A mechanical secondary carb setup is a waste of money for your motors. Do the math, your motors don't need the secondaries. If you opened the secondaries by hand at 3000 rpm, well yeah the rpms will jump just as they would if you opened the primaries. You proved nothing with that test. We are all telling you the carbs are working like they should yet you are determined to get those stinking secondaries open whether it needs it or not. Here is how I prove to customers the secondaries are useless:
Take boat out on water, go to full WOT RPM. Take note of rpms and speed. Then return to dock. Insert a screw or small bolt in the linkage forcing the secondaries to open at wide open throttle. Take boat out and advance throttles SLOWLY because the carbs don't have pumps built into the secondaries and the engines will simply die of fuel starvation if you try to accelerate to quickly. Go all the way to WOT. Unless something is seriously wrong you will still have the exact same rpm and speed. Your motors can only use the amount of air and fuel the math says they will draw. Most primary bores will feed that easily. I built a pretty cool 389 stroker motor for my boat and used the same 625 Holley. It doesn't open the secondaries.
This is the general formula for how much CFM your 5.7s need: Engine size (CID) x Max RPM /3,456)= CFM. Some folks like to figure in Volumetric effeciency so therefore: Engine size (CID) x Max RPM /3,456) x VE = CFM
A 350 with a VE of .75 (Yours are probably less) = 380 cfm. You have 625 cfm carbs most likely on there now. I guarantee the primary bores provide more than enough by themselves.
I suspect you're like a lot of folks who think the secondaries are supposed to open when the throttles are advanced all the way and the boat is accelerating. That's not how it works. Even Rochester QJets have a restrictor flap so the engine wont 'bog' when those huge secondaries open up fast. If you buy double pumpers and fall for some idiot in a speed shop who says "850 double pumpers are what you need yes siree, Sh** why not a couple of 1250's bro? Spin those props right around!" You may as well mail your money to me because that will do about the same amount of good. Secondaries only help at high rpm, and then only if the motor demands more cfm than the primaries can feed it. Forcing the secondaries open will cause a bog and a big loss in "getting on plane" performance. Just because someone says they hear that bog when they open does not mean they are getting a performance gain. Carburetion is without a doubt the most widely misunderstood system on any motor, bar none. Look at TBI and fuel injection throttle bodies... there are no secondaries. In fact my big ol Triton V10 just has a small single throttle opening.
So why are 4bbls even on these motors? They're a selling point. That's it. People demand them so by golly they're gonna supply them and folks gonna pay for them. Now, granted some 2 bbls indeed are smaller than what the motor can use, so going to a 4bbl that happens to have bigger primaries will get a gain in top speed. You can achieve the same results with a larger 2bbl and a lot less money.
I suggest you pay more attention to the ignition advance curve which can give you a noticeable gain if its not properly set up already.
http://www.automotiveu.com/PickingCorrectCarb.htm
