Your Evo SPX and 2X comes stock with a 13.5mm throttle body, even though the carburator is rated as 15mm. This mod will improve accelleration and high end performance.

If you put a calipers in the intake of your carb it will measure 15mm. But put it a bit deeper into the throttle body, and you will see it will taper down to 13.5mm.

This new 15mm throttle body gives you another 33% of flow area through the throttle body (remember the area increases as the square of the radius of the opening, so a small improvement of 1.5mm in the radius results in a significant difference in the area). You will have to transfer your throttle cable end retainer from your old throttle body to the new one (remove an easy to lose c-clip, pull the throttle end post, and reinstall). You will also have to readjust your idle mix (jewlers screw driver through the brass bezel that the throttle pivots around) and your big air screw.

When you ride, you find the engine keeps winding up at the high end. I only have a few of these in stock at the moment, but will be getting more in (with the throttle cable keeper). These look like original Active Engine Equipment