Vacuum secondary carbs are junk for jetboats, you need at least an 850 for them, we sell and use the Chinese blueprinted(by us) HP 970 copies with excellent results, they will work well on just about any modified jet boat engine, and are cheaper than the Hollies.  The Performer is one of the worst intakes for a jet. It has smaller runners than stock (and much smaller than the heads!). The Weiand Stealth, Crosswind, or Performer RPM Airgap, are miles ahead with any cammed performance engine, especially in a jet. Ya got a decent, mild, CAR setup though.  Probable HP, less than 1 HP/cubic inch, in the real world, unless pro built and fully blueprinted. Having a car engine, you are better to off sell it as such, on a car forum. I'm not baggin' on your deal, but internal clearances and most cams will be wrong for a boat too, and I wouldn't want you to get caught up in a pissing contest if you sell it to a boat guy and it doesn't work out.  Perhaps I was too harsh....  but it is what it is.  TIMINATOR

Vacuum secs typically do not FULLY open with a normal big block until about 6500 rpm, they are not designed to. Most jets don't turn that many rpm. If you use a light sec. spring to get them to open fully at normal jet rpms, a bog will occur during the transition. During part throttle cruise, the vac secs will open to the same amount as the primaries, the air flow speed will be low, and the economy will suffer.  Part of that is because the secs are jetted richer than desired for continous use to help any transition bog while they are opening. The Vac sec carbs will have much less throttle response than double pumpers too. Using a large double pumper allows staying on the primary bores during much higher and lower cruise rpms, the mixture velocity is higher at lower rpms and that promotes torque and economy. You can the use the wrong part on many applications and it will work, just not optimally. If you haven't tried both setups correctly calibrated back to back on the same day, you have no basis to compare. Many aftermarket companies still sell junk parts for their intended applications because they still sell, not because they are correct. There exists a manifold company that still sells several old outdated designs, and promotes them too, even though the newer designs COMPLETELY outrun the old ones in every application. Many peeps buy that stuff based on advertising or advice from peeps that ran the junk when it was state of the art "back in the day", then when the new guy wants more performance, the company will be happy to sell them that too, they got your cash twice, rather than them recomending the correct part from the start. Marine vac sec carbs are used mostly for outdrive applications to minimize the shock to the drive, and help keep from cavitating the prop on the launch. Jets don't need either crutch. If you have never seen how long a vac secondary carb takes to open in actual use or on a flowbench, you can't fully appreciate why many of us won't use one in most applications. My 21 Daytona with the 572 and dominator carb modified to allow much more airflow before the secs opened, could cruise at about 65-68 mph on only the primaries, and fuel mileage was much better than your average 18 footer with a 454.  P.S. it ran  113. TIMINATOR

At typical jetboat rpm, the secondaries will not be fully open. The main jets control mostly the top rpm/air flow calibration only. When the carbs sec is not fully open, the mixture is controlled mostly by the air jets, and emulsion tubes. Unless the carb has been modified that can't be done. If you pay to have it modified you could have easily bought the correct doublepumper instead. About your statement of unrestricted air flow... carbs are rated with flow cfm at 1.5 inches of mercury vacuum below the carb, in other terms, that is 20.5 inches of water restriction. Standard flowbench protocol is to flow at 28 inches of water(heads, manifolds, etc.). The intake tract will never see that much vacuum at Wide open throttle, nor will the carb, nor do you want it to. You say you want unrestricted air flow(and you should have near that), but you are then recomending a 750 vac sec carb that will never fully open the secondaries and cause a large restriction. HUH? I have not seen even a stock 330 HP jet motor that will not benefit from a good, larger than stock dual plane intake and a 850 carb. We have sold a BUNCH of the blueprinted(by us) Chineese HP 950 clone carbs for use on everything from stock Olds 455s, 454s, and 460s, to 675 HP+ roller motors with alum heads. In every case those carbs outran an 850 Holley. Just for all of your info, the most used by the GM jetboat engines as a stock carb is the Quadrajet 4 bbl. and that flows 810 cfm, the smaller 785 unit was popular on smallblocks. In the case of our foreman's Biesemeyer jet, as an early experiment we installed only a Junk Torker(our opinion) and the HP 950 on an otherwise STOCK 455 Olds with log exhausts, it picked up 2-3 mph. Plug color was too close to change anything, when his ported head, now roller cammed 455 was finished, we installed the same carb and an Olds Crosswind intake on it, again the jetting was very close, only requiring a change to jets of 2 sizes smaller. Most peeps wouldn't have even bothered, but we are anal(in our case we take anal to be short for analyze!). Every post is backed up by a lot of past real world testing. We don't deal in "I think" or "I read", we try everything as much as possible!  TIMINATOR