If the trouble happens when you transition from idle to hi speed it
may be your low idle adjustment.. I'm not sure how it's setup on the
Evo engine, But the screw would be in the middle of the throttle linkage
or it may be a air bleed type which would be somewhere in the front/side
of carb.. The starting point is somewhere around two to two and ahalf
turns out.. If you do try to adjust it.. First "count" the number of turns
it take to close it(lightly) That way you will know where your setting was
to start with.. And or make small moves(1/8" or so) at a time to lean or
richen it, and try it out to see if it helps..
But if nothing else has changed since it did run good, you may be having
a air leak somewhere.. From the needle valve, tubing and or tank...

Good luck in finding your problem!....
You know this is one of the many good reasons to belong to a club...
Most clubs give you unlimited help and knowledge... Not to mention deals
you may find:-)