Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
throttle plate v.tubes filled
F250 Restorer:
One more question: Why does this carb only have a idle bypass hole in one of the primary bores? yet have smaller bypass holes in both bores?
Kenth:
--- Quote from: F250 Restorer on September 13, 2020, 12:18:28 PM ---What is the horizontal slit in ONE of the baseplate's primary bores? Below:
--- End quote ---
The slit is the vacuum source for the EGR-valve.
Kenth:
--- Quote from: F250 Restorer on September 14, 2020, 07:04:51 AM ---One more question: Why does this carb only have a idle bypass hole in one of the primary bores? yet have smaller bypass holes in both bores?
--- End quote ---
The smaller holes are the vacuum source for the EVAP system (fuel tank ventilation).
I do not see any holes for idle bypass air in your picture.
F250 Restorer:
Thanks, you're right. The one bypass hole can't be seen in that photo. In this new photo below, the single hole is visible.
Thanks. I don't have the GM tank ventilation system, nor an egr, and having bypass in only one primary bore is puzzling. I'm thinking of using a rebuilt baseplate from the same year that has none of that. That way, if I need bypass air, wouldn't it be a simple matter to drill a hole in either primary butterfly?
Kenth:
That hole is added by someone to have full manifold vacuum in the port on float bowl usually having a ported vacuum source in the bore your plate lacks.
This hole does not lead to the idle bypass cavity in the float bowl.
You may still add idle bypass air by drilling the plate in the right locations.
Put on a gasket and you will see where to drill.
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