Cam timing is what I think.

240 is a little long for a mild 340. I would TRIPLE CHECK the cam WITH A DEGREE WHEEL to ensure it's where it's supposed to be. Having said that, even then I would advance it a couple of degrees to build some bottom end.

In fact, if you wanted to do a quickie you COULD just advance it 2 degrees from it's at and try it. But as long as the cover is off...I'd want to do it right.

By the way, NICE Duster!