OK..I know this has been asked before, but what is the FINAL answer on jetting an hp500 Carb on a single plane intake? Square jet it...or staggered? Its my understanding that the carbs came both ways. Thanks.

The Merc HP500 carbs all had staggered jetting from the factory.
There is no FINAL answer. Its all a matter of opinion and unlees you are running it on a dyno and trying to get every last bit of hp out of it, there is no way to tell for sure.

Unless you are running a hp500 totally stock, square it up.

just finishing my .030 500 w/850 and full headers where would be a good starting point on jetting ? thanks Rob

Call holley and find out what the out of box marine 850 jetting is. Then add 5 jet sizes to those numbers and you will start a bit rich and can lean from there.

BT