They won't open up unless the engine is under load. You won't be able to get the vacuum low enough without doing some damage to your engine. If you are suspecting that they aren't opening, you will need to do an on the water test with the engine cover open or off to see.
CUSTOM WHEEL & TIRE PACKAGES
$99 LED LIGHT PACKAGE - FREE SHIPPING
REBUILDING HOLLEY MARINE CARBURETORS - $235 DELIVERED
Email - firstname.lastname@example.org