot happens and the owner feels its a jetting problem. It is, but not the one he thought.
Look very carefully at your manual, note how that plate goes on the carb slide. A cursory glance will have most people installing it wrong
, and the symtoms can be similar to yours. Look at it carefully.
Be sure you have that fuel screw set-up right too.
All ti headers on modern thumpers will glow and can be seen clearly in the right lighting . However, the white plug is serious business, this problem needs fixed ASAP.
There's no reason in the world to run the motor longer than a minute, 2 minutes tops, while you check your work. Doing so could cause some real problems.
This problem is related to something that happened when you decided to re-jet and took it apart.
Let us know what you find. Doug