Carb selection

[Liberator 6]

I'm looking at purchasing a reman carb, but don't know what CFM is on the boat now. What would be a prudent choice for a 350 Magnum with thru hull exhaust? The long block is only 2 years old and is a stock setup.

Thanks!

 

[John_S]

Re: Carb selection

Merc supplied the 350 magnum with 750cfm Quadrajets and 750cfm Webers. Both are more than you need, even with thru hull.

 

[Cheetah 20]

Re: Carb selection

If you go to National Carburetors web site , they can tell you the model that came original.I purchased one and I am very happy so far.It only took 3 weeks to get.They don't build them until you order.My 350 (not Magnum) takes a 600 cfm Quad.Does any know what they did different to the Magnum?

 

[HT32BSX115]

Re: Carb selection

There's also Flyingfish http://www.flyingfishcarburetors.com/


Cheers,

Rick

 

[Bondo]

Re: Carb selection

The 600/650cfm Edlebrock Performer would be a Wise Choice,.......

It's a copy of the Weber carb that Merc uses,+ is Much Cheaper.........

And,...... I Agree,...... A 750cfm is Too Big for a SBC.........

 

[John_S]

Re: Carb selection

Cheetah, I am not aware of any 600cfm quadrajets, but 600cfm webers came on some 5.7L. The LX and Mag usually have the 750cfm. The mag would have a high-rise dual plane intake along with larger carb.

 

[180shabah]

Re: Carb selection

I thought "MAG" was mostly just marketing hype (like all of the VORTEC engines in the late 80s and early 90s) that depending on the year indicated the difference between 2 and 4bbl or later carb and efi.

I was not aware that Merc put 750's on SBC's

 

[Bondo]

Re: Carb selection

Ayuh,.........

When you run the #s,.....
A SBC can only pass about 550cfms at 5000rpms,.....

Using a Carb that's Too Big causes loss of vaccum signal to the carb at lower speeds.....
And,......
The Edlebrock/ Webber/ Carter Clones are a spread bore in the 650, which is smaller Primaries....
This allows alittle bit of Economy to be achived,+ Get Up'n Go when on the 2ndaries......
The 750cfm Carb is a square bore,+ has Big primaries,+ 2ndaries......

 

[John_S]

Re: Carb selection

The webers used by merc, had either 96XX or 97XX model numbers. The 96XX models are the 600/625 cfm model, while the 97XX are 750cfm. From the merc service manual, all 5.7LX and 350 mags had 97XX carbs. If you look down the barrels of the 600, the primaries are slightly smaller than secondaries. The secondaries on the 600 are the size of all 4 brls on the 750. Technicly you could call that 600 a spreadbore, but it is really more of a squarebore. The Q-jet only came with 750cfm on SB.

As stated, they are larger than needed. If they were mechanical secondaries, they would be overkill. But since the air-door opens according to the engine vacuum on these carbs, its really not a problem. They will open to only what is needed. Thats why the 600cfm weber can be used on the 4.3LX. If you run the VE numbers for a 600 on a 4.3l and a 750 on a 5.7l, the 4.3l/600 combo is more over-carbed. And Bondo knows a 600 can work just fine on a 4.3L.

Bottomline for SBC marine engines @5K or less rpms:
- Use a marine carb with vac secodaries
- Only need a 600-650cfm
- A 750 will work fine, but its more than you need
- Either needs to be tuned for application