I have a 2006 merc black scorpion 330hp that I have always had issues getting proper WOT RPM. Always been about 500 low. It is supposed to run up to 5200rpm. Broke down and got diacom and even after that I can't seem to see whats not right? Seems to run rich? Anyone have any baseline GOOD diacom readings for this engine? Would love to compare my live recordings to someone's that are on a for sure good running engine. I have the right prop per the boat manufacturer. This is a direct drive ski boat, but you guys seem to run the diacom more than any other forums. Thanks. Zach

To start I would pull the spark plugs and examine them and then proceed with a compression and leak down tests. Then post that info and possible pics of your spark plugs.

Also the wakeboard crowd who gets their boat work on by service shops all use diacom for Indmar and PCM engines. Also so does Ilmor in the mastercrafts. I know because I work on these types of boat for a living. Our area is just filled with them - Malibu, Supra, Moomba, Mastercraft, MB sports and so on.

What is your prop size (pitch/diameter)?

From my limited experience I have not seen many stock SBC motors (Merc or other brand) in I/O or ski boat configuration ever want to spin much more than 4800 rpm in factory trim, in spite of the recommended 4800-5200 rpm WOT range. Usually the factory cam and head combo run out of steam at that point. I had a scorpion style tunnel ram intake 350 Mag motor that had a WOT of 4800-5200. It would never go much over 4750 on a good day even when under propped and down hill:.

I'm not saying you don't have an issue, but it isn't out of the realm of possibilities. I only offer this a one data point. BUP has lots of sage advice though.

I never did a leak down test, but it did do a compression test and had between 160 and 165 on all 8. Plugs were all just a tad white. Can pull again and take a picture. Did do a leak down test on the injectors and lost about 4 or so PSI over about a 20 min period after shutting off the engine. I do get a gas sheen on the water when I shut down the engine? Did the injector test through diacom, but can only test one whole bank at a time and not individual on this engine, but had exactly 12PSI drop on both sides. Thanks for the help guys.