holley 750 carb swap
 

is there anything to be gained by swapping a holley 750 vac secondary onto a mild 420 hp 454 w/ stainless marine exhaust and other engine mods- right now has a qjet sittng on the edelbrock intake

Re: holley 750 carb swap
 

You will get a zillion answers on that one. The q-jet does have some good qualities, but so does the Holley. I think to know for sure, you are going to have to try it. You will have to buy a linkage adapter plate but that's not much. I think you will burn more fuel with the Holley, but I really can't say about the overall performance.

Re: holley 750 carb swap
 

I would get a mechanical secondary carb.

Re: holley 750 carb swap
 

If you're not having any problems with the current carburetor, and the carb swap is the only thing you're planning on doing, then you're not going to see enough difference on a marine application to make it worth while doing.

Re: holley 750 carb swap
 

my stock 454/420 has holley 800 couble pumpers with mechanical secondaries. That is how merc set them up.

Re: holley 750 carb swap
 

i just built the engines and am using the exsisting 330 hp carbs, which i know is not a good idea due to jetting issues, etc, but the engines are modified and was wondering if there would be any gain

Re: holley 750 carb swap
 

mcrsr
The way the q-jet is calibrated for the 330 HP is not going to be rich enough if you have modified the engine. I don't know how familiar you are with q-jets but this is the part that is really easy. Changing the secondary throttling rods to fatten up the secondaries is really easy and can be done even when the boat is running. Edelbrock offers a kit of different rods, the smaller being the richer. Dennis Moore's book on BB Chevy Marine Engines also list rod numbers and other great info on the carb. The primary side of the carb, you should really not have to change. I would not throw that carb away until you work with it a little and see if it gets the job done. They have great throttle response and are easy on gas.
Good Luck

Re: holley 750 carb swap/performance mods-bbc
 

i am very used to holleys, i rebuild qjets on a monthly basis for customers, but have never modified one, i do not even know where to begin, any suggestions?? i thought the holley, being a performance carb, would be easier to work with, better parts availabilty, and thought i would get more power- i used to build alot of cars and remember the throttle response and power w/ the holley was much better- any help out there or suggestions?? engines are 454's w/ speed pro hyper. pistons, 8.5:1 compression, comp cams xm 262h cams(218/224 duration @.050), edelbrock performer intakes, large oval port heads(but w/ small valves(ithink 2.06/1.65), harland sharp 1.7 roller rocker arms, comp valve springs(match the cam), thunderbolt 5 ignition, total timing is set at 32 degrees, stainless marine exhaust, no mufflers w/ stock exhaust system for the pachanga- dump forward into a 5" collector and then dump out the back through 5" hose w/ internal stainless marine flappers- any suggestions for more power outside of a blower?? what would be anyones guess as to power output right now? i would think over 400-420 hp- also- what about total timing- is 32 degrees good- i checked it at 3500 rpm- any help???

Re: holley 750 carb swap
 

I built up my 1989 330 with the same pistons, same cam, performer intake on top of the peanut port heads, and Imco risers on the stock exhaust manifolds.
The engine gained about 300rpm on top over stock with the same 23p prop. My 24' Pantera gained 3-4mph.
I would guess the engine made 50HP over stock.
I went up a size on the secondary jets like Whiteknucke suggested. That Carb is fine, Merc used it on the 370HP & 400HP/454s and a 390HP/502.
Have Fun!

Re: holley 750 carb swap
 

thank you for all the help, i probably will put on the holley in stck trim and do a plug reading from there, the thing i like about the holley is ease of availabilty of all the tuning parts- i bracket race a street driven 69 camaro convertible w/ a blown 383 w/ the holley- so gaskets, jets and powervalves are always handy!!! i just remeber on street cars that i've built the engine was always much more powerful and snappy w/ the holley. also, any suggestions on ignition timing?? right now i am running 32 degrees w/ tbolt 5, any suggestions???