Spread-Bore to Square-Bore

[woodyjim]

Hey guys. I've got an '84 Silverado that came with a 305 automatic with q-jet carb. I put in a 290 hp 350 crate motor and updated to and Edelbrock spread-bore manifold with the stock carb. The q-jet has been very erratic on both performance and mileage. I'd like to change to a 600 cfm square-bore Performer carb. Do I have to use an adapter plate? I'd appreciate any help.

[cjlinkster]

It'll depend on that specific edlebrock manifold. Some of them could accomdate both carb styles.
Posted via Mobile Device

If the bolt holes match, then you should be good to go. Otherwise you would need the adapter.

[woodyjim]

I'm at work and can't remember the model # on the intake, but it's the performer series. Does it have more to do with the shape of the carb. base plate or the way the ports line up?

[frog210]

i had a performer 2101 with the old q jet on a 305
swapped in a 650 cfm electric choke elderbrock
it fit perfectly direct bolt basically
i was also on that boat about the spread bore to square bore or square bore to spread bore
sorry i forgot lol
fit fine and works well

[THE "10" SILVERADO]

woody,

Depends, the most common Edelbrock performer is the 2101 which is a spread bore so if that's what you have you *need* an adapter for the sqaure bore performer otherwise you will get vacuum leaks out the wazoo.

When I upgraded to an Edelbrock intake and carb I got the 2701 Performer EPS which was a specific square bore intake. I didn't need an adapter because it was made for square bore carbs.

More than likely you will need an adapter plate because most Edelbrock performers are going to be spread bore. If you do need an adapter remember to put gaskets on top and bottom, otherwise you will have big vacuum leaks.

[INSIDIOUS '86]

I would just get a new intake rather than One of those power robbing spacers
Craigslist will take care of you
Posted via Mobile Device

[THE "10" SILVERADO]

Originally Posted by INSIDIOUS '86

I would just get a new intake rather than One of those power robbing spacers
Craigslist will take care of you
Posted via Mobile Device

Yep, picked up my brand new performer eps intake on craigslist. A lot of people stock up on parts that they end up not using and sell them for cheapo on CL.

[88Jimmy4x4]

Originally Posted by INSIDIOUS '86

I would just get a new intake rather than One of those power robbing spacers
Craigslist will take care of you
Posted via Mobile Device

how does the spacer rob you of power?

[motornut]

or use them and then upgrade and sell them....
mine used was -$50

[woodyjim]

Thanks for all the help guys. I'm a little curious about the spacer causing a power loss too. I sure don't need that. I'll sure give Craig's List a look though... don't know why I haven't already (duhh). I'll get my numbers together and see where I land. My parts dealer is a pretty big Chevy guy. He might even have some stuff laying around... Thanks again!

[Firebirdjones]

Hang on and I'll show the proper way to adapt the square bore carb to a spread bore intake without using those spacers......

[Firebirdjones]

Okay, first I'll start with a pic of this factory spread bore intake to get this out of the way first. There is no way to install a square bore carb on here unless you use an adapter,,,simply because the square bore bolt pattern isn't on this intake....make sense?



Now that we have that out of the way lets move onto the aftermarket intakes that have both bolt patterns such as the intake Motornut posted above. These will accept either carb "WITHOUT" the adapter but there is a certain way to do it. The reason so many people have vacuum leak problems when installing a square bore carb is because they slap a gasket on with the carb and go,,,,doesn't work that way

You'll need 2 square bore gaskets and a center plate. This used to be sold as a kit by Mr. Gasket and Holley just for this purpose for less than $10. Haven't seen them in a while as parts stores don't seem to carry anything usefull anymore (might try summit or jegs) but here is how you do it....

First picture is a square bore gasket, you can see the imprint of the spread bore dual flange intake underneath this gasket....



This obviously goes on the intake. Then you install the square bore plate. This piece is important, this is what stops the vacuum leaks that everyone has trouble with. It helps to apply pressure around the gasket for a tight seal, and covers the spread bore design of the intake completely..as you can tell in the first pic with the impression of the intake on that gasket,,,it has a perfect no leak seal.



Here it is sitting on the first gasket (just imagine a spread bore intake underneath



Then you set your next square bore gasket on top of the plate. Your spread bore dual flange intake is now ready to accept a square bore carb with no vacuum leaks. If you look closely you'll see the outlines on this gasket as well,,,,that's the bottom of a holley carb imprinted on there.



That is the proper way to install a square bore carb on a dual flange intake like Motornut has pictured above....

[1774btcrew]

Nice detailed pictures and description! I am sure this will help lots of people!

[Firebirdjones]

I hope that helps....there is something to make note of here however that I forgot to mention.

We all know how wide those spread bore intake mounting flanges are, and how narrow a square bore mounting flange is...this is where vacuum leaks occur hence the reason for this plate kit I have pictured above.

Notice the first gasket that went directly on the intake. See how wide the gasket is? This is to cover the entire spread bore flange on the intake. This is why the plate is needed. Simply throwing a carb on that gasket will not seal the sides because a square bore flange carb is not wide enough. This is the job of the plate and why it's needed.

Then look at the next picture with a standard square bore flange gasket. See how narrow the sides are compared to the plate?? The plate is wide on the sides to seal that bottom gasket on the intake. The gasket on top is only as wide as the carb is.

This may help to comprehend why people have vacuum leaks with a simple gasket and carb setup, just won't work well enough.

[INSIDIOUS '86]

Originally Posted by 88Jimmy4x4

how does the spacer rob you of power?

Poor design and fit. Causes Airflow interruption to close to the throttle blades
Posted via Mobile Device

[tucsonjwt]

Excellent tutorial.
I have used aftermarket square bore Edelbrocks (and Carters) on my old Chevy trucks and was given a big fat (.75"?) aluminum adapter for my stock spreadbore intake. That raised the carburetor a lot and screwed up the throttle linkage and air cleaner attachments. The thin adapter you show is the cat's meow.
In my experience, both the Carters and Edelbrocks had their quality problems and were not better, and probably worse than, a properly rebuilt Qjet. I remember the choke valve on one of the Carters would jam up and stall the engine. So, I filed off the valve, since it was oversized from the factory, and the engine would not stall, but a filed off too much and I could not choke the engine when cold.
I had a local carburetor shop rebuild the current stock Qjet I have and it seems to run better than any of the carburetors I have had so far. If it needs work again I will send it out to one of the Qjet specialty shops and see if that is better.
So, I think just staying with stock components in good condition works best for me. But each to his or her own.
I have always had 3/4 pickups, so I don't think you can get better than 10-11 mpg in the city (where I drive mostly). Since I drive a 454 I am just satisfied if it starts and goes ok, since I know I will suck a lot of gas anyway.

[5bassman]

I just picked up a brand new in box Edelbrock Preformer 2701 on EBay for $90 shipped. Also got a nice Edelbrock 1406 carb for $75 off Craigslist. The deals are out there, just keep an eye out.
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[Firebirdjones]

Originally Posted by tucsonjwt

Excellent tutorial.
I have used aftermarket square bore Edelbrocks (and Carters) on my old Chevy trucks and was given a big fat (.75"?) aluminum adapter for my stock spreadbore intake. That raised the carburetor a lot and screwed up the throttle linkage and air cleaner attachments. The thin adapter you show is the cat's meow.
In my experience, both the Carters and Edelbrocks had their quality problems and were not better, and probably worse than, a properly rebuilt Qjet. I remember the choke valve on one of the Carters would jam up and stall the engine. So, I filed off the valve, since it was oversized from the factory, and the engine would not stall, but a filed off too much and I could not choke the engine when cold.
I had a local carburetor shop rebuild the current stock Qjet I have and it seems to run better than any of the carburetors I have had so far. If it needs work again I will send it out to one of the Qjet specialty shops and see if that is better.
So, I think just staying with stock components in good condition works best for me. But each to his or her own.
I have always had 3/4 pickups, so I don't think you can get better than 10-11 mpg in the city (where I drive mostly). Since I drive a 454 I am just satisfied if it starts and goes ok, since I know I will suck a lot of gas anyway.

Quadrajets are excellent carbs, they are just intimidating to some people. Once you learn the in's and out's of them they can be tuned to perfection. I've tweaked on them for years and they do take a few specialty drill bits if you want to access things like idle feed restrictors (they are in the very bottom of the main body and a standard bit doesn't reach).
But with the right combo of power piston springs, rods and hangers and few other tricks they are excellent. I've been racing my bird with one for almost 20 years. I kept the original Quadrajet from the 454 on my 502 swap in my 79 pickup and simply tweaked it. They return good fuel economy when dialed in, and still provide great performance too.

[raceman6135]

Originally Posted by Firebirdjones

Edelbrock part #2732 is available from Summit Racing for $6.95

http://www.summitracing.com/parts/EDL-2732/

[THE "10" SILVERADO]

Originally Posted by 5bassman

I just picked up a brand new in box Edelbrock Preformer 2701 on EBay for $90 shipped. Also got a nice Edelbrock 1406 carb for $75 off Craigslist. The deals are out there, just keep an eye out.
Posted via Mobile Device

Check your floats and clean that Edelbrock out first before you install. I got my 1406 of CL and after I installed it, it leaked fuel ALL over my engine because dirt had got in an clogged just about everything. Leaked after I stopped the engine too. Even posting on here for help people said that it could honestly blow up in my face and to take if off right then.

Just check it out first.

[88Jimmy4x4]

Originally Posted by Firebirdjones

Okay, first I'll start with a pic of this factory spread bore intake to get this out of the way first. There is no way to install a square bore carb on here unless you use an adapter,,,simply because the square bore bolt pattern isn't on this intake....make sense?

what if you have a square bore edlebrock intake and want to use your original spread bore qjet carb?

[woodyjim]

Wow! You guys have sure given me a lot to think about! We just don't have that many shops around here that could do a quality rebuild. I'm going to look into it though... Firebird, thanks for all the info... very helpful! I'll let everyone know how it turns out. First things first... MONEY!!! Thanks again!

[Firebirdjones]

Originally Posted by 88Jimmy4x4

what if you have a square bore edlebrock intake and want to use your original spread bore qjet carb?

That's going the other direction That's a more difficult transition. I've never done this but I'd bet there is an adapter out there for it.

[88Jimmy4x4]

I have the adaptor already, just wondering if you had any tips

I chose this intake because I wanted the old type with oil filler neck, however my plan is to use a new jet qjet carb and run stock air cleaner.