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Old 05-17-2006, 08:16 PM   #1
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Spread Bore to Square Bore Carburetor Adapter Plate modifications?

I am getting ready to install a new Edelbrock Performer 750 CFM (electric choke) carburetor on my 1985 K30 Dually with a 454. Since I am using the stock cast iron intake manifold, with a spread-bore bolt pattern for a Q-jet, I am using a spread bore-to-square bore adapter plate, in order to install the Edelbrock Performer carburetor, which has a square-bore bolt pattern.

My question is that when I test fitted the adapter plate to the carburetor, the adapter plate plenum partially blocks both the primary and secondary carburetor venturie pathways. I was thinking of getting my die grinder out, and grind back the adapter plenum area to match the carburetor venturie pathways better. Has anybody done this modification before, and have any advice on it, or would it be a waste of time because this engine is only a stock engine anyway?
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Old 05-17-2006, 08:50 PM   #2
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improved airflow is NEVER a waste of time.
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Old 05-17-2006, 09:25 PM
Posted in reply to beater_k20's post starting "improved airflow is NEVER a waste of..."
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Lightbulb what brand...?

What brand of adapter are you using??..all the ones I've used like "Trans-Dapt" and "Proform" ones already have the spacer milled out to match the Q-jets shape and is just square on the carb side,and though they come close,they dont block off the flow of either set of barrels..but then I've only used 600 CFM carbs,maybe the 750 CFM is different??..(larger butterflies/venturies?)..

One thing I often see is the adapter installed with the wrong side facing the carb!..I bought a good truck cheap once thinking it needed a motor or engine work,and all along it was a vacuum leak from the owner installing the adapter wrong side up!...also many aftermarket manifolds need a flat plate to seal the base of a square bore to them properly..

They make several styles of adapters for square bore to Q-jet manifolds..some have 4 separate holes,and are claimed to increase low end torque..others are open,or have a divider separating the primary and secondary barrels..I tried both on an AFB on my SB 400,and the 4 holer seemed to reduce power in my estimation!..the one with the divider ran MUCH stronger..but every engine is different..I think the slightly taller height of the one with the divider is what made the difference mostly,not the divider..the one with the open center was about the same ...
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95 FORD Countour!..my first ford ever,after 30 years of driving! --a gift from the queen of bluegrass Rhonda Vincent!
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Latest addition--1985 K-10 Suburban,6.2 diesel,700R4,3.73's,12 bolt rear..still awaiting repairs and to be registered..
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Old 05-17-2006, 11:30 PM   #4
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What brand of adapter are you using??..
It is a very cheap Summit Racing brand made in Taiwan ($13), which may be why the fit is not exactly perfect. The spread bore side, which goes to the intake manifold, sizes up just fine, but the square bore side, which goes to the carburetor is the one that is off by about .020 of an inch.

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