stingraysal

Thinking of putting a set of 750 hollys on my 1988 c/c 312 stinger with7.4 mercs to replace the q-jets.
The engines have edelbrock performers, 4"thru hull exhaust, and extream marine cams. is it ok to use vacuum seccondaries? what power valve? jet size to start with? should it be a double pumper?

Big Block Billy

VAC secondaries work fine. Double pumper not necessary. I don't even have the choke on mine. I've gone on in greater detail, See the carb threads I've subscribed to. BBB

JohnJan

Don't be surprised if you do not see any increase in power and also a reduction in fuel economy.

Helmwurst

If you are going to use 750's, Buy the Eldelbrocks, they have a better midrange. and better low end enconomy.
My 2 cents...

Mobedik

I have had stalling problems with Edlebrocks on four wheel drive trucks around here. Even after using the optional float spring kits that they recommend for off road use, and considerable trial and error with float settings, etc. I don't know what sea conditions are like where you are but I wouldn't use them for boating here in the northwest. Also, I have had a lot of experience with both Holley and Rochester Q-jets and I think the Holleys are over-rated, except (maybe) for all out racing. They are tuneable but it takes a lot of time and effort. I have always felt the Quadrajet is way easier to fine tune. I think the early Q-jets, released well after Holley had built a reputation, simply got a bad rap that they have never really overcome. If you will notice, however, both Crysler and GM have used the Q-jet for years and it has alwyays performed very well for both. Lastly, simply changing the carbs won't do much, if anything, for your your anticitpated power increase. Cubic inches, valve lift/duration/timing, and proper and timely delivered spark are where the power comes from. Once you have those details covered you can use just about any method of getting fuel to the engine and do just fine! You mentioned extreme marine cams, if your cam lift and duration is very extreme you will have very low vacuum at low RPM's. This can raise hell with vacuum operated devices like distributor vacuum advances and vacuum operated power valves. If your cams are very radical you should be running a distributor incorporating mechanical advance only, and if you go with Holley go for the double pumper (and I would consider mechanical secondarys for a marine application). Holleys are rather fragile also, in that any backfire in the intake manifold is going to raise hell with the power valve. Although they are not as bad as they used to be (better materials I suppose), a few intake backfires used to take out the power valves pretty easy. (When that happens you end up with a very rich running engine.) Last thing, Q-jets are real good at keeping fuel in the carb where it belongs. I have seen a lot of leaking fuel bowls (either the bowl gasket or the accelerator pump diaphram) on Holleys; NOT a good thing on a boat. Well, thats my take on the subject, hope it helps.

Griff

If you are buying new ones, get mechanical secondaries. It is easier to control when the secondaries open.

I doubt his cam is that radical from the info he gave. I have never seen a vacuum advance on a Merc BB engine You can dial in powervalves as needed.

My bet would be that some small jet changes would be the only thing you would need to do on the Holleys. Keep them outta the secondaries and fuel consumption will be only slightly higher than the Quads.

stingraysal

Thanks for the replies on the holly's.I'll build a set of 750's I have and try them.but I'll keep the q-jets at the ready.