carb

[po51 21]

Getting a Holley ultra 750 double pumper for my 408 stroker with electric choke. What is the difference between vacuum and mechanical secondaries? Which is best? I'm borderline on my vacuum as I have vacuum brakes and a hotter cam. Where does the vacuum line for the secondaries hook to?

[po51 21]

Just realized that all Holley 750 ultra double pumper have mechanical secondaries anybody running one? If I want vacuum secondaries and electric choke in the 750cfm carb what are my options? Or should I go with the mechanical secondaries?

[1969_Mach1 333]

A lot of opinions with this topic.  I've had both vacuum and mechanical secondary carbs.  Both can be tuned to work well on a street motor.  My preference is a mechanical secondary carb.  I currently use a Holley 700 CFM double pumper on a 351W.  I am happy with it.  What trans are you using?  If it requires a kickdown linkage, the only mechanical secondary carbs Holley offers setup for that are the Ultra Double Pumper series.  But it sounds like you are looking at the Ultra Double Pumper 750 since those come with electric chokes.  If you get a classic series double pumper the primary throttle shaft will have to be changed and an electric choke kit added.  For a double pumper a 750 size sounds about right. 

 

For vacuum secondary Holley's, I know there are members here using the 770 cfm Avenger.  I'm sure somebody will chime in with more information.  Holley's classic 750 vacuum secondary, list number 3310-X (x is some generation level number) is okay.  The first and second generation 3310's, the 3310 and 3310-1 are actually 780 CFM, for some reason work great, and are highly sought after.  A 3310 or 3310-1 would of course be used and possible need a major rebuild.

 

I'm sure you'll get more input.  But, as long as thee are no kickdown linkage issues I wouldn't hesitate to put a 750 double pumper on a 408.  You might want to call Holley's tech line for their input.

[Rsmach1 71]

On a vac secondary the the rear 2 barrels open from a vac signal, mech secondarys are controlled by your foot. If looking for performance mechanical are the way to go, although it will require a lil more tuning.

[MikeStang 247]

Yep I'm going through this same ordeal right now.

Running an 870 Street Avenger...Its a nice carb and works fine but its Vac secondary and it took a bit of adjustment to get the secondaries to open all the way when they should... I am going to tune on it some more and get it dialed in for my 408 tho.

Oh I have 12 Hg of Vac on my 408 and the power brakes work just fine.

[1969_Mach1 333]

I think a big key to mechanical secondary carbs is do not oversize them.  They are not nearly as forgiving to being oversized as a vacuum secondary carb.

[Rsmach1 71]

I think a big key to mechanical secondary carbs is do not oversize them. They are not nearly as forgiving to being oversized as a vacuum secondary carb.

+1

[barnett468 418]

it depends on your trans type as mentioned and converter stall speed and rear gear ratio.

 

unless you are drag racing it, it is more practical to have vacuum secondaries with an auto trans but they are not best for a manual trans.

 

if you use vac secondaries, buy a set of holley secondary springs and try lighter ones until it starts to bog/hesitate a hair when you accelerate hard then go back up one size . . most people use the white one which opens them the soonest.

[po51 21]

Have no idea where the above post came from! I wanted to update my post I'm running a FMX with 3,25 trac-loc Looks like the way to go is vacuum secondaries, so the holley 750 ultra double pumper seems to be out. Looking for suggestions on carbs. Is anyone running mechanical secondaries with a FMX?

[barnett468 418]

.

you didn't mention the stall.

 

below is a carb that will work decent but will work better with progressive linkage . . it has annular boosters . . the holley ultras will be a bit rich.

 

http://www.summitracing.com/search/part-type/carburetors/brand/quick-fuel/fuel/gasoline/carburetor-flange/square-bore/secondary-type/mechanical/booster-type/annular?N=4294921892%2B400990%2B4294921890%2B4294921888%2B4294921886%2B4294881136&SortBy=Default&SortOrder=Ascending

 

unless you have a high stall converter, i would use a vacuum secondary carb . . you can adjust the secondarys to open very early if you want . . just buy a holley secondary spring kit and install the whte spring.

 

quick fuel ss680vs carb is good.

[1969_Mach1 333]

barnett468 you always recommend QFT.  Are you getting paid to promote QFT, haha?  Don't get me wrong, I use their metering blocks and their carbs have nice features.  I wish they would make their own parts, i.e. main bodies, bowls, and metering blocks here in the USA and not just assemble the carbs here.

 

I've never used a QFT carb.  Are they setup for Ford kickdown linkage?

 

I agree with the Ultra Double Pumper's.  I've had only one apart and I noticed large idle feed restrictors and small low and high speed air bleeds.  For some reason they seem calibrated to run richer than their classic series counterparts. Like they are setup to run on more aggressive engines than the classic series.

 

If the OP had carb experience I'd suggest finding an old 3310 or 3310-1, rebuild it and install a primary throttle shaft that is setup for Ford kickdown linkage.

[barnett468 418]

lol, no its just that qf carbs are not only better than a holley, they are cheaper so ts a no brainer.

 

yes, the ultra carbs are rich and many people remove the metering blocks and put on the old ones or tune the new blocks.

[po51 21]

I have a 2500 stall converter for the 408 with the FMX with 3.25 Trac-Loc rear

[po51 21]

 

OK thanks for all your input, I'm now looking at the below carb, what do you guys think?

 

 

 

Quick Fuel SL-750-VSF - Quick Fuel Slayer Series Carburetors

 

[barnett468 418]

ok, the high stall converter helps a little . . if you ran a mechanical secondary i would run a 680 to 725 with progressive linkage and imo, annular boosters would be the best . . unfortunately, there are no annular booster 680 or 725 cfm carbs, and the 750 annulars start getting a little expensive.

 

with the vacuum secondary, you can set the secondaries so they open at around 1800 rpm which will make it more similar to a mechanical secondary carb, or you can set them so they BEGIN opening at maybe 5000 rpm, which would make it like a two barrel 325 cfm carb until 5000 rpm . . this is one of the great things about vacuum secondary carbs.

 

if you were going to race it, you should use a mechanical secondary carb, but you should also use numerically higher gears which will also help keep the engine from falling o its face when you floor it from a dead stop.

[po51 21]

What do you guys know about this carb? How do you think it would work on my set up?

Quick Fuel SS-Series Carburetor 735cfm VS