throttle plate v.tubes filled

[F250 Restorer]

I bought a 17059247 on ebay. To my surprise, I found some of the v.tubes in the base plate have been filled with what I believe is plaster of paris.

I want to rebuild the entire carb, but first I need to determine if I can use the base plate. If that is rebuildable, I will go further. Does anyone know how to clean out the tubes? Is it possible? Is it worth the trouble?

[Cliff Ruggles]

Tubes in the baseplate?  Can you put up some pics?

[F250 Restorer]

Yeah, the v. tubes in the baseplate. I just took a drill and drilled into the white filling and it all fell out, in all the tubes.


The mix screws were still factory sealed so I though it good baseplate. Then I found that it was cracked at the throttle shaft, and the shaft had already been bushed.

Are you sure I can't use a baseplate from another carb? another 1705(9)

[Cliff Ruggles]

That small crack will not hurt anything.  You can drill and set a bushing deeper into the hole and it will be fine.  It's been dropped pretty hard at some point so the shaft will also need straightened after a new bushing is set deeper......

[F250 Restorer]

So...drill out the existing bushing and install the new one deeper so that it is further recessed with more of the 'meat' around it. OK.

What is the horizontal slit in ONE of the baseplate's primary bores? Below:

[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]

What is the horizontal slit in ONE of the baseplate's primary bores? Below:

The slit is the vacuum source for the EGR-valve.

[Kenth]

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?

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.

[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