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Vacuum Secondaries

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pmuller9
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Vacuum Secondaries

Post #1 by pmuller9 » Fri Jun 07, 2019 10:01 pm

There have been several 300 six engine performance engine builds with large valve, mild ported heads and street performance cams that are producing smooth, wide power bands but feel like they are lacking the low end kick that was expected.

I have not used a vacuum secondary carburetor on a 300 six because it responds well to mechanical secondaries without a hint of bog.
I would like to hear from those that are using Vacuum Secondary and Mechanical Secondary carburetors.

I would also ask if those of you that have Vacuum Secondaries if you wouldn't mind putting a screw and nut in the secondary linkage slot (or some other method) so the secondary is forced open after part throttle to see if the engine response changes.

Thank You in advance for you help with this research.
It will be a help to all with present engine builds and with future plans.

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Re: Vacuum Secondaries

Post #2 by wsa111 » Fri Jun 07, 2019 10:52 pm

Too many variables to answer. A large carb with an automatic trans, a carb with vacuum secondaries would be the best choice. Holley offers a spring kit. Start with the stock spring & if no flat spot try the next lighter spring.
A small 4 barrel might be better off with mechanical secondaries especially with a stick shift.
Of course the A/F on the primaries & the secondaries also influences your options.
For street use i prefer vacuum secondaries, because the carb only supplies what the engine requiries.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
2003 Ford Lightning daily driver. Recurving Distributors. billythedistributorman@live.com
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Re: Vacuum Secondaries

Post #3 by jason832 » Fri Jun 07, 2019 11:35 pm

This is a long post, but I've mostly ran a combination nobody has. Real interesting.

-300, 85 f150, 4 inch lift, 4x4, np435, 3.55 gears, 35inch mud terrains, 2100rpm@62mph
-h519p cheap hyper pistons
-91 octane required (any less it won't shut off)
-estimated 8.8 static compression
-Stock rods, bolts, mains, crank all that stuff
-Stock 84 head (with screw in studs, Clifford springs, stamped steel rockers, milled 0.060)
-Clifford shorty headers, true dual exhaust with magnalow mufflers you can see through
-Clifford intake
Comp 268 cam installed with comp gears dot to dot
-Street demon 625 cfm
-Stock duraspark 2 ignition

The truck is daily driven here in Canada and does a bit of everything. Towing 7500lbs, bogging through ponds, heavy summer traffic, 20hr drives even plowing over 4ft snowbanks. I have a wideband AFR meter to tune it, keep that in mind. Its with me everyday and my only ride to town regardless of temperature outside from 85*F to -40*F.

The carb is a 625 cfm street demon carb with a tuning kit (78-82 primary jets, 7 or so metering rod sets, 5 springs, all for the primaries). The primary side of the carb is tiny, about 1/3 of the total cfm and uses triple stack boosters giving it incredible atomization velocity and response. The mechanical secondaries are huge and tuned with just jets. Now to the tuning....

With the smallest 78 main jets, the second leanest of about 7 metering rods and second leanest of 5 springs on the primary side. The secondaries are way lean about 8 jest smaller than out of the box.
It cruises at 16.5:1
Very light acceleration about 14:1
Fully open~200cfm primaries 12.5:1
Full throttle above 2000rpm 12.5:1

Now for drivability. I have trouble with the electric choke not closing fully in Canada's January cold (linkage jamming or the spring not moving), but with the choke fully closed it starts instantly (you know its cold when the oil pressure doesn't come up to full for about 20seconds and it starts driving in neutral). Despite the 268 cam, the tiny primaries and triple stack boosters makes throttle response faster than you can smack the pedal and pulls harder than the stock 300 everywhere above 1000rpm. Below 2000rpm the secondaries will drown it but above 2000rpm it twists the truck and pulls hard. When I let off the gas in third at 4500 the truck shifts and the shifters move about 6 inches, it pullllls. I never really measure milage cause I drive it like a lunatic, but I've measured 14L/100km (16.8 USMPG) towing a 2200lb car trailer at 60mph for 4 hours of highway. Not bad for a 4 inch lift and 35s.

Long write up, but I'm very happy with this off setup. I tried a 500cfm summit carb (the primaries sucked and it had several problems). The 625 cfm carb is a bit overkill and I would not recommend it unless you have a wideband AFR meter and patience, but everyone that drives it or comes for a ride is stunned.

Next post will be a picture of the truck (two different paint jobs), link to the build thread and the odd carburetor NOTICE THE SMALL PRIMARIES THEY ARE GREAT ON A DAILY DRIVEN 300. Hopefully I will get my hands on one of gramps SR heads to play with and chassis dyno.
Last edited by jason832 on Fri Jun 07, 2019 11:54 pm, edited 1 time in total.

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Re: Vacuum Secondaries

Post #4 by jason832 » Fri Jun 07, 2019 11:48 pm

https://www.ford-trucks.com/forums/1498 ... heavy.html

Hopefully this helps towards your question Pmuller and interests others. Mechanical secondaries works good with a manual trans for people willing to take the time and set it up to work right.
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Re: Vacuum Secondaries

Post #5 by pmuller9 » Sat Jun 08, 2019 12:02 am

Thanks Jason.
Great info!
For the longest time 300 six builds have been under cammed and now most are being assembled with 268 cams and larger with very good results.
Now I believe we are limiting engine response by under carbureting these engines.
It sounds reasonable to have the secondaries controlled by vacuum but I believe the 300 six responds better with early opening secondaries.
Let the driver control the carburetor's secondaries instead as the above post by Jason demonstrates.

Looking forward to more responses that either support or bust this line of thinking.

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Re: Vacuum Secondaries

Post #6 by Max_Effort » Sat Jun 08, 2019 11:33 am

I like mechanical secondaries. I like 1:1 linkage ratio carbs on a road race, circle track or other engine that is driven aggressively at mid to higher RPM's.

The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

I doubt that more carb / more carb opening is going to add anything. But I've also learned to test everything...

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Re: Vacuum Secondaries

Post #7 by BigBlue94 » Sat Jun 08, 2019 1:10 pm

I'm pretty well a carb newbie. I have a small amount of tuning experience on a quickfuel 450 vacuum secondary carb.

Engine is 309ci with a 9.75:1 SCR. Dynamic at 7.47:1.
Large SI valves in head with bowl work.
Clifford intake and long tube headers
351w 'sportsman N' step dish pistons
Schneider 140H cam.
Manual transmission.

Itll run on 89 octane, but likes 91 better. I run 53 main jets here at 1000' elev. 6.5 PV. Normal AFR at cruise is 16.5:1 The secondaries seem to open at WOT a bit before 3000 rpm.

In low range, I have more power than I know what to do with, even down low at 1500 rpm

I've never driven a mech secondary carb
1985 Bronco. 309ci I6, NP435, 4.56 gears, Detroit locker and tru-trac, 4" lift, and 37" swamper tires. The 309 is 9.75:1 CR with a Schneider 140H cam, 4bbl, roller rockers, larger valves, and headers.

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Re: Vacuum Secondaries

Post #8 by guhfluh » Sat Jun 08, 2019 4:35 pm

I've run both the Holley 600 vacuum secondary and the Edelbrock Performer 500. I tune with a wide band also. I've chosen to stick with the Edelbrock for so long because of its drivability on both the primaries AND secondaries being much better than the Holley 600, but also because I was able to get the secondaries to come in much sooner with the Edelbrock than with the Holley while being able to keep it from stumbling. With the Holley, I couldn't get them to open much at all without a stumble and no way that I know of to be able to tune fuel at the transition when they open. With the Edelbrock, it took extensive modifications too the secondary air door, the secondary emulsion tubes, air bleeeds, etc, etc. and the AFR still isn't right, but the lean stumble is pretty much gone and the secondaries come in at about 2500rpm. I'd really like to have a more adjustable carb, like the Edelbrock AVS2 that I might be able to get tuned even better, as the one I have still isn't perfect.

I have always felt like it could always use the airflow much earlier and can still notice a big power increase whenever the secondaries open AND the fuel ratio is just right. For me, it seems like that rpm should be somewhere between about 2000rpm and 2500rpm. This is why I've really been wanting to try a large throttle body and EFI, where throttle body size doesn't need to be a restriction and doesn't affect fueling.

I also want to mention Canadian 91 octane and US 91 octane fuels differ from what Ive seen tested. US being able to make more power without detonation, preignition, pinging, run-on, etc. I use US 10% ethanol 93 octane. I've run US 100% gasoline 89 octane before and it has wanted to ping, diesel and run-on. I didn't try to tune around it or change plugs to fix it, just went back to 93.
1967 F-250 Crew Cab 2wd, 300 6cyl, T-170/RTS/TOD 4-speed overdrive
240 head, Offy C, EFI exhaust manifolds, Comp 268H, mandrel 2.5-3" exhaust, Edelbrock 500, Pertronix ignitor and coil, recurved dizzy. 200whp/300wtq

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Re: Vacuum Secondaries

Post #9 by THE FRENCHTOWN FLYER » Sat Jun 08, 2019 10:28 pm

Max_Effort wrote:I like mechanical secondaries. I like 1:1 linkage ratio carbs on a road race, circle track or other engine that is driven aggressively at mid to higher RPM's.

The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

I doubt that more carb / more carb opening is going to add anything. But I've also learned to test everything...

x2
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

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Re: Vacuum Secondaries

Post #10 by 54-4x4 » Sun Jun 09, 2019 11:42 am

This is a great topic and information.

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Re: Vacuum Secondaries

Post #11 by Harte3 » Sun Jun 09, 2019 12:26 pm

"Let the driver control the carburetor's secondaries..."

That is the key to running mechanical secondaries. My first carb was a Holley 450 Economizer with mechanical secondaries. It was configured similar to a Quadra-Jet with small primaries and larger secondaries and very lean running. When I stomped on the pedal from low speeds it had a hard stumble/kick-in unless running at 2500 rpm or more. I learned to push the throttle quickly to 2500 rpm and then could floor it from there without getting the stumble. I run a 465 Holley with vacuum secondaries now and even with the quickest spring in the vac module it does not stumble when stomping the pedal.
'83 F150 300, 0.030 over, Offy DP, Holley 4160/1848-1 465 cfm, Comp Cam 260H. P/P head, EFI exhaust manifolds, Walker Y Pipe, Super Cat, Turbo muffler, Recurved DSII, Mallory HyFire 6a, ACCEL Super Stock Coil, Taylor 8mm Wires, EFI plugs.

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Re: Vacuum Secondaries

Post #12 by F-250 Restorer » Sun Jun 09, 2019 5:22 pm

Max_Effort wrote:The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.


That would make sense. The 750 cfm Quadrajet flow 180 cfm on the primary side, and that was used on small and big block engines. The 800 cfm Quadrajet flowed about 210 cfm on the primary side. That would seem to indicate that smaller primaries are sufficient.

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Re: Vacuum Secondaries

Post #13 by SteveP » Sun Jun 09, 2019 7:53 pm

The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

156.25 @ 90% VE

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Re: Vacuum Secondaries

Post #14 by pmuller9 » Sun Jun 09, 2019 8:57 pm

SteveP wrote:The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.
156.25 @ 90% VE


That is only part of the story.
The 300 six has 34% more cylinder volume than the equivalent V8.
While the average volume flow may be the same for a given rpm and VE, the peak flow and "Gulp" volume is much higher for the six.
This is one of the reasons the 300 six works well with larger than expected carburetors.

This is also why I suspect a properly tuned mechanical secondary carb has a better engine response than a vacuum secondary.
The mechanical secondary tuning is also more sensitive to having the proper mechanical advance curves with cams that have higher valve overlap.
This is all in the context of street driven engines.

The info collected here is very good and hopefully will show a trend as the thread continues.

Would any of you with vacuum secondaries be willing to temporarily modify the linkage so the secondary is forced open early by the primary throttle?

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Re: Vacuum Secondaries

Post #15 by sandboxer » Sun Jun 09, 2019 9:44 pm

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

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Re: Vacuum Secondaries

Post #16 by THE FRENCHTOWN FLYER » Sun Jun 09, 2019 10:26 pm

SteveP wrote:The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

156.25 @ 90% VE


I doubt that the 300 has a vol eff of 90% anywhere in the speed range of the engine. Maybe 80% tops. So about 139 cfm?
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

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Re: Vacuum Secondaries

Post #17 by pmuller9 » Sun Jun 09, 2019 10:49 pm

sandboxer wrote: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.

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Re: Vacuum Secondaries

Post #18 by F-250 Restorer » Mon Jun 10, 2019 12:22 am

I have run ten different carbs, from 465 to 800 cfm. All but one, the 465 Holley, were run exclusively with a c6 and a small rv cam. Of those I have to say that the 500 Eddy and the 600 cfm Summit produced the best driveability. The performance provided by the 600 Summit, was in a class of its own. The two Quadrajets I ran both had tiny primary venturi, but with the auto trans I could not tell a difference in throttle response between those and the Summit 600.

Now, however, I have a manual trans and an engine with a cam that has much more duration, and the 465 Holley seems to leave me with a low end lack of zip. I was hoping that my trusty Quadrajet might cure the low end lethargy. Of course in the back of my mind I'm wondering if I need a carb with larger, higher flowing primaries. We'll see.

Pmuller: I'm a few days from putting on the QJ. But once I do, it is a simple matter to adjust the secondaries to open at different rpms. I'd be happy experiment with opening the 'toilet flushers' at varying points. The QJ has an adjustable spring that determines how much vacuum is needed to open them. The secondary butterflies are opened by the throttle linkage, but no fuel is metered out until the air flap begins to open. I'll just have to rig up something to only allow the flaps to open a small bit. I don't want to get the full 600 cfm secondaries popping all the way open and drowning my engine!

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Re: Vacuum Secondaries

Post #19 by sandboxer » Wed Jun 12, 2019 11:04 am

pmuller9 wrote:
sandboxer wrote: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”....

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Re: Vacuum Secondaries

Post #20 by pmuller9 » Wed Jun 12, 2019 11:29 am

Sandboxer
Average flow is not the issue and is a poor way to measurer the flow requirements of the induction system.
The 300 six has 50 cu inch cylinder volumes, equivalent to a 400 ci in. V8.
It is the peak flow, not the average flow that is the more important consideration with a big six.

The other factor is the intake manifold plenum volume.
The less the plenum volume the more the peak flow responsibility is shifted to the carburetor.

A big valve ported head 300 with a big cam still provides a lot of low end torque.
If the carb with less than 600+ cfm is used the upper end of the torque curve falls off early and the power band is shortened with no real gain in low end torque.

Here is another testimony.

[QUOTE=AbandonedBronco;18702899]I've often wondered about this, because in every formula I've ever seen, cylinder size is never a component. Unlike aftermarket stuff, which is almost always geared toward V8s, carb CFM size is used for every engine, with all configurations.Then, as all the pistons are moving up and down, it creates a steady airflow in through the carburetor, not individual pulses through the carb for each cylinder. It's simply pulling in 300 cubic inches of air (minus whatever for vacuum) for every other RPM.That's where the number 3456 comes from in the CFM formula. It's just converting cubic inches to cubic feet, then multiplied by 2, since a 4 stroke only pulls in air every other rotation. 12 * 12 * 12 * 2 = 3456

There's no accounting for number of cylinders or their size.That said, what you say always seems to be true. I've always had better luck with carbs that are bigger than the 300 should be using. My 390cfm had way less performance than the 600cfm I had on there.Most have better luck with 500 cfm Edelbrocks than they do 200 - 300 cfm carbs.Etc.Makes you wonder[/QUOTE]
Last edited by pmuller9 on Wed Jun 12, 2019 12:26 pm, edited 2 times in total.

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Re: Vacuum Secondaries

Post #21 by F-250 Restorer » Wed Jun 12, 2019 11:32 am

sandboxer wrote:
pmuller9 wrote:
sandboxer wrote: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.

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Re: Vacuum Secondaries

Post #22 by pmuller9 » Wed Jun 12, 2019 11:36 am

F-250 Restorer wrote: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.

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Re: Vacuum Secondaries

Post #23 by sandboxer » Wed Jun 12, 2019 1:02 pm

Ok:)
Convinced.
Thanks!

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Re: Vacuum Secondaries

Post #24 by sandboxer » Fri Jun 14, 2019 10:16 pm

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.

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Re: Vacuum Secondaries

Post #25 by jroach » Thu Jun 20, 2019 12:10 pm

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.

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Re: Vacuum Secondaries

Post #26 by 54-4x4 » Thu Jun 20, 2019 4:27 pm

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.

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Re: Vacuum Secondaries

Post #27 by F-250 Restorer » Sun Jun 23, 2019 2:52 pm

54-4x4 wrote: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.

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Re: Vacuum Secondaries

Post #28 by 54-4x4 » Sun Jun 23, 2019 8:12 pm

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.

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Re: Vacuum Secondaries

Post #29 by 54-4x4 » Sun Jul 21, 2019 8:31 pm

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.

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Re: Vacuum Secondaries

Post #30 by 1964f100240 » Sun Jul 21, 2019 9:58 pm

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.

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Re: Vacuum Secondaries

Post #31 by old28racer » Mon Jul 22, 2019 12:45 am

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?
Bad Day Racing Is Better Than A Day In The Stands :mrgreen:

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Re: Vacuum Secondaries

Post #32 by sandboxer » Mon Jul 22, 2019 1:07 am

1964f100240 wrote: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

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Re: Vacuum Secondaries

Post #33 by 1964f100240 » Mon Jul 22, 2019 5:45 pm

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
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Location: Valemount,B.C.

Re: Vacuum Secondaries

Post #34 by 54-4x4 » Mon Jul 22, 2019 11:27 pm

1964f100240 wrote: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?

1964f100240
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Re: Vacuum Secondaries

Post #35 by 1964f100240 » Tue Jul 23, 2019 9:50 pm

In the intake port.

F-250 Restorer
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Re: Vacuum Secondaries

Post #36 by F-250 Restorer » Wed Jul 24, 2019 9:59 pm

old28racer wrote: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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