I have rebuilt my 454 magnums, bored block 0.030 and installed speed pro, near flat top pistons, HP-500 cams, roller liters/rockers etc. Fresh lower end, and I am changing the rectangular port 990's to GM oval port 290's and i'm opening the valves to 2.19/1.88 with some mild porting, ( short side radius etc ) I have revolution marine exhaust. I plan to install the edlebrock air gap intake's. The boat is a Formula f-311, hydromotive 4 blade 24P props. Can i get away with installing a spread bore adapter plate and running my quadrajets or is a holley or some other square bore carb the only way to go? these are mark IV blocks. Thanks for any insight....

i ran a 750 holley 4150 dbl pumper on my 468 bbc ran great, 850 might be a bit much

You might find that the Quadrajet has too little of a float bowl for sustained high RPM running. I personally like the spread bore design for cars, but I think you'll find the 750 double-pumper to work nicely.

I've always run a Holley 850 jetted properly with great results you can get it w/ vacuum secondaries for really smooth transition. Personally I'd get some 850 D.P's., jet them right and have a blast. You'll only use as much carb as the engine needs and won't starve for CFM.

I would run a 800 cfm Holley 4150 style mech.sec. douvle pumper. Holley offers a marine coast gaurd approved model #0-9022. The jetting out of the box is usually pretty close for marine use. I would start there are jetting especially on a new engine. You would rather be a bit leaner during slow rpm break in period so you do not "wash" the rings. Set the float level by removing the bowl and invert it. Set float height with bowl inverted @ .375 with calipers or a 3/8 pc. of round stock. Set the idle mixture screws @ 1 1/2 turns from seated. And run it. This will a good baseline.

Dean Gellner

Gellner Engineering Marine Power

750 or 800cfm max. Anymore than that and you will get sluggish response on the low end. Get mechanical secondaries.

750 dp. will give you the best throttle response and enough air all the way through 5500 rpm.

Any other thoughts?

750 Berry Grant