dropping voltage wiring

Why don't you use a fuel pressure regulator?

What kind of car?

I do have a pressure regulator its on a Carbed yamaha R1 turbo my system only needs about 15psi max and my pump will do about 85psi I'm not 100 percent positive if it will run on 5 volts but I don't see why it wouldn't. If it doesn't then worst case is I buy a different pump.

After toying with my system for about 8 months the only thing I have left to try is lowering the flow/psi at the pump. My theory is that the pump is to powerful for my regulator and causes the spikes in my  fuel pressure when boost rises.

DC motors typically don't respond well to running at lower voltages then they are designed for.  It may run halfway decent but produce much more heat, it may hesitate as it spins and not spin consistently, and it may not spin at all.

If you want reliable speed control of the pump look in to pulse width modulation.  It will give you a much better chance of consistency wihtout risking burning up the motor that drives the pump.

I think the pulse width mudulation is out of my league if I need to make one. It would definately be the best way to go but I can only find one prebuilt unit and its to big to place somewhere on my motorcyle and its not protected from anything so I'd also have to make a protective enclosure  for it also.

Here is a DIY: http://www.aaroncake.net/circuits/motorcon.asp

You would have to verify that the transistor output is large enough to drive your load, but other then that it's actually pretty simple.

You may also need to modify it slightly to work in a portable setting (automotive grade power supply stuff).

I'm guessing if you search long enough you will find a prefabbed controller. 

Otherwise, you can try running it at a lower voltage, but I am guessing you will burn the pump out quickly (maybe within a few hours) if it runs at all.

I honestly feel that this problem will be best fixed mechanically, either with a different/better fuel regulator, or  by converting to a return fuel system (if you don't already have one).  The regulator should be able to pinch off the flow regardless of how much pressure the pump creates.

The windings of a 12vdc motor are different then the windings of a 5vdc motor.

Do a little research on DC motor control, as DC motor control is completely different then AC motor control.  AC motors you can typically control speed by varying the voltage.  DC motors usually like the PWM signal.