2003 4.3L V6 2 barrel carb

[Buckleybob]

thinking about a new carb for my boat. From what I have found the Mercarb that came on the boat is around 280 CFM. Carb calculator says about 315 is needed for 4,900 rpm WOT. Holley carb selector gives 500 cfm as recommended carbs. Any problems going up that much in size? Holley says the m-500 is tuned for small inboards and will work fine. Not changing anything else on the engine. Any response is appreciated.

 

[alldodge]

Happen to have a serial number?

 

[Scott06]

You may be better off going with a 4 bbl like edlebrock 1409 which is 600 cfm. Smaller primary bores ( than the two bores on a big two bbl) may give better throttle response. Plus since merc used the Webber version of this carb u can get a off the shelf merc throttle bracket for a super clean install

 

[Lou C]

Why not try rebuilding what you have?
I rebuilt my 30 year old Quadrajet and turned out very well. To me the only reason to replace a carb with new is if there is a lot internal corrosion or if the throttle shaft bushings are worn out causing a vacuum leak.
if you go with the Edelbrock 4 bbl keep in mind you also need to change the intake manifold to the 4 bbl intake. You can use the cast iron GM Vortec intake or a Edelbrock intake. But that adds cost and time to the project....the 2 bbl Mercarb I think is just a Rochester 2GV style carb....pretty simple....

 

[Buckleybob]

Has been rebuilt too many times, and is just loose all over, leaky, etc. I looked at Edelbrock, and intake, but am not really pushing for added hp, just want a good carb that works fine. I guess my only real worry is if there will be bogging at higher rpm with too much fuel and air getting to the engine. According to Holley, it is tuned for my engine. I guess we will see how it turns out. I can always go to the 300 Holley if it does not do any good.
Now, if only they would let us go back on the water!!

 

[Lou C]

Volvo Penta used both the Holley 2 bbl and the Holley 4 bbl on these engines after GM quit building Rochester carbs. Maybe find out which spec carb VP used since the engines were the same...

 

[Scott Danforth]

Volvo Penta used both the Holley 2 bbl and the Holley 4 bbl on these engines after GM quit building Rochester carbs. Maybe find out which spec carb VP used since the engines were the same...

350 cfm holley 2300 (smaller bore version of their 500cfm 2300) and the 500 cfm 2300's

depending on year, etc.

I would suggest the 4-barrel 1408 if you have to buy a carb.

However as 2-barrels go, the 2300 is half a 4150

 

[Lou C]

According to Holley the 4bbl was the 4160 that had no power valve and larger main jets to compensate. I tried one on my engine and it never ran right so I pulled out the old QJet (Which I foolishly thought was too old for a good rebuild) cleaned it out and put in a marine rebuild kit. 100•% better! That fool Holley is sitting in the box on a shelf in by garage.

 

[jimmbo]

A Holley 2300 500cfm is a bit over sized for a 4.3. FWIW, the 2 BBL 500, when Flow Rated like a 4BBL, is about 370 CFM(it is about perfect for a 5 liter at 5000 rpm). I haven’t done the math in the Holley 2300 350 flow rate, but it might a wee bit small for a 4.3 at 4800 rpm
If you want to go the 4 bbl route, get a very small one, or a spreadbore like the Q-jet. Some people swear by Carter/Weber/Edelbrock, I swear at them
The 4.3 has an Uneven Firing Order, due to it being a 90 degree Block and having 6 cylinders, and a Crankshaft design that doesn’t permit a Power Stroke every 120 degrees. This uneven spacing of power strokes can cause reverberations in the intake, resulting in odd fuel metering from a carb. Holley probably felt it was better/easier to omit the Power Valve than design on that would function correctly with signal variations from the Reverberations.

 

[achris]

Whatever you do, stay away from holley. Merc ditched holley circa 1970, and for good reasons. They stayed with Rochester, and later moved to Weber (Carter AFB/Edlebrock 14xx), before moving to injection.

I had the Weber 9600 on my 1994 4.3LX and no problems at full throttle with over fueling. Also, beautiful smooth steady idle from the smaller primaries.

The Edlebrock 1409 on those engines is a delight.

...The 4.3 has an Uneven Firing Order, due to it being a 90 degree Block and having 6 cylinders, and a Crankshaft design that doesn’t permit a Power Stroke every 120 degrees.....

RUBBISH! The 3.8L (229 cu in) was an uneven firing engine, but the 4.3, since Merc started using it in 1985, has offset crankpins, and the engine fires at exact 120 degree intervals.

... I haven’t done the math in the Holley 2300 350 flow rate, but it might a wee bit small for a 4.3 at 4800 rpm...

More rubbish! Here's the calculated flow requirements for a 4.3 at 4800... (run the numbers yourself, https://www.summitracing.com/newsandevents/calcsandtools/cfm-calculator)



Chris.....

 

[jimmbo]

I have never seen any Documentation stating GM increased the Offset of the Crankshaft Throws greater than the Original 18 Degrees. The 4.3's still utilize a Counter Rotating Balance Shaft. so I will assume the engine still has the same 108 - 132 - 108 - 132 - 108 - 132 variation in it's firing Order

When a 2 bbl carb is Flow rated for CFM, a Different Pressure Drop is used than used on 4 bbl

An example is The Holly 2300 500Cfm carb has exactly the the same Carb Bore, Venturi Bore and Booster Nozzle as the Holley 4150/4160 #3310 780CFM 4bbl carb ,so it is the front half of the 4bbl. However it is rated at a higher Pressure Drop than the 4bbl, which is why it is rated to flow 500 CFM. If it were rated at the Pressure Drop used to rate a 4bbl that Holley 2300 would flow about 360 cfm.
So the smaller Holley 2300 350CFM, when flowed at the Pressure Drop for 4bbls would be rated at around 250CFM

When I do my calculations for CFM requirements of an engine I use a factor of 75% - 80% Volumetric Efficiency for these engines. And as I said the Holley 2bbl 500 is just about spot on for a 5 liter spinning 5000 rpm, it would be a bit too big for a 4,3, but the 2300 350 will be a bit small.

I also know that the 2300 500CFM is a bit too small for 5.7, I was getting 4 1/2 Inches of Manifold Vacuum at WOT and 4800 rpm. Putting a Holley 650 Spreadbore on dropped the Manifold Vacuum to 1 1/2 inches and an increase of about 600 rpm, which required a prop change.

 

[achris]

I have never seen any Documentation stating GM increased the Offset of the Crankshaft Throws greater than the Original 18 Degrees. The 4.3's still utilize a Counter Rotating Balance Shaft. so I will assume the engine still has the same 108 - 132 - 108 - 132 - 108 - 132 variation in it's firing Order....

I guess since you haven't been to the sun and seen the hydrogen fusing into helium you don't believe that either! Just because YOU haven't seen it, doesn't make it untrue. Trust me, I have had the sump off a 4.3 more times than I care to remember, the crank pins are offset.

The reason for the balance shaft is the 90 degree V6 configuration, nothing to do with uneven firing. :facepalm:

Have a read of this... https://en.wikipedia.org/wiki/Genera...#4.3L_(262_CID)

Read the second paragraph in on the subsection titled '4.3L (262CID)'

Chris........

EDIT: I even found a picture of a 4.3L crankshaft for you...

https://www.nlamarine.com/products/...hp-205hp-1986-1990-v6-9517?variant=1153577072

 

[Scott Danforth]

the balance shaft was added in 1992 for the 1993 model year to counter-act the harmonic issue from an even firing 90 degree motor. the 4.3 has always been an even fire motor.

 

[Lou C]

So while it is a even FIRING engine, without the balance shaft it ain't exactly an even idling engine as anyone who has had one knows, at certain speeds it seems like it wants to leap off the soft engine mounts lol! if you watch it you can see an up and down front to rear movement...then a harmonic between 1700-1800 which is not really noticeable.
Yet they do last and are pretty rugged I'd say. No worse than any of the other small block Chevy family.

 

[jimmbo]

I guess since you haven't been to the sun and seen the hydrogen fusing into helium you don't believe that either! Just because YOU haven't seen it, doesn't make it untrue. Trust me, I have had the sump off a 4.3 more times than I care to remember, the crank pins are offset.
/QUOTE]

You are correct, the 4.3 has used an offset of 30 degrees making it a an even firing engine.

I was Wrooonnnn... er, Mistaken

My apologizes

 

[jkimball]

You may be better off going with a 4 bbl like edlebrock 1409 which is 600 cfm. Smaller primary bores ( than the two bores on a big two bbl) may give better throttle response. Plus since merc used the Webber version of this carb u can get a off the shelf merc throttle bracket for a super clean install

First post in a few years, but found almost exactly what I was looking for. So, sorry to hijack a post.
I have similar issues as the OP ‘97 4.3 with 2bbl that floods and is needing rebuilding, also have some corrosion on the manifold. So, I figure if I have to consider replacing the manifold anyhow why not go 4bbl.
National Carburetor will build me an Edelbrock 500 cfm for $275 and I plan to use the Edelbrock 2519 (marine) intake.

my question is, other than re-bending fuel line to rout to the new location, is the 4bbl throttle bracket the only other item needed for install? Maybe a PSI regulator to keep the carby happy at 6> psi

also, since pulling distributor and reinstalling - I assume I may need to throw a timing light on it?

thanks for the help