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tintingsandiego
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If you want to haul ass all the time(or have a big hp motor), you want at least an 850 double pump. I have a 460 mild cam with a holley 750 vac secs. fuel pressure holds at 5.5 psi. When krusn the mains are flowing = gas milage. When I stomp on it the secs open up and sucks the gas up. A double pumper will dump gas all the time, regardless of krusn or haulin ass. So ask yourself, what do you want to do or have as a motor?
 

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MPR FAB
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363 Posts
Discussion Starter #3
If you want to haul ass all the time(or have a big hp motor), you want at least an 850 double pump. I have a 460 mild cam with a holley 750 vac secs. fuel pressure holds at 5.5 psi. When krusn the mains are flowing = gas milage. When I stomp on it the secs open up and sucks the gas up. A double pumper will dump gas all the time, regardless of krusn or haulin ass. So ask yourself, what do you want to do or have as a motor?

do you have a metering block on the sec side
 

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tintingsandiego
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Yep, I use a guy here in san diego to rebuild all my carbs. He set mine up for my olds a while back then I went to ford. If its built right you should be cool. My plugs read perfect as well.
 

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A double pumper will dump gas all the time, regardless of crusing or haulin ass. So ask yourself, what do you want to do?
i disagree...u can stay on the primary's for along time before u start to feel the secondary's start to kick in...there are alot of peeps who run 750's & smaller mech sec carbs with great mileage :)...if the gas thing is an issue, why own a boat?...also...u could try the Edelbrock 750cfm...small pri, lg vac sec...rite out of the box, they run great...i have run one with good results (mileage, power)...just my .02 cents

FastRat
 

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MPR FAB
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363 Posts
Discussion Starter #6
dont care about the mileage to much just want to know which one is more efficient and whats the disadvantage is running a vac sec 750 holly
 

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Premium Member
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5,488 Posts
715 vac secondary on this 468 spinning 5400, my 850 DB really did nothing but use more fuel. I also feel my little 3x5 spark arestor is holding it back more vs the smaller carb.
 

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Living in a cage of fear
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16,464 Posts
Wait minute, You only have one carb??
Dual center squirters use the most fuel, who cares about anything else??:D:D

 

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dont care about the mileage to much just want to know which one is more efficient and whats the disadvantage is running a vac sec 750 holly
disadvantage is that you'll need vacuum to open up the secondaries off the line (time delay). DP's you just shove the go pedal and your off. Jet's load and go right way - unlike a car where you'll often see a bog off the start.
 

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E-7 Sheepdog (ret)
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6,834 Posts
dont care about the mileage to much just want to know which one is more efficient and whats the disadvantage is running a vac sec 750 holly
The disadvantage of a properly adjusted vac. seccondary carb in a jet boat is a SLIGHT loss of power, because it will never be too big, and generally be just a tad too small for the engine demand.
Properly adjusted there will be no bog with a v.c. Engine load will demand when the sec. circuit opens. There is, in fact, far more trouble and time spent adjusting a bog out of a big mech. seccondary slammed WFO from idle, which opens all 4 plates fully wide, where a vac. sec. will only open 2, the other 2 will open as soon as needed, BUT NOT UNTILL NEEDED.

A mech. seccondary will give you complete, full throttle opening at full throttle, no adjustment for airflow demands from the engine, so will make absolute maximum power the engine is capable of, which, again, if both are correctly tuned, is not a great deal of difference.

i disagree...u can stay on the primary's for along time before u start to feel the secondary's start to kick in...there are alot of peeps who run 750's & smaller mech sec carbs with great mileage
I'm one of the guys with "too small of a mech. seccondary carb", at least according to the guys who swear "850 or nothing". 700CFM 4150 Holley, primaries only untill about beyond 4,000RPM. With good water, my wife & I can cruise at that, making 45mph, and do mighty good mileage-wise. HEAVY old Taylor boat.
Stomp my foot all over it just playing instead, it was well capable of drinking 20 gallons in 2 hours, and a lot of that time was sitting floating, shut off.
I could always ski or tube kids for upwards of 4 hours solid before hitting the marina for gas.
Since my 454 now spins just short of 6-grand on Nitrous, instead of just at 5-grand it did for almost 30 years, the 700 is NOW too small, but was not beforehand. I have not been able to get Lowrider out to tune the new 850 yet, but, I STILL will be able to keep out of the seccondaries untill at least 4-grand amd the slightly bigger primaries will not cost me that much fuel consumption compare to the 700.
 

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Angry American
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3,103 Posts
I've tried different carbs on my engine, 750 dp, 850 dp and a 750 vs without any noticable difference. The 850 was actually the worst performing of all 3 probably because it wasn't tuned to my engine. Both of the 750's were from Bigs Performance and were tuned to my engine specs.
All three carbs were pretty much the same as far as the top end goes, just about 5100 with an A/B impeller running 76ish mph. Since there wasn't any real difference and the boat is used 95% of the time for cruising, I've been very happy running the 750vs. It's also won it's fair share of races against people with similar engines near the same power that say vacuum secondaries don't work.
Also, there's a misconception the vacuum secondaries run off engine vacuum, they don't. Vaccum secondary carbs use the high venturi vacuum in the primaries to open the secondaries. That vacuum will increase with airflow. You can change the vacuum level by changing the springs in the dashpot or you can get an adjustable dashpot so you won't need to change springs for different opening rates. I went with the adjustable type due to I like making things simple. If I knew I was going to be doing a lot of cruising, I'd turn the adjustment screw in about 1/4 turn so the secondaries wouldn't come on too soon. Turning the screw back out would allow the secondaries to open sooner and quicker.

I like the fact I could get decent gas mileage without losing any top end with the combo I have.
 

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E-7 Sheepdog (ret)
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6,834 Posts
Also, there's a misconception the vacuum secondaries run off engine vacuum, they don't. Vaccum secondary carbs use the high venturi vacuum in the primaries to open the secondaries. That vacuum will increase with airflow. You can change the vacuum level by changing the springs in the dashpot or you can get an adjustable dashpot so you won't need to change springs for different opening rates. I went with the adjustable type due to I like making things simple. If I knew I was going to be doing a lot of cruising, I'd turn the adjustment screw in about 1/4 turn so the secondaries wouldn't come on too soon. Turning the screw back out would allow the secondaries to open sooner and quicker.

I like the fact I could get decent gas mileage without losing any top end with the combo I have.
This steps over into cars, but, many, many years ago, I had a '74 Camaro, 400 small-block, 600vc holley (list 3310 if I recall right), TH400 automatic, 3.73-ish gears.
Made a 270+ mile road trip into SE Ks from home, through the flint hills. Before leaving, gassed the car full, and installed the lightest spring in the carb seccondary dashpot.
Carb would most likely be cracked into the seccondaries slightly, the whole trip at highway speeds.
Drove to SE Ks. to a now-ex-grandmother's house (ex-wife's kin).
Next day before leaving, gassed car, instaled HEAVIEST spring in seccondary dashopt.
Carb would NOT get into the seccondary at normal highway speeds (55-60).
Drove home, gassed up car.

Got WORSE mileage on the way back, with the 4-barrel basically locked down to a 2-barrell. Difference was 4 to 5 mpg (20-25%), from 20mpg down to 15-16mpg.

The engine was noticably down on power, required throttle inputs for the hills ups and downs I was driving through that it did not require the trip before, or larger inputs than before.

I under-carbed the engine enough to cause myself to get significantly worse fuel mileage, with a "smaller" carb.

It is all about how the combination fits together.
 

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Horsepower sells Engines
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402 Posts
i wish it was all abit easer with no spicific equasion to tell you what to run it just seems as if you need a dyno and a hand full of carbs diffrent sizes, diffrent designs and play for a day.
 

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Who says a V.S.carb is a bad thing? I see more reliable turn key year in year out boats running the 3310 . I run the holley modular 750 D.P. only because its shinny.
 

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Put A New Holly 750 Vacuum Carb On My Old Boat For My Buddy. Jetted Correctly On A Mild 454 Runs Like A Dream. Just Would Rather Have A Mechanical Carb By Preferance. It Was Just A Deal I Came Up On. No Regrets So Far.
 

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Angry American
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3,103 Posts
It is all about how the combination fits together.
that's what it's all about right there!

i wish it was all abit easer with no spicific equasion to tell you what to run it just seems as if you need a dyno and a hand full of carbs diffrent sizes, diffrent designs and play for a day.
That's why I've been using Bigs Performance, each carb is built specifically to your engine specs and is dyno tuned before it's shipped out.
 

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Just throwing in my two cents because I'm feeling like stirring the pot a little.:)grn
A double pumper carb is literally designed for more agressive throttle response, that's pretty much it..Whereas a Vacuum secondary was designed for ease of tuning, fuel economy (spread bore) and have some traits that make them less responsive to "mash'n the gas". The efficiency of the engine actually brings on more throttle, so it will not actually run out of it's tuned "comfort zone".

This has been beat into the ground on the whitewater scene and it was pretty much a decision of the driver, some people like to have the right now accelleration, some take full advantage of the vacuum secondaries to make lop count the same as skill (and get some MPG as a side benefit).

Vacuum secondary carbs are awsome, the square bores tend to have less to do with MPG and more with a controlled tip in on the throttle...:)Unsure

Spread bores work killer, Mercruiser has been using Quadrajets and Carter AFB's (Weber) for decades with great success. On a jet, the engine has to spool up before any work is done anyway, so the difference in neglegable, but on a vee drive, I would think a double pumper would be ideal if looking for the hole shot. Everyday cruiser, I would want a vac. sec.;)

GT :)hand
 

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Vacuum secondary carbs are awsome, the square bores tend to have less to do with MPG and more with a controlled tip in on the throttle...:)Unsure

GT :)hand
:)hand:)hand I concurr! But I have a feeling my Injection unit may spoil me in another direction.

Ray
 

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:)hand:)hand I concurr! But I have a feeling my Injection unit may spoil me in another direction.

Ray
I love EFI, but my pockets are too shallow.:)Violin

I drove an aluminum 410 small block with a port efi setup, damn...Pretty awsome. :)devil
 
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