After fighting with the spitting and sputtering on my '76 Mav 302 installation I finally bit the big one and pulled the intake manifold in hope of finding a leak.

I didn't find one but did find a few drops of water in the valley below the intake. This could be due to my spilling some out of the intake during removal or...? It appeared to be fresh so I'm not too worried about it.

I was concerned about a vacuum leak around the intake manifold (Weiand 8011 dual plane) since little else would explain the lean backfire through the carb during transition from the idle circuit to the mains. Since it is a relatively easy fix, I decided to change the intake manifold gaskets to Fel Pro deluxe super-seals (blue line) instead of the questionable felt gaskets that were on there.

My question...

Is it ok to block off the exhaust crossover through the intake manifold? As I am running an aluminium manifold it seemed kind of silly to have hot exhaust running though the intake manifold (!) heating up the incoming fuel charge. My new FelPro gasket set comes with blockoff plugs and I am strongly considering doing this unless someone can explain why it is there in the first place. My guess is that it helps warm-up when first started but I could be wrong.

Any help would be appreciated, I'm about at the end of my rope trying to cure the off-idle backfire.