Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Throttle blades open to far

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Garrett71monte:
So I just got running my 396 with 11-1 compression with a 240@50 solid flat tappet ground on a 108 centerline.  The problem im having is i can't get my throttle blades closed enough to get off of the primary  circuit  and stay in the idle circuit. My bypass air is drilled out to. 125 on each side.  I have no control of my mixture screws. Wondering if I should be looking at drilling my throttle blades.  Any help could be appreciated

Frank400:
what is the size of the idle components ? idle tube, down channel restriction, idle air bleeds ?  what is the carb number ?


  what is the timing at idle ?

Kenth:
BBC Quadrajets of the performance era has .138" idle bypass air from factory, so .125" is somewhat small.
I would start with .138" IBA, then look into the idle tubes and idle channel restrictions sizes.

Cliff Ruggles:
Before adding any more bypass air remove a small manifold vacuum hose and see if it helps.

More times than not there simply isn't enough idle fuel available to the mixture screws.

Not saying here that you may not need more bypass air, but it will only make things worse if you don't have enough idle fuel right to start with.

More specifics would also help:

What is the carburetor part number and exactly what has been done to it?

The details from the cam card would also help.

A 240 @ .050" flat solid cam really isn't that big, but if it's ground on a 108 LSA it will have enough overlap that idle quality will be degraded considerably in a the 396 engine........Cliff

77cruiser:
I ran a similar 240/108 solid cam in a 385 SBC. It had around 10" vac. I think the IAB was about .140.
Make sure the IAB is open from top to bottom & not blocked by a wrong gasket.
Idle tubes were .039, down channel .055 & mixture holes were .093.

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