why do big outboard motors have more than one carb

bktheking

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Re: why do big outboard motors have more than one carb

Cause they demand more fuel :D On the same token I believe it has to do with design, crossflow vs looper. Remember you are feeding oil to each cylinder, so one carb couldn't possibly do the job on a larger motor for all cylinders albeit it a v4, inline 6 etc etc.
 

scooper77515

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Re: why do big outboard motors have more than one carb

Cause they demand more fuel

That, but moreso, they demand more AIR!!!

Carbs have an inlet that only allows so much air flow. They usually rate them in CFMs (cubic feet per minute). If the piston/cylinder sucks in more air than the carb can flow, then the engine suffers loss of power.

So either bigger carb or more carbs to increase air flow to feed those monster cylinders.
 

cr2k

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Re: why do big outboard motors have more than one carb

Cause they demand more fuel :D On the same token I believe it has to do with design, crossflow vs looper. Remember you are feeding oil to each cylinder, so one carb couldn't possibly do the job on a larger motor for all cylinders albeit it a v4, inline 6 etc etc.

Actually you are feeding oil to the crank and rod bearings, which is where the fuel/air goes before it gets to the cyls. So you need to be able to evenly distribute the air/fuel/oil mixture
 

Mark42

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Re: why do big outboard motors have more than one carb

There are also efficiency issues that individual carbs have over one large, and the safety issue of redundant fuel delivery systems.
 

kenmyfam

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Re: why do big outboard motors have more than one carb

So you have to buy more rebuild kits !!!!:D
 

JB

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Re: why do big outboard motors have more than one carb

They don't. Many of the have fuel injectors.
 

F_R

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Re: why do big outboard motors have more than one carb

The "real" reason is efficiency/horsepower. The ideal two-stroke engine would have the fuel/air charge go straight into and through the crankcase to the cylinders.

As an outrageous example, supose we had a 6 cylinder in-line engine with only one carburetor in the center, manifolded to all 6 cylinders. The charge would have to be directed up and down to the top and bottom cylinders, making a Z-turn as it goes. Same thing for number 2 and 5 cylinders, but a shorter path. Also, same thing for number 3 and 4, but a shorter path yet. Are you noticing a distinct imbalance there? And notice all the Z-turns. Every time it makes a turn, efficiency is lost. Now if we were to put 6 carburetors on that engine, all 6 would have equal paths, and no Z-turns. Much higher horsepower because of less waste of efficiency. And less pollution too, because every time the charge splats up against a wall such as at the ends of the Z, fuel separates from the air and runs down hill. Down hill usually means into the lake.
 

Haffiman

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Re: why do big outboard motors have more than one carb

It is a bit more to it than just the cfm factor.
You need air-speed past the high speed nozzle to make it pull out the fuel.
You need as exact amount as possible to each cylinder both of air and fuel.
Remember that a 6 cylinder two stroke in principle is 6 individual 1 cylinder engines stacked in V or in-line on top of each other. The only 'common' part is the igniton system.
Due to its vertical arrangement gravity comes into play.
A lot of different factors.
 

Chris1956

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Re: why do big outboard motors have more than one carb

F-R has the answer. Kenny has the correct irony. It is all about efficiency. Don't you guys remember the big block (i.e. Mercury's 429-460 CU IN) motors from Detroit with a single 2 bbl carb? There was never any issue about providing enough fuel or air, and those motors were a lot larger than any outboard. The intake manifolds were the size of a large TV table, and had about the same efficiency, but one carb and a single dumb manifold was a lot cheaper than multiple carbs.
 
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