I have a big cam 400 and it reaves up to fast when you put your foot on the thottle

 

Does it exceed max safe rpm, like the governor isn't working or the throttle limited screw is misadjusted? Or does it increase rpm to quickly, like the clutch is slipping? I don't know that I'd have an answer for you anyway, but more information might be helpful.

 

On a 400 cummins it's probably the spring on the throttle. I've driven a few of them where it idles, and runs damn near wide open, there's no happy medium. The last one it was just a throttle spring