We tried about 7 or 8 air cleaner configurations on the dyno, the one with the most low profile bottom plate did best, doubling the size of the element made no appreciable difference, didn't have a big budget for parts but managed to borrow plenty for a full day on the dyno.

Cool beans....yeh, the bottom plate can make a good difference. Was the tests on the carb without the choke tower or the one with it ?

Started out with a completely stock 850 DP mech secondary's. Got our best number, with a 1" air filter spacer and the low profile base plate, 576hp IIRC, took carb off swapped metering plates and bowls on to proform HP style center section with billet base (promised 920 cfs I think) pulled an extra 22hp, I need to find the dyno sheets, they are around somewhere. Had to keep the 1" spacer to clear the throttle linkage, even so we had to adjust the base plate a little to clear.

edit 576 not 586

Looking at running a marine holley 850 part #80443. With that carb should I start of with the same jets that were in the 500's or go larger?

Start with jets in carb...as always...unless someone has a guaranteed starting point for that carb and your motor build.

DO NOT even think about putting in same jets as the OE Merc 800 Holley DP. Totally different carb !

What SB said !!!!

That marine 80443 carb should come with 88 primary, 96 secondary, with 6.5 power valves. Id run it out of the box and check from there. For the cost of that carb though, the quick fuel marine 850 with black diamond coating, and alot of other nice feautures, , is a much better buy for 650 bucks.

http://m.summitracing.com/parts/qft-m-850

Whatever carb you get, watch the AFRs closely.

I just put a new marine carb on, box stock with no changes and until it hit the power valve setting, it was running 13.5-14.3 AFR. Running that at 3500 rpm isn't too good. Opened up the secondaries and it was still over 13 even at 4400 rpm. I went up 3 jet sizes and it is dead on now. It was an off the shelf carb from a major player, not a carb from a custom shop.

Moral of the story is don't assume the factory calibration will be rich, be cautious and watch it.

And unfortunately, all I can get in that location is 10% ethanol gas, so plug color isn't a help.

Do you know what jets this comes with? and would you do the same with starting out with this out of the box?

WARNING! -- minor hijack!!!
Anyone had any experience with the "jet-less" adjustable metering plate? (has the little screw on top to adjust fuel)