Will switching out 2bbl carb with 4bbl help?

[Thumbtack]

OMC Cobra
MOD# 502aphub
ser#T1277370
V8
5.0LHO

I got a new block and it has been working great except that it has almost no get up and go. It will reach full rpms but it takes a while. I know I had issues with the carb in the past and it is getting to the point where I can't get the parts for it that I need to do a good clean up with it, so I'm assuming the carb is the problem. I am thinking of replacing the carb with a newer one anyway so I thought I might find out about making it a 4 barrel carb instead of a 2 since I was told it is a little strange to have a 2 barrel carb on a v-8 engine. I'm just looking for opinions on which carbs to try and if this will even be something that I can do or not so any help and advice would be great.

 

[mkast]

Re: Will switching out 2bbl carb with 4bbl help?

What size (weight) boat is this engine in?
A two barrel carburetor on a V8 is as common as dirt.

 

[Gary H NC]

Re: Will switching out 2bbl carb with 4bbl help?

If you can find the correct intake manifold then yes it will give you more power.If i remember correctly about 20 hp.

The 2 barrel was used a bunch even on the V-8 motors.
Size and weight of the boat is a big factor...

 

[Gary H NC]

Re: Will switching out 2bbl carb with 4bbl help?

I assume the new block you put in has the marine or truck cam in it?

 

[180shabah]

Re: Will switching out 2bbl carb with 4bbl help?

Also, what gear rato and prop are on it?

 

[Thumbtack]

Re: Will switching out 2bbl carb with 4bbl help?

Hey, sorry I didn't get back to everyone sooner it has just been a crazy winter so far.

1)I'm not sure what the boat weighs but it is a 23' searay.
2)The engine will eventually get to full RPMs.
3)I'm not sure about the marine cam or not but the one who put the engine in knew it was going into a boat.
4)I'm not sure either about what gear ratio the drive is (although it hasn't had this problem in the past) as far as the prop, we bought one with enough pitch to get us in the correct full rpm range but I forget what the size was.

I didn't realize that it was this common for the 2bbl carb to be on a v-8. I'm wondering then if maybe the accelerator valve in the carb might just be bad.

 

[tfret]

Re: Will switching out 2bbl carb with 4bbl help?

A 4 bbl will give you about 20 more ponies on a 5 liter. But more important is the intake manifold you select to go with it. If you are in fresh water or have a closed cooling system I would suggest an aluminum Edelbrock or Offenhauser "Dual Port" intake. (You can NOT use aluminum if its salt raw water cooled.) The dual port intake will deliver much better low end torque (needed to get on plane) than a single port intake can.

NOTE: I am assuming your engine is reaching the proper WOT RPMs it is just taking awhile to get there. If not, you may be over propped.

Just curious, what's the benefit of the aluminum manifold over iron besides weight?

 

[Silvertip]

Re: Will switching out 2bbl carb with 4bbl help?

Have you verified that the timing is set right and that the valves are adjusted correctly? If you have no idea what cam/engine combination this is you may very well be down on power compared to the old engine. Just so you know, when the small block Chevy V8 appeared in the 1955 model year, it had a two barrel carb as did the various displacement V8's through the 350. As was mentioned -- common as dirt. Lastly -- lots of people know engines and do a good job rebuilding them. That does not mean they know that a boat engine requires a different setup than one for a car.

 

[Seon]

Re: Will switching out 2bbl carb with 4bbl help?

If the cam can't support the 4 bbl, it'll be a waste of money to switch.

When shopping for my the new to me boat, I ran across several boats ads for sale that read, "new crate engine". When I asked the sellers, all said they bought it from the Chevy dealer which in essence are automotive built engines, not marine. Needless to say, I stayed away from them.

 

[Don S]

Re: Will switching out 2bbl carb with 4bbl help?

Thumbtack, you might want to let everyone know you have a 5.0L FORD, not a Chevy.

 

[mkast]

Re: Will switching out 2bbl carb with 4bbl help?

If the cam can't support the 4 bbl, it'll be a waste of money to switch.

If you look at the Mercruiser part numbers, the same cam was used in the two barrel and four barrel versions of most engines.

 

[Thumbtack]

Re: Will switching out 2bbl carb with 4bbl help?

Thumbtack, you might want to let everyone know you have a 5.0L FORD, not a Chevy.

1) Yes Don is right, sorry if this changes anything. I didn't realize I left that out of my description.
2)I guess the only way to know what kind of cam it has other than talking to the guy would be to actually look at the cam itself?
3)I don't think I checked the timing, thats something I will have to do as I think I just assumed the person who put the engine in did.
4)As far as the old engine having more get up and go, it will be hard to say because for a long time that engine was having problems (much worse problems) and wasn't used much. Before it had those problems it ran good with the same drive on it and a different prop. I was just assuming this problem was something left over from the old engine like the carb or something. But like you said I will check timing. I'll most likely check timing and see about getting the carb sent out and rebuilt (Rather than changing it) before I get involved with the cam issue as I'm hoping to not have to get into that.

 

[frank246]

Re: Will switching out 2bbl carb with 4bbl help?

What if the new replaced engine has 302 heads with smaller valves instead of the 351 heads and cam used on (later) marine 302`s.
Dunno if the omc 5.0 also used 351 heads and cam on their engines as i have a Mercruiser.
You can check the firing order, 1-3-7-2-6-5-4-8 for the 351 heads and cam,
1-5-4-2-6-3-7-8 for the (smaller valved?) 302 heads with the less 175 hp.
This might explain the loss in power.

Like Bubba mentioned above, A dual plennum (Edelbrock performer rpm) and a Holley 600 vac sec. would help, I also run this set up and made a big step from the standard ~188hp.

 

[mkast]

Re: Will switching out 2bbl carb with 4bbl help?

I don't know of any cast iron intakes that are dual port.

Dual plane?

 

[Thumbtack]

Re: Will switching out 2bbl carb with 4bbl help?

Hey guys thanks for all the help so far. So just to double check, a 4bbl carb might be something to try(after I check timing and send out the old carb), and I could put on the 4bbl without having to get a different intake manifold (unless I wanted to)? Thanks again for all the help.

 

[Silvertip]

Re: Will switching out 2bbl carb with 4bbl help?

And just how do you intend to bolt a four barrel carb onto a two barrel manifold.

 

[Thumbtack]

Re: Will switching out 2bbl carb with 4bbl help?

And just how do you intend to bolt a four barrel carb onto a two barrel manifold.

I just thought they made adapters but I guess I would have to look into that, but I plan on checking the timing and sending out the old carb first.

 

[Bondo]

Re: Will switching out 2bbl carb with 4bbl help?

I just thought they made adapters but I guess I would have to look into that, but I plan on checking the timing and sending out the old carb first.

Ayuh,... They make Adapters that Don't work,...
If you wanta bolt on a 4bbl.,.. Get the Right manifold...

 

[ENSIGN]

Re: Will switching out 2bbl carb with 4bbl help?

How's the water shutters? That engine has to be able to breath in and OUT.

 

[fossill]

Re: Will switching out 2bbl carb with 4bbl help?

While no engineer, I personally think 2 bbl carbs are more efficient than the 4 bbl. Case in point is that I have owned several GM 350 V-8's in identical vehicles back in the day (unmodified) and the ones that were equipped with the two bbl carbs always outperformed the 4 bbl ones in get up and go as well as holding power climbing a hill. I could never figure out the hoopla over a 4 bbl carb other than it made more nose and pushed through more fuel. I've never seen the 4 bbl used on industrial V-8's either come to think of it. May work better on a boat, don't know.