Author Topic: What makes a "marine" carburetor?  (Read 7968 times)

Offline Schurkey

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What makes a "marine" carburetor?
« on: December 19, 2008, 10:57:22 PM »
Just curious--what's special about a "marine" carburetor beyond a Coast-Guard stamp of approval?

I'm guessing that they have either special throttle shafts or some kind of seal on the outboard pivots for the shafts.  Beyond that--no idea.

Can a "normal" automotive carb be converted to marine specs--or would it require submission to the Coast Guard for approval?
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Offline Cliff Ruggles

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Re: What makes a "marine" carburetor?
« Reply #1 on: December 20, 2008, 02:33:36 AM »
Marine carburetors are not highly "specialized" units, but do use different calibrations and offer a few features not found on automotive units.

The vast majority of the Marine units we've had sent in here for rebuilding have been side inlet divorced choke models. ( I've seen a few with electric chokes, but they were really late production and still used the early style castings.)  They have all used very small main and idle airbleeds, along with relatively small jets and metering rods.  In other words pretty much calibrated like a pre-emissions carburetors from the late 1960's.

They differ distinctly from automotive units in that the baseplate is not equipped with an APT adjustment, and most do not have any idle bypass air passages drilled.  The shafts are coated with the green teflon coating found on the later automotive units.

They also have a slashed front vent, with the slash facing back toward the rear of the carburetor.  This helps to insure than if/when any flooding would occur, it would go into the engine and not the engine compartment.

The front vacuum fitting in the upper left corner of the main casting is drilled completely thru into the right side main venturi area instead of stopping where it would provide manifold vacuum to the fitting.

For some reason that I will never understand, they continued to use the early large float and early fulcrum position as would be found an any q-jet model prior to 1969 and any Oldmobile carburetor prior to 1975. 

The early arrangement is OK, but lacks the leverage on the fuel inlet needle of the later designs.  One would think that the later arrangements would be more advantagous in a Marine application considering the environment that a boat operates in?

In any case, don't worry too much about the Coast Guard inspecting your carburetor for compliance.  Boarding officers are not trained specifically in carburetor identification.  They are trained however in any and all applicable Federal Laws concerning safety equipment.  They will be looking for a certified/approved flame arrestor on top of you carburetor, and a working ventilation system in an enclosed engine compartment, plus a working bilge pump or method of emptying the bilges.

In all cases, check the Federal requirements for the length/type of vessel that you own for specifics, and make sure everything is on board that is needed.  The Coast Guard also does NOT have juristidiction unless you are operating in a navigable waterway subject to Federal laws/regulations.  This includes the Great Lakes, most of the large rivers, some large lakes that have multiple adjoining states, and all ocean waters within the territorial sea.

In any case, I would not use an unmodified automotive carburetor on a Marine application.  The venting is there for your safety in case of flooding, etc.  The flame arrestor should always be used as well.   A marine engine also spends most of it's life in a heavily loaded situation.  This requires a calibration that is going to keep plenty of fuel flowing through it to avoid any lean conditions and potential engine damage......Cliff

Offline Schurkey

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Re: What makes a "marine" carburetor?
« Reply #2 on: December 25, 2008, 06:31:44 PM »
Appreciated.  I thought there was more to the "marine" carbs in terms of sealing the fuel system to prevent fires in enclosed engine compartments.  I guess I was wrong.
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Offline Cliff Ruggles

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Re: What makes a "marine" carburetor?
« Reply #3 on: December 27, 2008, 05:02:29 AM »
One thing to add.  I've seen attempts by some of the commercial remanufacturer's (all of which are out of business now) to build "Marine" carburetors from late model cores. 

They simply installed a slashed front vent and a "generic" calibration.  They should be avoided, as the calibrations they used were not that good for Marine applications, some even retained the HUGE main airbleeds as they used later 1970's Chevrolet cores.  Trying to get one to work on a Marine engine could result in engine damage as they do not delivery enough fuel anywhere in the loaded rpm range......Cliff

Offline makomark

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Re: What makes a "marine" carburetor?
« Reply #4 on: January 07, 2009, 07:31:19 AM »
Just to add to a bit more on this.

That drilled thru vacuum fitting going into the venturi is to connect the "sight tube" that runs from the fuel pump (another marine unique item). Typically, this hose is semi-transparent tygon, yellow in color. If the fuel pump diaphram fails, the fuel goes into the carb via this hose instead of ending up in the crankcase.

Holley claims to have modified throttle shafts to satisfy the safety requirements. After Cliff stated he had NOT seen anything like that in the marine q-jets, I took a couple of mine apart and they are just as he described above.

Also, I took my favorite carcass core apart and started measuring bleeds, etc., and again, Cliff's right on the ball. All those air bleeds in the air horn are really small.

In terms of the USCG inspecting your stuff, overall Cliff's summary is consistent with my years on the water. The "risk" in using an auto carb isn't really with them, it's with the insurance company IF there is a mishap. Every policy I've read WON'T cover anything if non-approved parts are used - something you may want to keep in mind if you ever go looking for a used boat with a gasoline engine.

Finally, would anybody offer up a 'recipe' for these early casting marine units? My core has a bunch of corrosion so the numbers I just got are probably a bit tight but not by a huge amount...I don't think anything in the airhorn was bigger than 0.040".

Offline Cliff Ruggles

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Re: What makes a "marine" carburetor?
« Reply #5 on: February 06, 2009, 02:03:25 AM »
The stock calibration should be pretty good unless a large camshaft has been added that would require some additional idle fuel and possibly some idle bypass air?.....Cliff