Jetting!!!????

[graham300ex]

I have to leave friday for a race and need to fix my jetting. I put a 400ex carb on my 300ex. It has 148 main. Idle to 3/4 is fine but when I get it wide open it stumbles and picks back up. Is it too rich? The spark plug i couldnt get an accurate reading.
Look at my engine mod list below.