I just purchased a gen VI 454 HO crate motor. I believe the engine is rated at 425hp at the crank. I think the lift on the cam is .510 intake and exhaust. I will be using the standard merc dual plane intake with or without spacer. 088 rect port heads. What should my jetting be for a holley 750? Primaries? power valve? Senondaries? I wouldn't mind if the end result for this motor turned out to be close to a stock 454 mag. I will be running gil dry exhaust (because that is what came with the boat).

I will let you know tomorrow, I have the same setup except with a 850.

Ok. thanks. I don't have much experience with carb motors. My last two were fuel injected. What is the purpose of the tube fitting coming out of the base of the carb? It looks like the hose has a fitting on the end that goes to the valve cover. I would post a picture, but the posting rules state "You may not post attachments"

Mine are 850's done by Nickerson mounted on merlin single plane intakes and 1" spacers. They are 84's in primary with 5.0 pv and 90's secondary with a pv plug. The chokes have been milled off and what ever else they do.

Merc racing used a holley 800 on the HP450 (454 with same internals as the HP500). Jetting was

75 Primary
81 Primary
89 Secondary
92 Secondary
6.5PV in primary only, secondary pv plugged

Im not sure, but i believe the #92 went over the #7 Cylinder, and the 81 over the #1 cylinder. Someone who has taken one of these carbs down stock should be able to tell you. The staggard jetting was probably to compensate for the fact one bank usually runs a bit leaner than the other.

You can probably go with a couple jet sizes down from those numbers, since your carb is smaller.

I was off on that a bit. Here is the info from the Merc Manual

Holley No. 80466 - 800 CFM

Primary - Port 81, Stbd 75
Secondary - Port 89, Stbd 93

Power valve - front 6.5, rear - none

Accelerator pump - Primary - 50cc, Secondary - 50cc

Accelerator nozzle - Front - #37. Rear - #37

MILD THUNDER, did the HP450 use a single or dual plane intake?

In my manual (Merc #9) it shows the 440 and 460 cyclones using the aluminum dual plane with an 850 ran 84's all the way around, 6.5 PV in front w/rear plugged. I would think 750 would run 1 or 2 sizes smaller.

Here is the jetting from my old '86 Merc 420s with 188 or 088 GM square port heads, dual plane Weiand Stealth intake, Stainless Marine exhaust and old Barry Grant (Holley) 750's

The butterfly's have 172 stamped on them, Front PV is 6-5, blocked in the rear, 30cc accel. pump, jets are: Front 77's and rear 87's.

Thanks for all the input. looked up my id numbers, the carb is model number 4150, id 80537, mech secondaries, 30 cc acc pump. If anyone else knows what this setup should be for my motor which is pretty much the 454 mag setup. Mine is gen vi but I believe the gen v's from mercruiser had the same setup but maybe a different brand carb.

Here ya go. This is what holley says stock jetting is.
http://tgsperformance.com/holley.htm

80537 750 (F)73,(R)81 6.5 4150 Marine