Should I buy new 2bbl carb or upgrade to 4bbl carb and intake?

[msj1979]

In reference to this older topic, http://forums.iboats.com/forum/engin...ild-or-replace

I have a 1999 mercruiser 5.7 with a two barrel carb; I had it rebuilt and it ran better but still not right. I was looking into a new carb, which was about $750 brand new, then I was looking into edelbrock four barrel marine carb and edelbrock marine intake which was about $750 also. What should I do? Upgrade to the four barrel or just buy a brand new two barrel?

 

[airshot]

Unless the rest of the engine is set up for the richer fuel mix (timming, cam shaft etc) you will just be dumping fuel into the engine with little gain in hp. The four barrel versions have other mods so the engine can make use of the extra gas and air mix.

 

[dan t.]

Unless the rest of the engine is set up for the richer fuel mix (timming, cam shaft etc) you will just be dumping fuel into the engine with little gain in hp. The four barrel versions have other mods so the engine can make use of the extra gas and air mix.

Wrong! Same cam, same dist. between 2and 4 bbl engines. Swapping to a 4bbl will give you more HP but only on the top end and possably a better hole shot. If you are happy with your present performance leave it alone. If it were mine I would use a cast iron intake and a Rochestor Quadrajet carb,far better than Edelbrock .

 

[gm280]

Too many times folks think swapping to a larger four barrel is their answer to more get up and go power. Not necessarily. In fact you could really harm your present hole-shot if your present carb is working correctly now. When you install a four barrel, you increase the CFM capabilities but if your engine can't suck in the increased air openings/CFMs, the engine will bog down instead of get up and go. And the reason for that is because the addition CFM capabilities actually opens atmospheric pressure (not what you really want) to the engine and therefore the fuel/air mixture is not drawn in like the engine requires. Too much atmospheric pressures instead of a good vacuum draw. So that is why a lot of four barrels have smaller primary barrels and then as the RPMs rive up and have an increased vacuum draw, the two secondary barrels come in. Then you could get higher RPMs and hence more HP. But it will never be Earth shattering for sure... You really need to balance everything together for noticeable better power and performance. That being pistons, cam, ported heads, rocker arms ratios, balancing the engine and most import, free-flowing headers to be able to use the increased carb CFMs to their benefit... When I was young and dumb (dumber then I am presently now, in my teens) I swapped a very good and perfectly matched factory four barrel carb for a whopper 850 CFM mechanical secondary double pumper huge Holley carb because I thought (totally wrongly) that more was better. Boy was I wrong... It went form good performance to a sucky performance and I was so disappointed but didn't understand why at that time either. I mean more (bigger) was better ..right? NO!

 

[JustJason]

I had it rebuilt and it ran better but still not right

Unless the carb you have is specifically leaking down on you, get it to run right first before making any changes.

 

[gm280]

There was a very good book that the Holley Company put out some years back that was the absolute best information book anybody could even imagine about carbs in general and their brands specifically. I highly suggest that anybody interested in carb fit and performance for any engine, to pick up a copy and read it cover to cover. The amount of quality information about carbs is beyond anything your could ever want or imagine. It covers everything concerning carburetors and why things are made the way they are... If you seriously have any carb questions, your answer is in that book... And it really isn't a sales pitch for their carbs either, but serious engine information and demands from a carb and how those demands are satisfied... :thumb::thumb:

 

[airshot]

Wrong! Same cam, same dist. between 2and 4 bbl engines. Swapping to a 4bbl will give you more HP but only on the top end and possably a better hole shot. If you are happy with your present performance leave it alone. If it were mine I would use a cast iron intake and a Rochestor Quadrajet carb,far better than Edelbrock .

Just basing my suggestion on my neighbors conversion last summer, he went directly to mercruiser for info, had to exchange cam and recalibrate dist after switching to a 4 bbl as it just stumbled like a pig. Now it runs like a scared jackrabbit but ouch the fuel it uses...

 

[thumpar]

I think you should figure out why it is not working before making changes. The 4bbl might give you an extra 20-30hp but that will probably only get you 2 mph on the top end.

 

[keith2k455]

Carb tuning is almost a black art. Squirters, jets, pump cc, pump cams, mixture, vacuum or mechanical. My advice would be to buy the book or find another source of knowledge, tune what you have and then decide to upgrade. The hot rod community understands this well, you can start with a bench test, but you then have to get it on a running engine, get a vacuum gage and timing light and start making runs after you get the idle set right. There are other things to change as well, plug heat, timing advance, etc.

You see, switching to a bigger carb may not overcome any of the things I mentioned. Also, don't forget a 2 bbl has one set of adjustments, a 4 bbl has two sets of barrels. Don't think getting the primaries right is all that's needed, because a lean bog from an 850 will cost you more power than just putting a 409 cfm 2 bbl on.

 

[achris]

In reference to this older topic, http://forums.iboats.com/forum/engin...ild-or-replace

I have a 1999 mercruiser 5.7 with a two barrel carb; I had it rebuilt and it ran better but still not right. I was looking into a new carb, which was about $750 brand new, then I was looking into edelbrock four barrel marine carb and edelbrock marine intake which was about $750 also. What should I do? Upgrade to the four barrel or just buy a brand new two barrel?

Your 5.7 litre engine turning at 4800rpm (max recommended by Merc) will need about 420cfm. Too much more (carb) than that and it's a complete waste, and will degrade the performance of the engine, particularly at low/idle speeds. The 'smallest' 4 bbl you can get is around 650cfm (Edelbrock 1409), and that also needs the right manifold. By all means upgrade to that carb, but don't expect 'chalk and cheese' improvement.. I did a similar swap on a customers' engine. The 2 bbl manifold was rotted out, and a 4bbl complete with carb was half the price of a 2bbl manifold, so that's the way he went. Increased the boat speed by about 1.5knots. everything else remained the same.

BTW, airshot. On a Mercruiser engine, the cam, heads, pistons, distributor and ignition advance curves are the same for both the 2bbl engine and the 4bbl..

Chris....

 

[Moggy23]

I was running a 2 bbl Mercarb with single idle jet on my 97 5.7 ltr Mercruiser, the best thing I did was to throw it in the bin, it had the worst idle system I,ve ever seen on any carbie, I was to the point of drilling & tapping in extra idle jets, but this couldn't be done as the circuit was too high up in the carbie.

My problem started during cold weather as it would run rich until some warmth was created in the manifold, choke is of before anyone jumps on that. Then run ok for the rest of the day

After a couple of seasons of stuffing around, cleaning & resetting carbie, I threw on a Marine Holley 500 2bbl, it run like a injected motor, started, run clean during the warm up, but lost some fuel economy & couldn't be tune for economy, the Mercarb was returning 1.2 ks /ltr, Holley was 1 ks / 1 ltr,

So I bit the bullet & purchased a Edelbrock 1409 & manifold, the Edelbrock is a rebadged Weber, bought all the necessary bits like accelerator bracket & Mercrusier spark arrestor of ebay

The new deal fitted up nicely,just had to drill the EGR port on the 1409.
Tune up time, set the idle, choke & base timing on the muffs, then set the idle mixture again on the water in gear.
Hooked up a vac. gauge & ease into the throttle & on our way up to 3,900 rpm were it leaned out, so the strongest metering spring was fitted from my calibration kit and took 2 degs timing out to match the injected motors.
Next run went straight to 4.900 rpm @ 40 kts were it just leaned out, so next size secondary jets were fitted.
I never worried about trying it again as I was happy with the results in the Aussie 23 Caribbean, the 2 bbl was WOT at 34 knts @ 4,400 rpm

Now I cruise at 24 knts, 3,100 rpms, returning 1.4 ks / 1 ltr, love it.

 

[Maclin]

There was a very good book that the Holley Company put out some years back that was the absolute best information book anybody could even imagine about carbs in general and their brands specifically. I highly suggest that anybody interested in carb fit and performance for any engine, to pick up a copy and read it cover to cover. The amount of quality information about carbs is beyond anything your could ever want or imagine. It covers everything concerning carburetors and why things are made the way they are... If you seriously have any carb questions, your answer is in that book... And it really isn't a sales pitch for their carbs either, but serious engine information and demands from a carb and how those demands are satisfied... :thumb::thumb:

^^^^THIS^^^^ ^^^WHAT HE SAID^^^^

I have that, it is from last century but so are Carburetors lol. I feel naked without it when thinking about Carbs. Great book, terrific resource for any carbureted system and even better for Holleys.

 

[Maclin]

More specific to the OP's question...

You do need to get it running correctly now. Putting on a 4bbl will not fix any underlying problems.

There can be gains from going to a 4bbl, but do not think about it as solely for more power. The primaries on a 4bbl are smaller than the 2bbl in almost all cases. The smaller the carb bores the better lowend and midrange response and economy can be had from said carb. So in a good 4bbl setup the total CFM is not too large, and the primaries are setup for a good takeoff and economical midrange cruise, and the secondary tuning is set to open as the engine demands more but only open far enough to match engine demand.

 

[Tail_Gunner]

That thread you referenced to was a 5.7 but a non vortec and that makes a difference. Your engine will respond far better to a 4barrel than his if you DONT boat in salt water jump on the edelbrok stuff.

Have a read: http://forum.grumpysperformance.com/...rb+size#p39779

https://www.perfprotech.com/new-gm-...d-ignition-components-2541-scc/product/243880

 

[jkust]

There is a good article back from around 2002 or so in Boating magazine. They swapped the 2bbl to a 4bbl and intake on a I think a 1999 5.7liter. The point of the article was to give buyers a point of comparison when considering an EFI boat/engine (presumably MPI if the article was from 2002) and buying a carbed engine and just doing the upgrade. Recall MPI was new for model year 2002. The gist of the article was that they ended up getting a lot of mid range and high end power to where they had to keep re-propping it. They summarized that at least looking through 2002 (or whatever year the article was written since I've forgotten exactly) eyes, the upgrade was a better bang for the buck than buying MPI. As I understand and have read on here over the years, you get much less bang for the buck on 5.0's and less yet when doing the conversion on a 4.3.

 

[Volphin]

I think the 1409 is a fine carb for that application, and will run just fine. I've done several 1409/Eddy mani upgrades (including my own boats) and never had an issue with the end result. Setting up a carb is nothing special. It what we called drivability back in the day. The Holley book is a good start, but any old Chilton book will tell ya the same. You will spend most of your boating on the primaries at cruise. When the secondaries are called for they will give the engine what it requires... more fuel. IMHO running a V8 on a 2bbl is a waste. Letting the engine breathe deep while maintaining proper A/F mix is perfect for better throttle response and more overall power. You won't get a lot of increased MPH, but the goal is power.