why at around 4,000 rpm does the motor nose down, then inrease normally with it nailed?

 

What do you mean "nose down?"

And please read the FAQ and SEARCH for your questions before posting. Most common problems with FC's have been addressed many many times and can either be found in the FAQ at the top of this section or by using the search function.

 

It sounds like to me, if I had to take a stab at things you're probbably getting the 3,800 RPM hesitation. There is a search button at the top and take a look for "3800 RPM hesitation" or "3800 hesitation" Im sure it will come up with more then you want to know..

Mostly its has to do with grounds, so clean them all up. Don't forget to check the firewall to bellhousing ground.

Also you might think about using subjects to reflect the problem. I almost didn't check out the thread since it was not looking like a question....

Something like "4000 rpm nose dive" Just a thought...

 

Could also be hitting fuel cut...