Are you able to pinpoint when the bubbles appear in the clear fuel line? This might seem like a silly question but when trying to get our 760 running for the first time we had this problem.
It would start okay, throttle up but as soon as you let off the throttle bubbles would appear while idling, then stall. This turned out to be the axial seal on the pump drive shaft, but like you say yours should be fine as it's new. The way we found the leak may help you.
Take a priming bulb attach it directly to the pump inlet and create a vaccum with it, after doing this a hissing noise could be heard and it was traced to the seal. Pressurising the pump didn't reveal anything so it was air getting in rather than fuel getting out that was the problem. I hope this helps.
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