Can anyone tell me the difference between blower carbs and regular carbs? I've been told that blower carbs need to be mechanical seconday due to the blowers having generating little to no vacuum. other than that, how hard is it to modify my Holley 850 marine mechanical secondary carbs to work on an 871?

They can be rejetted to work OK without mods. But I think it's accepted that boost referenced carbs work better.
Boost referencing involve drilling a port to the power valve and using vacuum or boost under the blower as a signal for the power valve to open, rather than the pressure under the carb.
My $0.02
Gary

Worthwhile reading.
http://www.offshoreonly.com/forums/s...oost+reference

The boost referencing is one of the differences in a "Blower Calibrated" carburetor vs. a naturally aspirated carburetor. The secondary butterflies on a vacuum secondary carburetor actually open based upon the air velocity going through the primary butterflies, and engine load not vacuum. That being said the biggest issue with trying to run vacuum secondary carburetors on a blower with sideways mounted carburetors is distribution. Most people believe the supercharger will mix the air and fuel up enough to give good cylinder to cylinder distribution. Not the case! Also make sure the carburetors you're dealing with are Marine carburetors, not automotive.