[solarite_guy]

Agree, it is going lean at low vacuum when the secondaries open.

Agree also, the missing valley pan is not good on a street motor, but not a contributor to this problem.

I sedriously doubt your sam lobes are at issue here too. If there was a problem with wiping a or a couple lobes you would know it. The car would run like crap at higher vacuum too.

Without re-reading everything, have you sprayed the carb cleaner where the manfold and heads meet?

Which intake manifold gasket are you using?

Okay, so let's brainstorm a little bit.

1. Put the brown pump cams back in.

2. The 280H comp will want more main jet than what the stock jets are in the carbs. This could be a contributor. I am making the assumption you are in the mains at this point.

We need some enrichment to occur here. I have been trying to avoid this, but it is coming down to a couple more ways to provide some enrichment. An obvious thing and what the engine needs is some more fuel at this vacuum point.

2a. Options: More jet and rear plate OR rear block, with jet and power valve (one of your first thoughts).

I may have mis-interpreted but can you get into the secondaries slow enough that it wont backfire? If so, what vacuum readings are you seeing?

Let's wait for your feedback on these questions before buying anything.

Cheers