Anyone ever heard of a vacuum brake? Me neither untill the car would not start properly and had a hesitation on acceleration.

So I looked under the air cleaner and there was this plastic part in two halves at theright front of the carb,

I ordered 1 from Zip, they show the correct part but shipped the wrong one.

Two tries later I got the correct part from Advance.

I have a stock 74 with carb # 7044206 but the correct replacement was listed as for a 70 Carb.

Did Bubba change parts on my carb?

Glad to say it works fine again.

Anyone have a clue? Is my carb a mix of differant years?

 

Sound like your talking about the choke pull off.

 

Hi, yes it's the choke pull off, seems to work fine now but I dom't no why a stock 74 would use a 70 pull off, But thats what I replaced it with and the part works fine andit lets the air valve open in less than 2 seconds so the car accelerates just fine with no bog so I'll leave well enought as is. I never new a lot about carbs so this was new to me.
Thanks for the help. Pete.

 

Hmm. I actually have the same problem. I was told it might be my secondary main assist valve opening late, but I'm a firm believer in ockham's razor. How do I check to see if my choke pull off thingy is working? I've taken off the air cleaner and the two plastic rings look really dirty and feel almost brittle, but I'm not sure what they should look or feel like. I also know the air cleaner has enough room to freely spin around, even with the bolt tight.

 

How it works? It seems to get vacuum as soon as you start the car then when you go to WOT it looses the vacuum and the rod moves out to let the air valve open. Thats all I could figure out, PG

 

The "vacuum break" is what slightly opens the choke as soon as the engine fires. With a cold engine, the choke should completely close when set. If it doesn't, then it's not adjusted correctly. The Chevy Service Manual gives instructions on adjusting both the choke and vacuum break.

 

Thanks 73 shark, I'll check that out, PG.

 

Your welcome. It does also as you say re: secondary air valve in that it keeps it from slamming open as soon as you go full throttle. Sort of analogous to Holley's vacuum secondaries.

 

The choke thermostat (in the manifold on the early carbs) is what close' the choke shut so it will start when the car is cold.And the choke pull off (front of carb.) is what opens the choke slightly as 73shark said so the car wont load up and pour out black smoke till the choke thermostat opens up when the car is fully warmed up.Simple test is to put vaccum to it and see if it moves open without leaking.Or have someone start the car and watch it move. Thats its only job.

 

That is correct but when you go to WOT the choke pull off looses its vacuum and it extends which lets the rod to the secondary throttle plate move and allow them to open, then when you let off of the gas the vacuun is restored and the rod helps pull the secondary throttle plates closed.
Thanks for all the help, PG>