I’m doing a custom build by swapping my f4i motor into my kei truck. Can I just remove the return throttle cable since the throttle bodies have some spring tension, and just run the singe pull cable? Or should I try to connect some kind of custom return spring instead of a return cable?

 

The only realistic answer is suck it and see.

It will depend on the length and routing of the pull cable so, once you have the engine and throttle installed, if it does not return reliably the add a return spring.

 

Now that's a build I want to see pictures of. Is there a spot on the forum for other things that have our bike engines in them. I have a GSXR swapped quad and everyone on the gixxer forums got mad at me for asking questions like this because "I ruined the purity" Some admins were even deleting my posts. lol.
To answer your question, the 2nd cable works as a safety for if the throttle bodies butterfly's get stuck open. Twisting throttle backwards will close them. I had to do this on a bike that sat over winter. It tried to rev to the moon first time i cracked the throttle. If you do decide to delete it, I would put a stronger spring down on the throttle linkage or add another one somehow just to be sure. Your pedal will also be harder to pull back to 0 than the standard twist grip.

 

Gotta laugh and ask if you typed suck it or was it text to speech?
I'm guessing you meant TRY it and see.

 

Another fine example of a common language divided by an ocean a very common saying in the UK which means the same as try it and see or give it a go etc...
Of course does have other meaning in a different context

 

Two nations separated by a common language :-) Stuck it and see is a very common way in the UK of saying that you need to try something to know whether it will work.