Question for the carburetor guys?
 

Whats the deference between a marine and a automotive carb besides the j tubes?. I've notice if you look at a holley say the 850 dp its jetted way more
then a automotive one . Any reason for this?.

I found the marine bowls and floats are different as they are designed to be more stable in waves and wont trickle over and create stumbles as a automotive one will. some have a fuel inlet in the event your fuel pump diaphragm erupts it will take the fuel to the carb (engine) instead of the bilge.. Jetting is base calibration usually unless it is model specific for the application .

I would stay with the marine version. you may want to jump a wave or two..or three.

O rings on the shafts as well

Also, while almost all "marine" carbs come equipped with "j" tubes, they are not required by the USCG.

As for the richer jets..... A marine engine is under constant load (even while idling in gear) as as such needs the extra fuel.

No such thing as a "light" cruise in a boat.

For a vehicle, you want to jet the primaries lean enough for a slight stumble at cruise, and then add a few #'s richer so that it doesn't stumble. Obviously, then jet secondaries for best power and then a step richer to keep it together.

Don't do this on a boat...it's always under load and thus wants richer a/f's. 'Cruising' in a boat is not the same cruising in a car. Cruising in a boat is still under a ton of load.

Every marine Holley I've had also had these funky "spring-loaded" bowl bolts. Bolt on the business end, screw top on the other and a heavy spring between. J-tube cuts down CFM a little. (a marine 750 is rated at 719 for marine) I haven't used a marine Holley in years.
(as was mentioned above--Boat runs "uphill" all the time--set accordingly)

J tube also plays with metering....no, the carb people don't account for this....so, if you run into a carb (usually from a 'Tuner') that is having problems keeping the a/f miture atleast semi flat, then you may want to look into this.

Why ?

The air going past the float bowl opening at the end of the j tube may actually be creating havoc on the fuel bowl pressure - which is always assumed / supposed to be ... atmospheric.

Probably another topic...but what the hell...figured I'd throw it out there...LOL.

I agree. I hear things like "when Im not in boost" regarding supercharged marine engines. Insinuating, as long as the engine isnt in "boost", that its a light load scenerio. This isnt true!

Example. I could be running along at 4000RPM, at be at 0 on my boost gauge, going about 65mph. At that point, if i had some 400hp engines, that would be balls out . So, while that may looked at as a "Cruise" situation, making 400HP per side to run 65mph, is in NO WAY comparable to a car's "cruise", at 65mph. A car can cruise at high vacuum at 65mph, and maybe need enough fuel to support 60hp(guess), while I need enough to support 400hp at my cruise. Heck, most cars can cruise on the idle circuits alone, without even getting into the high speed circuits. When I am cruising in my boat, I am at a heck of alot more throttle position than a car cruising.

This is why I cringe, when I see these lean AFR numbers thrown around, and high timing numbers, insinuating "its not in boost, so I can lean it down". Sure, maybe your buddy who has a chevelle cruises down the road at 14.7:1 afr or leaner, with no issues. But, it's no where near the same load scenerio.

speaking of load on a boat,it got pretty loaded on the sea ray saturday night!