Vacuum Secondaries

F-250 Restorer

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sandboxer":2c48c2u0 said:
pmuller9":2c48c2u0 said:
sandboxer":2c48c2u0 said:
Anyone have experience with a 450 double pumper? I need to decide on my induction soon, so this thread is of the highest importance at this point for me.

10:1
Schneider 142F
300 rods

I have two Carter BBD (285 cfm) carbs that are an option to a 4 barrel mechanical.

The 450 double pumper is a distinct option though.
Thanks
My last build had a 232/232 .050" duration .534" lift cam with a big valve ported head.
I had very good results with a Quick Fuel HR 650 cfm carb, mechanical secondary.
I would recommend a 650 cfm carb for your engine also.
Thanks. I’m reading mixed messages throughout this thread, as everyone has a different setup, but I’m definitely worried about carb overkill.
I spoke with Tom at Stovebolt and he was adamant about using the Holley/Weber progressives (3 of them) for drivability. I hear all the other voices of experience as well, so it’s a lot to digest.
I tend to agree with the notion that 80% VE for the 300 is probably near the top end of things, so I was planning on staying in the 400 cfm range.
I’ll pick up a 650 though and try it. I’m worried about used ones that have been “tuned”....

I would recommend a 500 cfm Edelbrock for bolt on performance and reasonable economy. Get the tuning kit with it with a selection of metering rods and jets. The book is easy to understand, and the jets/rods/springs are easy to change. It is a great little carb.
 

pmuller9

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F-250 Restorer":3rxon1ep said:
The tuning kit with it with a selection of metering rods and jets. The book is easy to understand, and the jets/rods/springs are easy to change. It is a great little carb.
He will be using a big 242 degree solid lifter cam and will need more than a 500 cfm carb.

The Quick Fuel HR 650 is very tunable and includes jets in the power valve circuit which the standard Holley does not.
 

sandboxer

Well-known member
I have a Holley 1968 GT390 4speed 600 cfm vacuum secondary carb that needs a rebuild. I’m also picking up a Holley 650 double pumper 4777c tomorrow that is missing 2 idle screws for cheap...
I’ll see what kind of budget hot-rodding karma I can channel to see this work out.
If not, it’s a new Quick Fuel 650DP.
 
I m running a a quadra-jet in my f1-50 with a manual trans. I have yet to tune it at all because im still doing all the metal work and not really driving it yet. so far I know 3 things (1) I have the carb set up too lean (2) i need to re-configure the throttle linkage as the forward facing primary orientation that is mandated by using a quad on a c-series causes the stock throttle cable to bind. (3) i can idle the truck down to about 10mph in fourth gear and mat the throttle without any rattling/spark knock. this is an otherwise stock engine with a hedman header and an intake and the quad.
 

54-4x4

Well-known member
Nice to see this topic is still active.
The small primaries in a quadrajet carb are a real asset for good drivability to pull hard at low rpm.
 

F-250 Restorer

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54-4x4":2jqsqyse said:
Nice to see this topic is still active.
The small primaries in a quadrajet carb are a real asset for good drivability to pull hard at low rpm.

Many QJ's had a by-pass air system. Rochester and GM found that with the tiny primaries, when they were placed on engines drawing a certain amount of air, the velocity of the incoming air pulled fuel out of the boosters at idle. Remedy? They allowed a small amount of air to by-pass the venturi, and that lessened the pull on the boosters/fuel. I wonder at what point (HP) the by-pass is needed.

Although the QJ secondary butterflies are opened manually with a link from the primary throttle, no fuel is delivered until the air flaps in the top of the carb are pulled open by vacuum. So, I guess it is a hybrid manual/vacuum secondary carb?

One other interesting tidbit I thought I'd throw in: From 75 on, GM put the 800cfm model QJ only, with a larger primary bore, on small engines.
 

54-4x4

Well-known member
I like the Qjet carbs.I have used them in the past and still have a couple in the cupboard.I found the older ones easier to tune with the exposed idle jet screws.They also were more progressive bringing the timing in.
I would like to try an Edelbrock carb or a mechanical only secondaries carb.
I like the Mikuni pumper I put on my bike.It seems more like I'm hooked up to the engine with the throttle.
 

54-4x4

Well-known member
A friend gave me a mehanical secondaries Holley 450 cfm 4360 carb and adaptor plate so I will have a go at tuning and getting the feel driving a carb with direct hook up compared to a Holley 550 cfm vacume secondaries.The price was right.Don't know what it came off but it looks in very good shape.
 

1964f100240

Well-known member
I have had good luck with 650dp 750dp and mechanical fuel injection but i have to say standalone EFI is the way to go I run a 240 12.5 255@.50 .650 lift roller on e85r I can drive it on the street now.
 

old28racer

Famous Member
Will a quadrajet fit a DP intake and work well on a stock 300 with headers & HEI distributor? Or is a summit 600 vac sec a better match?
 

sandboxer

Well-known member
1964f100240":3kyve5fw said:
I have had good luck with 650dp 750dp and mechanical fuel injection but i have to say standalone EFI is the way to go I run a 240 12.5 255@.50 .650 lift roller on e85r I can drive it on the street now.
Not to divert the subject, but can you give us the skinny on the cam and lifters?
Thanks
 

1964f100240

Well-known member
I didnt want to change the subject I guess what i was saying is i have tried every combo carb from 2bbl to split dominators on a 240 300 and the efi is bye far the better way to go. the cam im running in this engine is a crane bbf lifters manton push rods
 

54-4x4

Well-known member
1964f100240":2fgnmveg said:
I didnt want to change the subject I guess what i was saying is i have tried every combo carb from 2bbl to split dominators on a 240 300 and the efi is bye far the better way to go. the cam im running in this engine is a crane bbf lifters manton push rods
Do your injectors locate at each intake port or where the carb normally sits?
 

F-250 Restorer

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old28racer":1qg08tqw said:
Will a quadrajet fit a DP intake and work well on a stock 300 with headers & HEI distributor? Or is a summit 600 vac sec a better match?

To use a QJ on a DP intake you'll need a $10 adapter. I like the Summit 600 (copy of the old Autolite 4100), but using one in the DP configuration left the primary bowl hanging above the header. On hot summer days that produced vapor lock, even with a shield. They also make a 500 cfm version of the Summit carb, but the venturi are the same diameter on both carbs. Summit merely put in a larger booster on the 500 cfm version. The larger booster hinders flow 100 cfm. The jets and air bleeds have been tapered down as well.

The QJ is highly tuneable. However, unlike the Edelbrock where you can make metering rod and booster spring changes without taking the top of the carb off, the QJ requires that you need to pull the top to make the changes. It's a great carb, but it does require patience and persistence. I would stick with the 1976-1980 QJ since it has a nice feature that allows you to adjust the cruise A/F ratio with the turn of a screw. The 76-80 QJ, identified by the 1705 prefix vertical beside the secondary throttle linkage, comes in either 750 or 800 cfm. The difference is all primary venturi size. Look down the throat of the primaries. If you see a bump in the venturi wall in each bore, it is an 800 cfm carb, flowing 210 cfm on the primary side. Good luck.

If you want to use a carb that is bolt on and play, in the correct configuration for a DP intake so it won't hang over the header, I'd use the Edelbrock 500, or a Holley 600, or a 465 Holley.
 
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