Any and all of what you mentioned.....   Usually improper loading will erode the impeller about 3/8" to about 1 1/2" past the leading edge of the blades. Aluminum worse than bronze. The efficiency will fall off and the hole shot suffers, thats your clue to have it looked at. About two years ago we had a customer not heed any of our advice, and buy one of our 496 motors for a boat he just bought. We offered to look at the pump for free, he didn't take us up on our offer, as a matter of fact, we never saw his boat at all. His bro (also a customer with his own boat), said the pump worked OK as long as he pussyfooted it out of the hole, and didn't try to run faster than 75 or so, above that, it loaded and unloaded the pump continously. I recomended a different grate, and an inducer, he didn't have the cash for that, so he would just do it over the winter. A few weeks later they were racing each other and everybody else, and during a high speed run the side of the suction housing collapsed a hole big enough to put two fists thru. Made a big splash and some noise, but didn't sink the boat because of the big pump setback. The hole stopped right where the custom transom plate was siliconed to the suction housing. He was lucky. Instead of just purchasing a grate ($200.00), or inducer ($479.95 with shaft machining), he bought those, a used suction housing, a complete rebuild kit, an impeller and shaft, plus labor.  With a non setback pump the hole would have been inside of the boat and sunk it, immediatly. The pump will tell you what it wants if you are smart enough to know where to look at it, listen to it, or ask someone who is.  TIMINATOR

How many years was it in the ground before it was pumped up here? TIMINATOR

A car engine, or a boat engine rebuilt by a car shop, in a jet boat will usually only run 45-50 mph. Car engines have smaller valves, less compression, and even the most popular "RV" cam used by the car guys is much smaller than the stock jet boat grind. As far as dists go, read my post in the need "copper wires",  it may explain a lot. A boat also needs less fuel line and pump than a drag/ street car because the tanks in a boat are even with or in front of the engine and much closer, so the fuel doesn't need to be pulled foward against the "G" load of the launch for 12 feet. Check the cam and dist.   TIMINATOR

The biggest problem with stock BBchevys is that the valve guide is tall and won't allow much of an incerase in lift, may be only to .490. You need to check it by having some one measure from the top of the retainer to the head, then while you compress the spring until it hits the top of the guide and remeasure, subtract .060 and thats all of the lift you can have without machining the guides down. Do both the intake and exhaust the amount of clearance will probably be different. Your big problem is the cast hypereuctetic  pistons, even the trucks have them. They will explode at about 400-450 HP, or with ANY detonation. Intake and carb- you can usually get away with, cam usually explodes the Hypereuctetic pistons. Even the stock 330 HP boat motors have the junk pistons too. Also a car or truck ignition curve is way different than a jetboat wants, it not only limits the available horsepower, but kills the fuel mileage too. Good luck.  TIMINATOR

Need more info for an informed answer. Speed, weight, intended use? That said, most pump parts/mods with a near stock motor (with the exception of a divertor) and a properly operating pump in good condition, will benefit nothing but the seller. At about 500+ HP, and 75+MPH you can see a small diference. I have done extensive testing at stock, 500, 750, and 1000 HP in lake boats for most mods you can think of, and my conclusion is that everything you can do will make a small difference in speed or holeshot, but NO ONE PART, other than Nitrous will make any significant difference in speed. Jet speed is a blend of about 25-30 seperate small interconnecting issues. Every time you change one part, you need to change everything else again to find out what the overall new combo wants. We build pumps, do setbacks, do bottom/fiberglass work, and unlike anybody else we know, have a COMPLETE engine machineshop and custom fabrication facilities. I like to think we are unbiased. The first mod other than a diverter, is to add horsepower. When a pump combo is right and more HP is added, it changes very much of the setup. No other single change will have as great an overall effect on the entire setup. For max speed, add as much horsepower as you can afford. More than you think. Either do the cam/intake/carb/exhaust deal and stop there, or if more cash is available, do a stroker shortblock with good heads, and add the external bolt ons as more cash is available(thats the preferred way). We rent nitrous kits for customers to get a real world idea of their goals. Most find that they need more HP than they expected, and can't get to their goals with just bolt-ons. Sorry about the long post, but this info is relevent to many peeps on here.  Let the discussion begin! 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

FTN inserts will change almost nothing assuming your ride has the correct pump setup from the factory, and almost every untouched unit we have seen was correct. The nozzles make the most difference on a light boat that runs over 70 or so and has more than stock HP. The lighter, faster, and more HP makes a bigger difference. If you will never build a "stomper of a motor" and are prepared to do a lot of setup changes and mods, spend the cash on an inducer. TIMINATOR

True story. That. Pumps will, at full throttle, boat not moving, suck stuff off of the bottom from 4 to 5 feet! If you are in reverse, the reverse water blows stuff up right under the intake! I make a practice of never hammering the throttle from a dead stop unless the water is about 10 feet deep, but then again, I don't like throwing money away! We had a customer that always beach launched his ride, and would warm it up on the trailer by revving his engine, he killed his new pump in 2 months! TIMINATOR

2bolt cap BBChevys with ARP studs, and a PROPERLY line honed block, will live a long and useful life at 600-650 HP assuming NO detonation. Above that HP level or with detonation the caps will "walk" side to side and spall metal back and forth to the block and the cap. When this side to side "walk" occurs, the cap becomes pinched in at the parting line, and that will close up the bearing clearance across it. When this closing up occurs for long enough, the edge of the bearing will scrape the oil from the crank, and this will cause the bearing to weld itself to the crank, and the result is a spun bearing. Modern bearings, especially CL-77s are made with a large, shallow chamfer for the last 1/4" or so to allow that to happen a small amount before destruction. This is called a "Delta wall" bearing. The more HP a bearing is designed for, the more Delta wall is included. We use CL-77 "P" (for up to 500-550 HP in a big block) or "H" (for over that HP level) in every engine we build. The "H" bearing has more "Delta" and more crush to retain the bearing and promote heat transfer. Remember- if the line hone is big by .001" the crush is reduced by .00314" and disaster will occur at a lower HP level. TIMINATOR

Keep in mind that the internal clearances need to be bigger in a boat engine: valve guides, ring end gaps, piston/wall, rods, and mains. Also make especially sure that the oil drainback holes are opened up and radiused. Deburring/polishing the valley and heads is preferred to painting them, it can't fall off! 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

We do all of MRERACERS engine work and the fab work that he can't. We did the drive coupler too, but you can buy those now. His headers are old boat 351 Cleveland units from back in the day, with the LS flange replacing the Ford ones. Side mounts can be fabbed by anyone with a saw, drill press and welder, he uses a 3 point system. Preliminary runs were in the mid/upper 70s. The major fly in the ointment is lack of torque. He runs an AB impeller with a cast aluminum V-blocker grate to keep the engine in the power band, among other things. THE ORIGINAL INTERNAL CLEARANCES AND BOAT CLEARANCES ARE WAY DIFFERENT!!! I.E. piston/wall, guides, ring gap, etc.  His hull is my old Baker tunnel V, its heavy, but it rides well, and is efficient at speed.  TIMINATOR

With the proliferation of aluminum heads on the market, it is seldom a cost effective plan to rebuild a set of ANYTHING stock heads, rather than replace. ANY of the replacement heads will out flow any stocker heads even if the stockers are ported (that last statement is not true for stock port sized, street replacement, entry level heads, but then again, who wants those?). We just had a customer insist on rebuilding a set of the Super Cobra Jet heads: new valves, thick wall bronze guides, springs, retainers, locks, surface, bake, blast, magnaflux, street/strip port, 5 angle competition valve job with back cuts, and unshroud the chambers for better flow around the valve. The cost was more than Pro-Comp heads (with my choice of good quality American-made parts), they flowed 35cfm less than the aluminums on the intakes and still weighed 30+ pounds more too. We flowed a pair of D2VE heads with big valves and street/strip porting for a comparison too, they flowed only about 20 cfm less than the ported Super Cobra Jets! In my opinion not worth the cash to modify any of them. BUT!!!! Never buy complete heads of any brand from any factory unless they are specifically sold for marine use!! Have a REPUTABLE boat oriented machine shop select the proper combination of quality marine compatible parts. The valve seat widths, and guide clearances will be different from auto (even race) specs, those will have to be modified from as delivered car specs. The proper spring pressures are very different too.  Sell your Super Cobra-Jet heads to a collector, I wouldn't spend a dime on them. For what those guys will pay, you can probably have a set of much better performing new aluminum heads with parts for $400.00 or so! Whatever you spend on parts for your old heads can go for parts on new heads too. The aforementioned post reflects my opinion, one that has been formulated from 40 years in the boat oriented machine shop industry.  I could be wrong.  TIMINATOR