View Full Version : engine/ Carb Gurus. 502 crate motor carb choice?

03-12-2013, 01:02 PM
Hey guys,

The carb off my OMC 7.4 330 H.P. motor seems to not be to happy. Rather than a tear down and rebuild I'm thinking a new carb may be in order.

Anyone have suggestions on carb choice size, jetting etc. to get me started. As mentioned int he subject line it is a crate motor (marine) 502. ...no cam etc.



03-12-2013, 01:05 PM
I would guess that a 1410 Edelbrock marine carb @ 750cfm would be pretty close right out of the box. Is it a spread bore or square bore flange on the intake?

03-12-2013, 01:40 PM
no idea if it is spread or square bore. ...not even sure the difference really.

It is an edlebrock air gap intake that much I know.

I can tell you that I want to be able to idle this thing real low just in case I have issues with the shift assist module on the OMC boat.

I like my stuff to run well and have typically been a fan of not over -carbing motors but most of my experience is with triple down draft webers not four barrels.

Thanks again,


03-12-2013, 02:20 PM

03-12-2013, 02:53 PM
Air gap intake will be square bore. That's a good thing - gives you more options. Spread bore means the secondary holes are larger than the primaries, ie on a Quadrajet.

As far as "plug and play" goes, it's hard to beat Edelbrock, and if you decide to tune that's fairly straightforward as well. Tuning the idle is a matter of adjusting two screws on the front of the carb for mixture and the idle speed screw on the throttle arm itself. Super easy.

The Edelbrock website has the instructions for their carbs online, well worth the read as they are pretty well-written.

03-12-2013, 03:43 PM
a 7.4/330 is not a 502....

03-12-2013, 04:47 PM
a 7.4/330 is not a 502....

My understanding is that he is swapping parts from his old 454 over to a new crate 502, but not positive.

joseph m. hahnl
03-12-2013, 05:50 PM
You always go with the spread bore manifold :chilipepp: it's Bigger:incredible: and a square bore carb will fit:D

Pat McPherson
03-12-2013, 09:04 PM
My 502 crate engine has a 750 Edelbrock on top. I've been told she may spin a few more rpm with a 800 or 850 Holley but those cost about twice.

03-12-2013, 11:08 PM
a 7.4/330 is not a 502....

No it isn't but the 8.2/415-ish crate motor that I installed is a 502:)

03-13-2013, 08:48 AM
The Holley will make more horsepower and once u get it set up will last a long time. The initial investment will pay off. The air gap manifold and Holley carb combo is really tough to beat.

03-13-2013, 11:12 AM
I'm curious roughly what HP/RPM the crate motor was intended to make.

03-13-2013, 02:07 PM
What's everyone's feeling on mechanical Vs. vac. secondaries? I had a Barry Grant mighty marine demon on the minx and loved it. No choke to deal with. pump the throttle a couple times, turn the key, fired right up. With the "idle ease adjustment" it was a snap to install and ran great through the whole RPM range.

In cars the vac. secondary probably makes more of an efficiency difference because boats are always running under so much load anyway. You can also figure out exactly where the secondary will start to open on the throttle for cruising purposes (get the boat on plane then back it down to where you know the secondary is closed) so why bother with a Vac. secondary?

And a related note, I also had an electric fuel pump and I don't ever want a boat with a carb with a mechanical pump again.

03-13-2013, 03:16 PM
Never had a choke on any of my boats(except outboards) nor my hotrods. Easier to just pump it a few times and maybe have to restart it a second time than mess with a choke. I would think mechanicals would be better even on the water. First off you have much better throttle response, especially if running a cam. Second you can dial in where the secondaries open so you know to stay under that RPM for cruising. Air gap should be square. I would not go too crazy searching for the best, etc as this is not a car. We are always under load and get 4mpg at best so dialing it in is not going to be a science. Don't run too low of an idle as this will ingest water. Low idle on BBC's can also wipe out cam lobes.

03-23-2013, 02:24 PM
I have the carb off my 1992 OMC 454 right now so if you need some measurements let me know. It has a 750 Holley

BTW my 502 came with a jetted up 750 Edelbrock (free) and I'll prob upgrade next year once I get things dialed in. It will run with that carb but if you plan on doing anything crazy you may need more.

Dr. David Fleming
03-23-2013, 04:56 PM
My 502 crate engine has a 750 Edelbrock on top. I've been told she may spin a few more rpm with a 800 or 850 Holley but those cost about twice.

The choice between Edelbrock and Holley is one that Mercury faced also. The Edelbrock carburator was designed by Carter in the 1950s then bought up by Webber of Italy. MerCruiser used the 750 cfm version on all the engines in the 1990's. Mercury Racing developed the line of blue race motors and they had the 750 Holley.

The Edelbrock is a great stock carburetor it does not leak gasoline easily and makes good power. It was used by Chrysler Corp on the big 440 and 426 engines for years. The Holley was a racing carburetor developed by Smokey Yunick a famous engine tuner in Miami. Holley picked up the design and it has been the premiere racing carburetor It has vertical gaskets on the float bowls and will leak gasoline if not kept up in good shape.


03-23-2013, 08:57 PM
Love those Holleys when they are tuned right...

But... all my boats get Edelbrocks. Plug and play, no hassle. I have run them on everything from 318 Chryslers to 502 Chevys...

03-23-2013, 11:58 PM
The Holley will make more horsepower and once u get it set up will last a long time. The initial investment will pay off.

This is really the third time I have heard this today. Got to get it set to the engine. They are all broke out of the box. These words come from those that know.

joseph m. hahnl
03-24-2013, 08:53 AM
Holly's definetly need fiddling out of the box. Vacume testing is key.
The leak factor in a Holly is HUGE and in my mind,it out ways any performance gain that I personally would gain. I went with Edelbrock 750 cfm for my 400 cid.

I opted for electric choke. When a block has a :chillpill: that's 40* F at the break of dawn.
Takes a little more than a few squirts to get her going :tooth:

Annnnnd ! Like I said, spread bore all the way on the manifold. More gulp:alky: on the primary side.
The GM performance spread bore intake manifold used on Mercruiser 350 Mag, is among the best dual plane manifolds for a natuarlly aspirated motor.I suspect it would out perform a square bore Crosswind. But,I have no evidence to support it.
Any manifold can be made into a Crosswind by Via, Dremel or thermal spacer.

My set up includes a 1" single plane thermal spacer:cool: Edelbrock spread bore dual plane intake and a 750 cfm carb :drinkbeer:. The Minx has a little more head room under the hatch;)

03-24-2013, 12:07 PM
There are some really good archived threads around here on carb selection. Randy C. wrote a treatise on picking a carb that I referred to when picking parts up this winter for the family hammer wagon. He recommended generally smaller carbs for marine applications because of the high load / low vacuum signal.

I went with an Edelbrock 1409 600cfm square bore, vacuum secondary carb for my 350. I know it will be closer to "plug and play" out of the box than a comparable Holley, even if I leave some HP on the table vs. a perfectly tuned Holley. I have had a lot of different Holley carbs on hot rods over the years, and loved them when they were set up correctly. Unfortunately it generally took a lot of testing and tuning to get them there. Great if you've got access to a dyno but a lot tougher otherwise. Just so many parameters to get set, and hard to tell sometimes what you're screwing up. Jets, metering blocks, power valves, needles & seats, boosters, idle circuits, etc. It can be overwhelming for a non-expert.

Also, the single feed Holleys are notorious for leaking at the o-ring for the fuel line that connects the primary and secondary fuel bowls, and to a lesser extent the fuel bowl and metering block gaskets themselves. I wouldn't run one in a boat for that reason alone - almost had a disaster when one of those o-rings let go on my hot rod in high school. Smelled gas coming through the vents and stopped and popped the hood. Had a big puddle of gas BOILING on the intake. Very close call. If I was going to run a Holley it would be a dual feed no questions asked.

The Carter/Edelbrock is less prone to leaking and easy to set up right out of the box - just adjust the idle circuit with two screws right on the front of the carb and set the idle speed with the screw on the linkage.

If I ever build an engine and have an opportunity to dyno tune it I will probably go with a Holley. Until then, I'm going to stick with the Edelbrock.