changing from a 2 barrel to a 4 barrel carburetor

[rolmops]

I have a 1989 4.3 liter OMC Cobra with a refurbished engine(2024) and EST ignition.
It runs very smooth and with the current prop it usually tops out at 4200 rpm. This engine pushes a 1989 221 Starcraft Islander with a 9.9 Suzuki kicker (128 pounds) and about 90 pounds of battery weight which is situated in the front of the boat, the usual piles of fishing gear (great lakes salmon trolling setup) and fifty gallons of fuel. This rig tops out at 30mph if I push it hard .
The salmon fishing is often offshore which means a 10 to 20 mile trip each way.
Would it do me any good to switch from a 2 barrel to a 4 barrel carburetor in order to speed up a bit, or would the gain be very little?
Thank you for your opinions and advice.

 

[cyclops222]

22 foot boat.
Check that tachometer and GPS speed for being very accurate.
Does the bilge pump EMPTY ALL the water down there ?

 

[jimmbo]

A 22 ft is a lot of Boat for a 4.3. Myself I wouldn't use anything smaller the Big Block in a boat that Size. That said...

Tops out at 4200 Rpm. What is the WOT range for your Engine, I mean the WOT range that was Specified for it in 1989?
A 4.3 is not going to need a lot of CFMs. At 4200 rpms it is only going use about 250 CFM. The 2bbl 350CFM Holley will flow enough air even at 4600rpm. A 4bbl isn't going to do much. Over Carbureting an Engine will take pretty much all the Fun out of operating it

 

[Pmt133]

I picked up about 4 mph on mine but thats a vortec headed 4.3l. I haven't finished dialing in the carb yet but I ran the same 21p with the 2bbl and 4bbl. The 21 on the 4bbl performs the same as the 19 did with the 2bbl as far as hole shot cruise etc. For the price I don't think its worth it but I got all the conversion parts free so it was a why the hell not proposition.

I'd verify the tach and operating range too first. But it ain't magic. That being said my gains were on the top end of what most would expect as well and I really didn't think the bottom end would fatten back up but those small primaries on the afb are pretty good.

 

[Scott Danforth]

yes, you will pick up power in your 4.3

what year is the engine? original 1989? newer ? upgrading to vortec heads when you switch to a 4-barrel will move the boat along at a bit higher speed

the islander doesnt weigh much, similar in weight to my bud's 24 foot crestliner hawk that hits 45mph with his 4.3 when running to the fishing grounds (also on lake Michigan)

 

[Lou C]

It was a factory option…the 2bbl was rated at 175 hp and the 4bbl was rated at 205 hp. The thing is that they used a Quadrajet which is a good carb but the originals are all old now. So your choices if you’d want a new carb are a Holley 4160 or the Edelbrock 1409. Used 4 bbl intakes do pop up on eBay from time to time. Adapting the Holley is a bit easier than the Edelbrock because it was used on those engines after GM quit making Q-Jets in about 1990. Both OMC & Volvo used the Holley whereas Merc switched to the Weber. How much does it add? Hard to say but 30 hp is about what i recall from both OMC & Merc specs.

Going by OMCs specs I think your WOT rpm is a bit low. You should be seeing 4400-4600 or so.

 

[cyclops222]

What is the Prop Slip at WOT ? That tells where a lot of hp and speed could be wasted.

 

[Lou C]

The other thing is those one piece batwing exhaust manifolds are pretty restrictive, when I switched to the center riser exhaust (4.0 diameter vs 3.5) the engine definitely seemed more responsive. I would try dropping your pitch 2” first. I run a 15x17 on mine.

 

[rolmops]

So the official WOT range is 4200 t0 4600 and the current carb is a 2 barrel Rochester. The current prop is a 15-17 a slightly smaller prop would bring me up to 4400 rpm. But this was before I moved the 100 pounds of battery forward,so now it may be different
Does an Edelbrock or Holly fit onto the intake instead of a 2 barrel or do I have to get an intake manifold that has a larger hole for the 4 barrel? I will check the block numbers to make sure about the production year.

 

[dingbat]

the islander doesnt weigh much, similar in weight to my bud's 24 foot crestliner hawk that hits 45mph with his 4.3 when running to the fishing grounds (also on lake Michigan)

Is the 4.3 that much of a dog?

My glass boat is a good bit heavier. A 200 Hp outboard pushes it @30 mph at cruise, just touches 40 mph at WOT

 

[Lou C]

You would need a 4bbl cast iron intake which has the spread bore opening for a quadrajet, both the Holley and Edelbrock are square bore carbs so both require an adapter. There is an Edelbrock aluminum square bore manifold but I recall reading that it won‘t fit with the OMC high mounted alternator. The Holley fuel inlet enters on the port side, and the choke assembly will just clear the OMC shift bracket and trim relays. The Edelbrock I have not yet tried to adapt to the OMC 4.3 but from looking at it, I think it’s a bit more involved because the OMC shift bracket/bell crank is in the way of where the fuel line on the Edelbrock exits. I think you’d have to make spacers to lift that bracket higher up and away from the carb for the fuel line to have enough clearance. Both the Holley and Edelbrock have electric chokes vs the old style choke on the Rochester carb, and you’d also have to adapt the throttle linkage. It is a fair amount of work to get everything right. While a Quadrajet would be easier, because no adapter needed and it clears everything, there are no new ones and remans, I would advise you to be very cautious about. They often do not work well!
I might first try going with less prop pitch, it won’t increase top speed but the engine will reach closer to the max RPM and will definitely feel more responsive coming up on plane.

 

[airshot]

Check tour engine for top performance as your wot sounds slow. My 1983 22' Islanders also has a 9.9 kicker, three group 27 batteries and loads of fishing gear ( walleye trolling) and the boat has a 3.0 140 hp and will top out at 34 mph with 3 adults on board according to both GPS units.. A good motor tuneup, proper prop and trim should get you more than 30.....