Here's an off-beat suggestion... I once replaced the fuel tubing, and accidentally substituted a piece with a very narrow ID... I have no idea where that piece ever came from... I just grabbed it and used it...

Replaced that piece of tubing, and things worked fine....

Also, it could be that you are using fuel tubing with too large an ID and you might be leaking where you expect that the joints would be tight. I bought some Du-Bro tubing that was like that... mea culpa for buying the wrong sized stuff.

Like I said, this is an off-beat suggestion.