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Quadrajet for 253

the_boozer

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Darren_L

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there are some differences but they aren't difficult to change. The 308 Quaddy might run a bit rich on a 253 without some changes
I know the metering rods are a different size
 

Aussie V8

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As far as I am aware there were no factory 253 red motors with a quadrajet so there is no difference at all cause if it doesn't exist it can be different can it.

Blue 253s had the quadrajet and as Darren says it is leaner in the jetting. On the primaries 1 jet and 1 metering rod number from memory, I don't know what secondary rod 253s had but it would also be leaner and leaner for longer as 253s will never, on the street, use a quadrajet to its full potential. There was an article in a magazine a few years ago about jetting a quadrajet and it said what the ideal jet and rod combinations were. From memory it was information that Larry Perkins supplied. I have it somewhere but I don't have a clue where atm.
 

Smitty

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As far as I am aware there were no factory 253 red motors with a quadrajet so there is no difference at all cause if it doesn't exist it can be different can it.

Blue 253s had the quadrajet and as Darren says it is leaner in the jetting. On the primaries 1 jet and 1 metering rod number from memory, I don't know what secondary rod 253s had but it would also be leaner and leaner for longer as 253s will never, on the street, use a quadrajet to its full potential. There was an article in a magazine a few years ago about jetting a quadrajet and it said what the ideal jet and rod combinations were. From memory it was information that Larry Perkins supplied. I have it somewhere but I don't have a clue where atm.

part correct....

the primary jet is one size smaller on the 253 quaddy compared to the 308 (70 v 71)
but BOTH 253 and 308 Quaddies used the same primary needle (49s)

The 253s use a richer (yes richer) secondary needle labelled CX with the 308s using the DP needle
Both 253 and 308 Quafddies use the same secondary air valve flap adjustment. Having said that
if you tinker (as I do) you can get a 253 honking enough to utilize all that Quaddy goodness via
the modded secondary setup... :)

hth
 

ari666

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i read an article once about holleys that were too big for an engine and the method to jet them UP to suit the smaller donk. read it 3 or 4 times like this :confused: but supposedly the lack of venturie vacuum from a less dense volume of air rushing past means the bleeds dont meter enough fuel and the jets need to be INCREASED to get the mixtures right.

is that same thought process for the richer needle?
 

VR38

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just tighten up the secondary butterfly spring real tight so it rarely opens.
Is that suppose to be funny? It's vacuum operated. Why would you want to starve the engine?
 

Guzzoline addict

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i read an article once about holleys that were too big for an engine and the method to jet them UP to suit the smaller donk. read it 3 or 4 times like this :confused: but supposedly the lack of venturie vacuum from a less dense volume of air rushing past means the bleeds dont meter enough fuel and the jets need to be INCREASED to get the mixtures right.

is that same thought process for the richer needle?

Yes same thought process. Stronger vacuum, smaller jets/bigger needles,weaker vacuum bigger jets/smaller needles.
 

codfishface

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Is that suppose to be funny? It's vacuum operated. Why would you want to starve the engine?

Yes, it is vacuum operated, but the spring applies pressure to the secondary air flap (not the secondary throttle plate) and allows you to tune out the dreaded "bog" common to 253s running any carb rated too large for them. The secondary throttle plate will still open when called upon, but the amount of air available to it will be restricted....not starved as such.
 

Aussie V8

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Yes, it is vacuum operated, but the spring applies pressure to the secondary air flap (not the secondary throttle plate) and allows you to tune out the dreaded "bog" common to 253s running any carb rated too large for them. The secondary throttle plate will still open when called upon, but the amount of air available to it will be restricted....not starved as such.
And as such supplying a stronger vacuum so the fuel can be drawn into the venturi.
 
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