Welcome to Yamaha Synth!! Thanks for the question...

I am looking to use the fc7 pedal to control wah wah clavinet sounds on my XF8. Please help.

There are already a couple of VOICES setup to do this. We can recommend you use them to learn from _ then you can applied what you learn to the Clavinet sound of your choice.

First Experiment
Recall the VOICE:
PRESET 1: 079(E15) Pedal Wah Clavi FC2

Plug your FC7 Pedal into the Foot Controller 2 jack on the back panel of your XF8.
Press [F6] EFFECT to take the shortcut to the EFFECT Edit page
Press [SF2] INS A to drop into edit on the VCM Pedal Wah type which is assigned to INSERTION EFFECT A

You can see how the FC2 is assigned to control this Pedal Wah effect by navigating to the CONTROL SET
Press [F4] CONTROL SET

The Virtual Circuitry Modeling effect here is an accurate reproduction of classic Wah pedal - this is recreated by modeling the resistors and capacitors used in classic wah-wah pedals.

Also USER 1:009(A09) Clavi PedalWah FC2&MW
Here the VCM Pedal Wah is assigned to INSERTION EFFECT B

Press [F6] EFFECT to take the shortcut to the EFFECT page
Press [SF3] INS B to drop into edit on the VCM Pedal Wah type which is assigned to INSERTION EFFECT B

You can see how the FC2 is assigned to control this Pedal Wah effect by navigating to the CONTROL SET
Press [F4] CONTROL SET
The ASSIGN 1 Knob, the FC2 and the MW can all be used to apply wah-wah!!!
The ASSIGN 2 Knob will adjust the Resonance parameter of this VCM Effect.

ALTERNATIVE METHOD
As an alternate method of setting up a Wah-Wah effect you can assign the FC7 pedal to control Filter Cutoff directly.

Setting up a Foot Pedal (FC7) as a wah-wah via FILTER CONTROL - Plug it into Foot Controller 2 jack

EDIT the Voice by setting up a Control Set as follows:

Press [EDIT]
Press [COMMON EDIT]
Press [F4] CTL SET (Control Set)
ElementSwitch - activate it for each Element

Source FC(04) Foot Controller 2
Destination Cutoff (FILTER Frequency)
Depth +1 ~ +63 (set to any positive value to your taste, maximum is +63)

This will make the FC7 pedal plugged into the FootController2 jack control the cutoff frequency of the filter.
Then you need to go to the individual Elements and program the degree of control you want from each.

Press Track [1] to select Element 1 EDIT parameters
Press [F3] FILTER
Press [SF1] TYPE
Select a LPF (of your choice) 24dB will be deepest

Set the Cutoff (frequency) = to a low number (to taste) a setting of “0” will give you the deepest wah effect when the pedal is heel down. Set to your taste.
Repeat for each Element. Touch the illuminated track buttons 1-8 to select possible Elements.
By setting the Cutoff Frequency to a low number you are “biasing” the control to the FC. The Filter’s cutoff frequency will be determined by the position of the wheel. When the pedal is heel-down the filter is closed and when the pedal is toe-down the fiter is open.

If you want to reverse the direction of this:
Set the Cutoff of each Element to 255 and set the Control Set Depth to a negative value -1 ~ -64 (set to any negative value to your taste, maximum is -64).
The direction of effectiveness is purely a matter of choice.
This is a completely silent wah-wah, if you miss the noise of the old Morley or Cry Baby, you can use the other Dual Insertion Effect to dirty it up. Or combine it with the Pedal Wah Insertion Effect for more defined filter cutoff - experiment!