Carb question?

[Greaser]

I need to buy a new carb for my southwind, and I was wondering if a holley 750 cfm, with vacume secondary and manual choke would be a good choice. Right now I have a 850 double pumper, and everyone I have talked to says it is to much. I figured I would ask you guys. My motor is a pretty much stock 460.

[napabob]

This is from the Holley site. I installed a marine 750 CFM mechnaical secondaries and electric choke on my stock 454
Engine Size (CID) X Maximum RPM / 3456 =
CFM @100% Volumetric Efficiency
EXAMPLE:
350 CID X 6000 RPM = 2,100,000 / 3456 = 608 CFM
Approximately 608 CFM would be required for this engine. However, most street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, good intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage.

[Greaser]

:confused: wow, that was pretty technical, :confused:
can anyone translate that in laymens terms:D

[Oldsquirt]

Napabob, for some reason most jetboaters find that that formula results in too small a carb for all out performance. Can't tell you why but that has been the general consensus.
Greaser, You might see if anyone has a 750 to try before you buy one. Compare WOT rpm between the two. I'll bet some Ford owners will chime in to say you'll be happier with the larger carb.
Where you boat do you really need a choke? If so, you will be happier with the electric. A manual choke carb can be retrofitted to electric pretty easily, tho. I did that with a 750VS years ago, but eventually removed the entire choke assembly. Just didn't need it.

[FOURQ]

get the mechanical secondaries not the vacume

[MudPumper]

Mechanical Secondaries, No choke!!!!

[riverbound]

Originally posted by Greaser
I need to buy a new carb for my southwind, and I was wondering if a holley 750 cfm, with vacume secondary and manual choke would be a good choice. Right now I have a 850 double pumper, and everyone I have talked to says it is to much. I figured I would ask you guys. My motor is a pretty much stock 460.
I do have an extra 750 w vac. secondaries. I recently changed to a 700 dp. I was told the 700 dp would work better for my application. shoot me a pm if you are interested in it it is a holley 3310s (shiny finish) 1 season on it (3 trips)

[Hallett19]

talk to Ty at www.tpcracing.com, he set me up a great holley 780 vac secondary carb for my boat. He put a billet baseplate and bigger boosters and even set it all up for me, although out of the box it ran just fine, he set the float levels for me. The formula you used probably wont work. I have a "for the most part" stock 460 and it ran almost 200 rpm slower with a 650 mech secondary Edelbrock than it does with the 780 holley. The 780, after the rebuild got me almost 200 on top of the 200 I gained with the 780 carb !!! He does great work and ships anywhere.

[PLACECRAFT20]

I HAVE TRYED BOTH ON A 454. THE 850DP RUNS A LITTLE HARDER ON THE TOP END BUT THE 750 ACCELERATES A LITTLE FASTER OR IS A LITTLE MORE PUNCHY IF THAT MAKES SINCE. BOTH OF MY CARBS ARE MECHANICAL. THIS IS THE ONLY WAY TO GO IN A JET BOAT. I AM CURRENTLY RUNNING THE 850DP.

[OMEGA_BUBBLE_JET]

Greaser. I have a 650 holley double pumper with mechanical secondaries I pulled off my boat this past weekend. I went with a 750 holley dp. the 650 spun my 460 ford to 5300rpm no problem. I would be more than happy to bring it to the lake with me this weekend and let you borrow it and if you like it we can make a deal. I rebuilt it last season it has only maybe a dozen trips on it. Oh yeah it has no choke and is already jetted for a big block.
Omega
edit: I definitely would stick with a mechanical secondary. vacuum secondaries can be made to function on boats but work better in cars.

[Greaser]

Omega, I appreciate the offer, but I think that going from a 850 dp to a 650 is to much of a jump down. Im kinda leary about going that small. From what people have told me I need a 750 . It sounds like mechanical secondarys is the way to go. And I really dont think I need a choke, but how do you disable the choke?
Omega, let me talk to my jet boat guru, and ask him about that carb, I will let you know. Only thing is I was hoping to get one this week, so I could be ready to go on Sunday. The carb I have on there now is completly shot, and will not run safely.

[OMEGA_BUBBLE_JET]

Originally posted by Greaser
Omega, I appreciate the offer, but I think that going from a 850 dp to a 650 is to much of a jump down. Im kinda leary about going that small. From what people have told me I need a 750 . It sounds like mechanical secondarys is the way to go. And I really dont think I need a choke, but how do you disable the choke?
Omega, let me talk to my jet boat guru, and ask him about that carb, I will let you know. Only thing is I was hoping to get one this week, so I could be ready to go on Sunday. The carb I have on there now is completly shot, and will not run safely.
trust me the 650 will work beautifully on a stock 460. don't fall into the huge carb=more speed mentality. you can OVER carb an engine very easily and it will effect performance negatively. the 850 is way too much for a stock engine and might explain some of your problems now.
I'll tell you what this carb I have is ready to go right down to having the double pumper line and fuel pressure guage on it. All you would need is a gasket which should only cost a couple bucks. I'll bring it to the lake with me and you can try it out. If it doesn't work out for you bring it back to me next time at the lake. the only thing you have to loose is a $2 gasket. the damn thing is just sitting on the shelf in my garage (in a box of course) and I plan to get rid of it anyways.
I think this little carb will surprise you and I know the throttle response will be incredible. So should I bring it with me on Saturday????:)
Omega
edit: sorry it is very easy to remove the choke. unscrew the linkage off the side of the carb and unscrew the choke plate from the rod. slide the plate out and then slide the rod out. you're done. If you need help doing this I can hook you up.

[napabob]

Originally posted by OLDSQUIRT
Napabob, for some reason most jetboaters find that that formula results in too small a carb for all out performance. Can't tell you why but that has been the general consensus.
Greaser, You might see if anyone has a 750 to try before you buy one. Compare WOT rpm between the two. I'll bet some Ford owners will chime in to say you'll be happier with the larger carb.
Where you boat do you really need a choke? If so, you will be happier with the electric. A manual choke carb can be retrofitted to electric pretty easily, tho. I did that with a 750VS years ago, but eventually removed the entire choke assembly. Just didn't need it.
oldsquirt
I used the formula and decided to install a 750 CFM (mech. secondaries and electric choke).
454 CID x 4600 RPM = 2088400 / 3456 = 604 CFM
I too was suspicious of it (result) being to small for an Edelbrock Performer 2 - O (oval ports).

[OMEGA_BUBBLE_JET]

Originally posted by OLDSQUIRT
Napabob, for some reason most jetboaters find that that formula results in too small a carb for all out performance. Can't tell you why but that has been the general consensus.
Greaser, You might see if anyone has a 750 to try before you buy one. Compare WOT rpm between the two. I'll bet some Ford owners will chime in to say you'll be happier with the larger carb.
Where you boat do you really need a choke? If so, you will be happier with the electric. A manual choke carb can be retrofitted to electric pretty easily, tho. I did that with a 750VS years ago, but eventually removed the entire choke assembly. Just didn't need it.
I have heard this same myth and wonder how it got started. I am sorry but that engine doesn't know if it is in a boat, car, tractor or what and should make no difference either way. In fact most marine applications rarely see over 5500 rpm while some automotive engines see well over 6000 rpm. Anyone care to explain this to me?:confused: BTW I think it is important to mention that the size of the carb refers to the amount of AIR it will flow not fuel. I always thought that jet boats needed to run a little rich:wink:
Omega

[napabob]

Originally posted by OMEGA_BUBBLE_JET
I have heard this same myth and wonder how it got started. I am sorry but that engine doesn't know if it is in a boat, car, tractor or what and should make no difference either way. In fact most marine applications rarely see over 5500 rpm while some automotive engines see well over 6000 rpm. Anyone care to explain this to me?:confused: BTW I think it is important to mention that the size of the carb refers to the amount of AIR it will flow not fuel. I always thought that jet boats needed to run a little rich:wink:
Omega
This site is for props, but it sounds that it could be a basis for WOT, RPM and impeller matching.
http://www.deepcreekyachtclub.com/WebPage/PropellerTune.html

[Moneypitt]

napabob, I think some basic carb re fresher info is needed. The engine under the carb is an air pump, nothing more. The fact that we try to pack this air pump with an explodable mixture of fuel and air and the use a spark plug to make it blow up, doesn't change the basics, its an air pump!.......The vacuum impulses that the carbs sees from this air pump are what makes a carb work, or not work. When you reduce the air flow,(cfm) down to what the air pump can flow, you have a much better chance of matching this magical air/fuel mixture to your application. The formulas that the carb builders use are for real. These limitations are established by the size of the air pump. IMO, go with the loner carb, what have you got to lose? And yes, just about everyone that buys a carb for a jet boat buys one way to big and runs them rich......Terrible fuel economy, sound familar? A dialed in carb of the proper size will not run lean, it will run just right........MP

[OMEGA_BUBBLE_JET]

Originally posted by Moneypitt
napabob, I think some basic carb re fresher info is needed. The engine under the carb is an air pump, nothing more. The fact that we try to pack this air pump with an explodable mixture of fuel and air and the use a spark plug to make it blow up, doesn't change the basics, its an air pump!.......The vacuum impulses that the carbs sees from this air pump are what makes a carb work, or not work. When you reduce the air flow,(cfm) down to what the air pump can flow, you have a much better chance of matching this magical air/fuel mixture to your application. The formulas that the carb builders use are for real. These limitations are established by the size of the air pump. IMO, go with the loner carb, what have you got to lose? And yes, just about everyone that buys a carb for a jet boat buys one way to big and runs them rich......Terrible fuel economy, sound familar? A dialed in carb of the proper size will not run lean, it will run just right........MP
well it is good to see I am not crazy;) one more thing to add. the holley formula is based on 100% volumetric effeciency which is something no stock engine will achieve. try more like 80%.
Omega

[napabob]

Originally posted by Moneypitt
napabob, I think some basic carb re fresher info is needed. The engine under the carb is an air pump, nothing more. The fact that we try to pack this air pump with an explodable mixture of fuel and air and the use a spark plug to make it blow up, doesn't change the basics, its an air pump!.......The vacuum impulses that the carbs sees from this air pump are what makes a carb work, or not work. When you reduce the air flow,(cfm) down to what the air pump can flow, you have a much better chance of matching this magical air/fuel mixture to your application. The formulas that the carb builders use are for real. These limitations are established by the size of the air pump. IMO, go with the loner carb, what have you got to lose? And yes, just about everyone that buys a carb for a jet boat buys one way to big and runs them rich......Terrible fuel economy, sound familar? A dialed in carb of the proper size will not run lean, it will run just right........MP
Thanks for the carb. function clarifications.

[waterslinger]

Ford guy to ford guy run the 750 dp no choke

[OMEGA_BUBBLE_JET]

Originally posted by waterslinger
Ford guy to ford guy run the 750 dp no choke
Ford guy to ford guy you better get that orange FRAM filter off there.;)
Omega

[GM Killer]

Remember a few things. Bigger is always better, so dont hold back, go with a tunnel ram and dual 1250s. Why be a wimp about it, Fords can handle it!!!
Built FORD TOUGH!!!!
Go for it DUDE!!!!!!!!!!!
:D :D :D :D :D