Technical Rochester 2GC carburetor question

Hi group, I have a 1965 Pontiac 326, bone stock, with a Rochester 2gc carburetor. The engine is freshly rebuilt and I rebuilt the carburetor myself. The engine runs like a top except for one problem. When the engine is running the throttle body gets very tight. So tight that my strongest spring combination for the throttle return is barely enough to return it. Or if it is it's too tight to press with your foot. This kind of throttle body assembly has no return spring built-in, but that shouldn't matter with the external throttle return spring set up.

The thing about this is that when the engine is off, the throttle is very loose and easy to move. I took the carburetor off a little while ago and looked over the whole throttle body set up. There is no binding that I can see at all and it works perfectly easy when the engine is off.

 

Did you remove the throttle blade when you rebuilt it? You have to work with them a tad to make sure that they are perfectly centered and are not making contact with the sides of the throttle bore or not so close to one spot as if they are an RCH off they may hang up.

 

Just to be sure it's the carburetor, unhook the linkage and see what it feels like. It would have to be a pretty good vacuum to be holding that tight, does heat have anything to do with it?

 

That gasket looks thin, maybe a spacer to help rid some heat.

 

Thank you guys, there is no contact with the butterflies to the throttle body. I actually could not make it bind by forcing the throttle one way or the other.
I dont believe heat to be involved since it will do this cold.
I also checked for the accelerator pump and linkage to be binding but it is not.
I know it sounds unbelievable but with the engine running, it's like s parking brake is being set on the throttle body shaft.

 

disconnect the kickdown and try it.

 

No change with transmission kickdown removed, in fact no change with the whole throttle linkage removed.

 

Try moving dash pot to the side so only idle speed screw controls idle rpm.

 

Is there anything about the linkage for the choke or the fast idle cam that can be installed wrong, or a choke lever that can be installed 180° out of phase?

 

At the Carb? or at the pedal? with the whole throttle linkage removed

When it binds next time, shut it down and quickly undo the mounting bolts a bit [the mounting surfaces of the carb or manifold could be warped and heat expands it more than gasket "crush"]

A simple replacement base from another carb could fix it. There are plenty of 2g bases out there from people that have done a tri-power conversions.

 

Did you remove the plates and shaft when you did your rebuild?

 

If it’s not a heat related issue, then is the engine twisting and binding on something when running?
What does that slotted thing in foreground do ? (2nd pic)

 

Difficult to tell from the picture, but I believe Clay (post 9) has the answer.

Jon.

 

The OP only stated that he didn't see any interference between the throttle plates and the throttle bores. But I'd also like to know if he did remove the throttle plates from the shaft during his rebuild.

 

That is the transmission throttle valve/kickdown cable.

 

Clem’s idea seems pretty logical. It only does it with the motor running. It’s got to be a bind from engine movement.
(OR)Could the dash pot be holding it open when running.

 

Thank you for your input.
I did not disassemble the butterflies from the throttle shaft when I rebuilt it.
Last night after thinking about your comments, I went out and took a long narrow flat head screwdriver (engine not running) and inserted it down through the throttle bore and put a little downward pressure on the center line of the throttle shaft simulating manifold vacuum conditions.
What I found was as soon as I applied that downward pressure on the shaft, it tightened right up just like it does when it's running.
There's one more fact I forgot to mention. When I got this carburetor the throttle bore was seized up. I took the throttle body and immersed it in automatic transmission fluid for about a week and a half.
When I was able to break it free, and had good movement so I assumed it was fine.
Now I'm wondering if rust or corrosion in the throttle shaft bores could be causing friction when vacuum is applied from the motor, instead of a smoothe turning.
What do you think?

 

I was thinking vacuum too, but thought maybe the hot air choke was at fault.
With choke rod disconnected does the throttle still freeze up?

 

Does the throttle bind if a light lube is sprayed at shafts while running?

 

All gaskets in and under carburetor exactly the same as before?
Typical kits come with several variations.

 

If the butterflies were frozen shut I would have removed the plates and shaft…..my experience is I’ve never been able to flush clean them. Personally I like the Rochesters because they have a cast iron plate and steel shafts. None have ever been worn enough for me to have bushed even after being frozen. Remember to peen the butterfly screws if you remove them when putt in them back in. Also leave them slightly loose, close them and tighten the screws for best alignment. Mark them for side and top/bottom..good luck.

 

Thank you Jimmy, I'm going to disassemble it right now. So if the shaft is polishable and the bore in the throttle body is still okay, would it be advisable to put a little copper anti seize when reassembling?

 

Maybe before disassembly of the throttle body, you might check the adjustment and operation of the choke and fast idle circuits.

Cannot tell conclusively from the picture, but it just doesn't look right; like maybe fast idle cam is 180 degrees out???

EDIT: When the choke butterfly is completely closed, the fast idle screw should rest on the highest step of the fast idle cam. When the choke butterfly is completely open, the fast idle screw should not contact the fast idle cam.

Jon.

 

I would think that with the engine running, under most conditions the manifold vacuum applied to the dashpot would pull the plunger away from the throttle arm. Which should allow the engine to return to curb idle and not apply any force against the throttle lever.

But it sure wouldn't hurt to check it for proper operation.

 

Well,
I've got rust.

 

Scotch bright pads work great. I buy cheap ones at the dollar store. Blow everything out good. Air in a can for computer key boards works good with a nozzle if you don’t have an air compressor. Brake Cleaner too.

 

Will do. I'm going to clean all this up.

 

I don't believe the dashpot is normally connected to manifold vacuum. Isn't it a self-contained device that acts as a "shock absorber" and controls the engine's return to idle when the accelerator is suddenly released? It keeps the butterflies from slamming shut and stalling the engine..

 

Hey fellas, I just wanted check in to put a period at the end of this thread. I got my issue solved.
I cleaned the throttle shaft with a 3m scotchbrite wheel and chucked up a bore cleaner into my drill and cleaned the throttle body housing and reassembled.
It works like new.
The symptoms were exactly like I described so I hope this thread helps someone else.
Your comments and advice made the difference.
Thank you.

 

Thanks for coming back to explain your findings.