Intake
 

Have 454 Mag engine(385hp) with Bravo outdrive, Was MPI injected, removed the injection unit and replaced it with a Victor Jr. intake and 800 double pumper carb. with a 1in spacer under the carb. Also while it was down replaced the flat tappet cam with a crane roller (226 and 234 at .050 lift), had the heads shaved .020 and a good 3 angle valve job and the valve pockets ported. New springs and roller rockers. The ingnition was upgraded to crane HI-6 and the engine has EM Thunder exhaust out the rear of the boat. Spinning a 23 pitch prop and the boat weights about 4800 lbs. Was running about 64+/- at 4800rpm befor all the changes and now after all the upgrades still runs the same no change in RPM or speed only a big flat spot in the running under say 4500RPM.(But seems to run and start smoothly)
Was thinking the intake manifold may be the problem along with the double pumper. What do you guys think. Would love to run around 70. is this combo right for this?? Thanks for all the help/

This will be an interesting thread as I know there are people who wanted to do this. My only thought is maybe you are not in the power zone of this motor now. You may have to drop prop pitch and get it spinning around 5200 or so to see. Obviously, the simple things are check to see that the carb is opening fully and check your full ignition timing. Should be 34 degrees BTDC or so. I dont see anything wrong with that intake or carb, most people prefer the air gap.

Where is the cam degreed to?
Have you verified the correct initial timing and the correct advance curve selected in the Hi6?
Did anybody set the float level, verify complete opening of the secondaries at WOT?
Did anybody do a plug chop to check carb jetting at WOT or did it just get bolted on from the box?
Do you know what your fuel pressure at the carb is at WOT?

You should be staring down the throat of a solid 450 hp when that combo is sorted out.

What are the rest of the cam specs? IE: lobe seperation, lift, centerline. Also, if that motor is a 454 Magnum MPI, didn't it have a hydraulic roller from Mecruiser?

The cam spec's are: lift intake 587, exhaust 610 duration 226 and 234 @.050. Crane's part number is 139731, a pretty popular cam. The lobe center line is 112 and timed in at 109 after top dead center.The engine was a Gen 6 but had a flat tappet cam. one of the last one's made with this cam. but all the rest is the good stuff from the factory. it does have the large rectangle port heads.The timing is 34degrees and in a 3200 rpm the plugs are dark but not that black. and yes the carb is wide open. So you guys don't think the problem is in the carb or intake, though about a single plane Edlebrock RPM and 780 vac. secondary. What do you thinks. and thanks for the info help

I do!
 

I think that the intake manifold is the problem. I would guess that you are 20 horsepower down on power with that single plane over a dual plane Performer RPM. Call Edelbrock and see if they agree.
Dennis Moore
PS
I like the rest of your set up, you picked a good camshaft! DP Holley carb is fine.

Thanks for the help and will call Edlebrock tomorrow and post what they say or recommend.

Tests
 

I can't recall where I saw it, but about a year or so ago, there was an intake comparison test in one of the hot rod magazines. If I recall correctly, the test was done with the old RPm manifold (not the air gap), and the Vic Jr was also used. All in all, the RPM made a bit more torque on that combo, but not much (maybe 5-10 lbs) at lower RPMS, then after 3000-3500 the vic made more torque & HP, then around 5000 they were pretty close till about 6500, then the vic picked up again. They also tested a wieand, and a dart if I recall.

It was just interesting to see that between the single & dual that there was not too much difference, it could have been the cobo though.

Just something I recalled after reading this post.

So I wouldn't be so quick to discard the intake.

As for carb spacer. Try taking it off ? Some combos it helps, some it hurts. No way to tell.

Just my thoughts

I would loose the spacer and try that first. That is free.

I vote for trying it without the spacer also I think that makes three of us now.

Your not turning enough RPM for that manifold to be effective. You don't usually see any gaines over the dual plane until after 4500 RPM. If you can prop the boat to run higher RPM you might gain some. Most likely you'll end up changing that manifold for a good dual plane. Keep the spacer, you'll probably need it for that set up. (I assume it's an open spacer)