I have a question that is way off topic because the carb and engine in question is in a BOAT and not a Camaro but I am a Camaro owner who has been impressed with the knowledge I have seen on this forum so I thought I would try my question on you carb experts.
I have an inboard boat with a 4 barrel carb. My only interest with this boat is cruise efficiency (MPG) as I log lots of miles. Top end performance is of no concern. I have to run the boat near the RPM range where the carb secondaries will begin to open but I try to hold it there to prevent the gas guzzling that would occur if I allow the secondaries to open. The boat works better though if I run 100-200 more RPM but I think this would put me in the secondaries. My question: If I disable the mechanical linkage that opens the secondary throttle plates (effectively making it a 2 barrel carb), will I be able to run more RPM without too much more fuel consumption? I know that more RPM will mean more fuel but I'm hoping to avoid the drastic increase in fuel consumption that occurs when the secondaries open. I also know this will hurt top end performance but I don't care.
Is there more to this than I'm thinking?

Thanks for any help!!
Grant

Yes, by disconnecting the secondary linkage, secondaries should not open up, unless engine load pulls them open. I don't think this would happen.

Ditto.

Without the secondarys rolled in..the primarys will be yanked on harder to maintain the the same rpm under load.The same amount of a/f is reqd to maintain speed regardless of its source.Aside from the secondary pump shot..the carb is a passive device and reacts to manifold vacuum.With some secondary throttle angle..the signal will drop under the primary boosters and that ckt would lean out but the a/f would remain the same with the secondary boosters offsetting.Depending on the manifold design..your VE may decrease with the 2 barrel and distribution issues at higher engine speeds.Stay with your oem setup.it will use the same fuel but just require a little less throttle input than a 2 barrel setup.

Thanks. I suspected there might be more to it than simple black/white.

67funcar

George I'm with ya on theory. But I think if he rigged up a little longer rod on his mechanical secondaries it would help him. Yes it may lean him out, but he stated his main goal is efficiency. And leaner is efficienter:D.

Long as it doesn't go so lean it gets hot and does Bad Thangz:sad:

The mechanical secondary carbs open the sec before primary gets to 90° eh?

Hey, a different approach... downsize your secondary jets to same as primary:hurray:

Some linkage needs to be there for a failsafe shut off of secondaries. Hi engine vacumn can pull thos secondaries open , and stick open, Now What? I had a 3916 three bbl do this, cause I thought I could save gas , using only the primary. Guess what happened in town away from the stop light :eek:

[quote=77wolf10.85;848777]George I'm with ya on theory. But I think if he rigged up a little longer rod on his mechanical secondaries it would help him.

it would allow more throttle angle before secondary engagement and grab that desired 1-200 rpm increase Grant seeks.Not much changes though.The primarys boosters will be pulled on harder and the main jets will flow more..resulting in the same amount of fuel consumption.Identical 4 corner jets and power valves are fine.If he leans out to above 15/1 under load..he may be draining the pistons out of the pan on the next oil change.I am no boat guy...but maybe a prop change would allow you to plane out at cruise and stay on the primary ckt only..at the cost of some out of the hole performance???

Goerge has it. Assuming everything is jetted exactly right, it will take no less fuel to produce "X" HP on the primaries alone than it will to produce that same amount of HP on less primary and a little secondary.

Note also that "It will guzzle gas if I let the secondaries open even a little" simply cannot be a true statement.

Again assuming everything is jetted right, fuel consumption is simply a matter of power produced, particularly at cruise.

Have you check and experimented with timing? There may be some efficiency gains there. Is the jetting spot perfect? A wideband gauge, or perhaps some dyno time may help you get the engine tuned for max efficiency.

What about trim? Your trim can have a dramatic effect on a boats speed and fuel usage.
Perhaps a different prop would help as well. A lower pitch prop will cut the load on your engine, at the expense of speed.

I've never really played with this stuff from an efficient cruise point of view. When I'm just putting across the lake, I tend to keep her trimmed down low, which increases load and reduces speed, but smooths the ride. My "passengers" like that. When it's just me, the throttle os firewalled, and she's trimmed up so high she's dancing on the pad, I like going across the lake with my hair on fire!

i wouldnt give throttle up. or neither disable 4 barrels.when you half throtle and up on plane cruising.and than floor it what does your rpms go up too? most boats others tampered with and put wrong pitch on boat or either wanted to get faster quicker.too say a 17 pitch will get you too plane quicker but or gas mileage a 19 willl do you better slower on to plane but better gas mileage.for better i need prop you got what year make model and io with motor size.also outdrive you got. for sertern years they got rpm range with the motor and outdrive.with the length of boat and all. give me all specs also weight of boat if known. i got a program which will dial you in closer that you have. from what you saying you probally 2-4 pitches off. pm me with the specs