I'm wondering if you have enough initial timing. A vacuum gauge would reveal that as far as a base setting goes. Increase timing to max vacuum and back off 1/2". Works good for a start.

Other problem could be the power valve springs could be to tight. Easy to check. Loosen the power valve retaining screws just enough to slide the retainers over to the side so you can see 'em. Crank it up and make sure they pull down and stay that way.

Stock springs can be a bit tight for a lopey cam and allow the enrichment rods to jump up and down.