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!

Well my question is this a stock marine carb has 88/94 power valves front and rear. This holley 850hp new out of box has 80/80 power valve only in the front.. How can they be co different ?

88/94 with front AND rear powervalves, is a heck of a lot more fuel than 80/80 with pv in front only.

Theres more to carbs than jet size. Things like booster design, venturi size, air bleeds, powe valve restriction diameter, idle jet size, and other factors that determine a carbs calibration.

Point is, comparing jet sizes between different carbs is useless. You can slap a 750 on with 75-81 jets, and get the same air fuel ratio as say a dominator with 92/98 jets for example

As stated. when a engine is under constant load is looses vacumn in the intake. Boats are loaded.. low intake vacumn as compared to same engine combo in a chevelle with a 3.73 gear at 55 mph.

Not trying to restate what has been nicely said above.

You need to compensation for lack of vacuum that a constant loaded boat engine experiences. Vacumn is what signals the pull over fuel from the metering circuit and venturies under cruise speed to keep fuel ratios a a good and safe level.

Carb air bleeds are more sensitive to enrichen and jetting is increased to give a marine carb a richer condition with less vacuum in the intake. (air fuel calibration ) . You don't want a hesitant. lugging.. pinging (if you can even tell with open exhaust) big block under load.

Stay with the marine application set up .

Anything with square jetting (same sec jets as primary) should have 2 power valves. So, I don't get the reasoning behind that 850HP.

As far as why carbs have different jetting but with similar cfm ratings:

1) Jets are not the only thing that influence fuel rates. We have different booster designs, different venturi and throttle plate sizing. Inside the metering blocks are emulsion tubes that can differ, channel sizes and restrictions, etc,etc,etc.etc list goes on

2) Different use applications ranging from: sing carb drag boat, dual carb drag boat, single carb smaller cid, single carb larger cid, different range of camshafts, etc,etc,etc,etc yeh, list goes on

====================================

Good example,
There was/is a 830cfm Holley for Automobiles. I can't tell you how many people bought this thing for there street big blocks. Was absolutely horrible carb on these. Time and time again, the talk was 'it's too small' or 'it's too big' and etc,etc,etc as it thru fits since people couldn't seem to tune them correctly.

So....a person like me (after giving up) sits down with their Holley reference material, does some digging, and finds out it was desigjned for small block road course racing.

Bingo !

LOL. I guess coming back to finish my post after starting it a long time ago was a waste. Hah !

I see you answered what I did.

Easier if I waited and just replied 'X 2.' LOL.

The off road guys use a different vent that serves the same purpose as a J tube without the issues. Both vents are connected with one tube that has holes across the top with an offset to miss the air cleaner hold down bolt.

Best bet is to send your carb to a marine carb. performance shop and have it flowed and setup to your engine specs.They cut the choke horn off, seals on the throttle shafts, and alot more to the metering blocks as for jtubes they found that they caused a restriction so they switched back to straight tubes. They did a stage 5 on my carbs, 6 years ago.. never a problem, never a hesitation, backfire etc.
I used http://nickersonperformance.com/marine/marine.html Ask for Brad or Dean if interested. Located in Bristol PA

Are you talking about the QFT M850? If so, it doesn't have a PV in back, only the front. I have one and have had it apart.

It was this one. https://www.holley.com/products/fuel...arburetors/hp/

Ever put an o2 sensor on your engines with their carbs?

Yes I have for my friend with a single engine Checkmate, Had to pull a total of six jet sizes out to clean it up, 2 from the primaries, 4 from the secondaries. and leaned out the secondary high speed jets 2 sizes. Now my friend may not have givin them incorrrect info to begin with? don't know. You won't burn pistons thats for sure! you might wash down the cyls though! Not bashing them this was my one time instance with them. may have been a fluke!

Yup, Holley has one and some aftermarket tuners / holley aftmkt parts manufacturers make some.

Many look like this:
http://ep.yimg.com/ay/spideraccessor...-470-cfm-1.gif

when comes to jetting the booster venturies change it as well, they have a few different ones now and the way the air signals the carb can be way different so jetting for the same CFM carb may be way different.

What about when the secondaries open, my holley secondaries start opening just off idle. One 850cfm on top of a 177 blower. Not like the linkage shown above.

I have a 99 388 Slingshot with 500HP's in it. Only had it a short time, first time with a carb boat so it is a little Greek to me but along the lines of carbs the factory 800's that were on it were in need of rebuild bad. I was going to send them to Nickerson but had a buddy talk me in to letting another guy build me a set of 850's for it. Idles like a dream, smooth on power but feels maybe down on power? 76-77 MPH48-4900 RPM, 1.5 gears and stock 30's. Best I have seen is 80 with the old set up (when it would run). Compared to another friend who had the same set up and was seeing 5200 RPM with the stock set up and 82-83 MPH. I figured I'd have more power? More air/more fuel ='s more power? Trying to keep from doing anything else drastic to these motors as I want to re-power maybe next summer but it has been frustrating.

Stay with the factory carbs on the HP500's. No reason to change.

Yeh, that's coming from a 'Stock Sucks' kind of guy...but, it's not like the HP500 is OEM per say.

has a 1-1 linkage, not a good setup on a boat...

Hard to compare two carbs, without a dyno, or some air fuel ratio numbers. Bolting on a bigger carb, that may be set up too rich, or too lean, can hurt power. Not because the CFM of the carb, but because the calibration may be off.

Thanks. Guy that built the carbs test drove the boat with the tech down in Clear Lake, TX. and adjusted it out, did whatever he does etc. Idles like a dream, power is smooth and crisp, great docking manors etc...but just feel like the top end isn't there. I am following the thread you started MILD THUNDER, thanks for the info!

To be more exact: More air with the correct amt of corresponding fuel = More power.

You will never be sorry if Nickerson Performance rebuilds and Flows Any carb you have.
They know what they are doing!

Tim

Yeah, I have learned this the hard way. I was trying to get ready for Tickfaw and Nickerson was 3-4 weeks out at the time and this other guy was recommended and local to my old home town (Houston). Said "I can do 850's...blah blah blah, do this trick, that trick, works well with your set up etc. Put them on as we were pulling out for the run, ran horrible, brought it back, he "rebuilt" them again another way and they are much better, but feel the loss in top end. Long story short, should have sent my factory 500 carbs to Nickerson instead of buying and building the 850's. Sold the factory carbs so this is all I have to work with unless I go EFI.