Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

throttle plate v.tubes filled

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F250 Restorer:
Thanks. I think at this point the carb may be trash. When someone has drilled one, chances are they have drilled others, or enlarged some, and I don't know what the specs are for each hole. So, rather than spending hours rebuilding a carb that may never be tuneable, I'm going to throw in the towel. What a waste.

Cliff Ruggles:
That hole wasn't added by anyone the later Buick carburetors used it to supply manifold vacuum instead of ported vacuum to the fitting just to the right of the fuel filter housing in the main casting.  It just happens to come out in the same passage where idle bypass air could have been used.

Do NOT drill the throttle plates to add bypass air.  The bypass air system is excellent and a very precisely controlled vacuum leak to effectively lower the throttle angle if/as needed.......Cliff

Kenth:
I see the throttle plate as the assembly where axle shafts sit, and the part that in some cases needs to be drilled for idle bypass air..
The throttle valves are often referred to as throttle blades.
1969 RAIV -70 Pontiac Ram Air carbs has .118" holes in each throttle blade and .073" idle bypass air in the throttle plate.
Also, many 1969-74 Oldsmobile Quadrajets have holes drilled in the throttle blades added to the idle bypass air in plate.
And, if your carb has a hot air choke it will have some .092" bypass air thru the air path in it.

I have found 1967 Qjets drill in that manner for full manifold vacuum, later units usually uses a slit under the throttle blades for the same function.
Looks like Buick went the old style vacuum source for 1979 units.
Learn something everyday!

F250 Restorer:
Thanks Kenth and Cliff. What I think Cliff is saying is: Run the carb and determine whether bypass air is needed. That is what I will do. Thanks.

Cliff Ruggles:
Yep, far as I know the later Buick carbs are the only ones to use a drilled round hole in that area to supply manifold vacuum to the fitting on the front of the main casting.  All the others will use a source either above (ported) or below (manifold) the throttle plates for vacuum signal.

As far as adding bypass air it is best to add it using the installed idle bypass system if your carburetor has it.  Some early units didn't have it so it's just easier to drill the throttle plates instead. 

Bypass air is actually a very nice feature and makes getting a Q-jet to idle with a relatively "big" cam much easier.  Adding bypass air lowers the throttle angle doing two things, gets the main system off-line and it covers up more transfer slot.  It's important to get all the idle fuel coming thru the mixture screw holes and what amount of transfer slot is left.  If too much transfer slot is exposed we can still have little if any control with the mixture screws with some set-ups. 

So for the most part LESS throttle angle is better than MORE.

I will add that bypass air is NOT a cure for not enough fuel to the mixture screws in the first place, but part of the "recipe" for success with that sort of thing.......Cliff

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