[dhcarguy]

Is anyone familiar with this carburetor specifically the part in the pics. It is called the Auxiliary throttle valve assy. I do not know what car or(boat) or year it is from.

I have seen a few Rochester 4GC's without that part in them. Was wondering if it is needed and what does it do?

Thanks, Dave

[steve25]

Please post the picture!

[dhcarguy]

Originally Posted by dhcarguy

Is anyone familiar with this carburetor specifically the part in the pics. It is called the Auxiliary throttle valve assy. I do not know what car or(boat) or year it is from.

I have seen a few Rochester 4GC's without that part in them. Was wondering if it is needed and what does it do?

Thanks, Dave

oops

[steve25]

Ok I dig out my Rochester book and that air valve was part of every 4G carb, as it’s what makes them a the vacuum secondary precursor to the Q-jet.
I guess if you run the carb at a high enough rpm all the time to be well into the secondary’s, or on a big enough cid motor you can do with it being in there.

One other thing I will say is that by the looks of it if you leave it out the carb may or may not flow more air.

And yes there is a air flap tension adjustment on that whole deal .
I would like to see a A/B flow test done to determine such if it where me!

[steve25]

Another interesting thing about these carbs I never new is that the primary float is vacuum assisted!

[Kenth]

It is needed for the application, and works like the airvalves on a Q-jet smoothing out the rate of incoming air at WOT to avoid the dreaded BOG.
A few early units with small secondaries, mostly 1952-56 Cadillac, did not use these airvalves.

[JohnnyAction]

Hello Dave, The purpose of that valve is to regulate air flow through the secondaries when the throttle plates are opened. They prevent a "Bog" when the secondary plates are opened. You can adjust the spring pressure on this valve to tailor it to the air flow required. If you look at a Rochester Quadrajet carburetor, they have similar plates in the secondaries to control air flow. John

[dhcarguy]

Thanks EV1, I should have posted a picture of the throttle body also.

[Kenth]

Chevy unit, and wrong (too large) throttle body gasket.

[1968GTO421]

Originally Posted by JohnnyAction

Hello Dave, The purpose of that valve is to regulate air flow through the secondaries when the throttle plates are opened. They prevent a "Bog" when the secondary plates are opened. You can adjust the spring pressure on this valve to tailor it to the air flow required. If you look at a Rochester Quadrajet carburetor, they have similar plates in the secondaries to control air flow. John

Absolutey Johnny. I've got one somewhere in my grarage that I had on a '60' that I raced in the early/mid '60's. We left the airvalve in as we knew about low speed bogs on big cars. Still with a little tuning it ran well with an Isky cam. As a sidenote, mine says "Quadrajet'" on the side very clearly. Id' go find it in the garage but just had surgery and am "housebound". When I recover, I'll retrieve it and add it to this thread or start a new one about these carbs. They were great 1/8 mile carbs. I used a Stewart Warner electric pusher pump on my carb.

[tom s]

the secondary air flaps on this carb like a factory AFB have no adjustment EXCEPT for adding some weight or taking off some weight to allow them to tip in.The engineers designed them to work with the intended application of the carb.Tom

[steve25]

So this info in the Doug Roe Rochester carb book is wrong?

[shaker455]

Originally Posted by dhcarguy

Is anyone familiar with this carburetor specifically the part in the pics. It is called the Auxiliary throttle valve assy. I do not know what car or(boat) or year it is from.

I have seen a few Rochester 4GC's without that part in them. Was wondering if it is needed and what does it do?

Thanks, Dave

That flap valve is part of the carb and belongs in place.
I work these units on a regular basis as well as live test.
They are great carbs that work well for small early 4-barrel units.
Neat to see how Rochester Products evolved as the good features were integrated into the next design.

[tom s]

It looks like the book was right.I have run the 4GCs on 4 cyc tempests years ago but NEVER pull the air valves to compare it to the AFBs.Shaker could speak if he had ever adjusted that spring.Nice catch Steve.Tom

[shaker455]

I leave the tension where they are set.
It is a thin wire with very light tension and I have been satisfied with it's operation.
Especially when they are set for the stock application they were intended for.
Doug Roe was a sharp guy & engineer but I have not seen these carbs used in anything other than a stock application.
I recently did one for a 57' Caddy and in fall a 57' Star Chief with 347.
Owner took me for a ride in his Cream puff and all he did was burn the tires everywhere!
They are very sensitive to float height and will stumble if not set right.
The pontoon style float take time and patience to get set correctly
The base to main body gaskets have to be the right ones as there are about ten different styles

[carbking]

The first 4GC carbs did not have the auxiliary air valve. Without checking, memory says they were introduced on some models in 1956.

The spring, when fatigued, may be replaced (same spring as the much later 4M carbs); and I doubt you will find this in any reference..................the initial setting was 1/2 turn of tension past touch-closed.

The 1956 and newer versions were really great carbs, if one didn't need more than about 700 CFM. Very reliable.

Shaker - a LONG time ago, we bought a close-out on the large marine versions. In the early 1970's, street racing was still very active, and lots of the Chevy dudes put 327's, 350's, whatever, in their tri-5 Chevvies and doctored them to look like 283's. We sold quite a few on the large 4GC's to the "cheaters". "It's just a 283, look at the Rochester carb; the 327's used Carter AFB's".

Jon.

[dhcarguy]

Boy ,am I learning a lot. When I reluctantly agreed to my buddy to rebuild his carb. , ( he knows I've rebuilt my tri-power carbs a few times over the years), I had no Idea what I was in for.
He knows it may not be perfect when I'm done, But I like to mess with stuff(lol).

Kenth,
The gasket in the picture was the one on the car when I took the thottle body off. I can't figure out why it is not right. If it is the wrong one, could that be the reason the car ran so bad. ?

Thanks again EV1, anymore info is appreciated.

[Kenth]

Chevy 283/327 used two different sized bore 4GC´s.
Someone mounted the large bore gasket on your small bore throttle plate.
To have a properly function carb it is a good practise to use the correct gaskets.

What are the cast numbers on nozzles and what size jets in your carb?
Is there 4 digits stamped on rear drivers side corner of air horn?
This would help to ID your carb.

[shaker455]

Originally Posted by dhcarguy

Boy ,am I learning a lot. When I reluctantly agreed to my buddy to rebuild his carb. , ( he knows I've rebuilt my tri-power carbs a few times over the years), I had no Idea what I was in for.
He knows it may not be perfect when I'm done, But I like to mess with stuff(lol).

Kenth,
The gasket in the picture was the one on the car when I took the thottle body off. I can't figure out why it is not right. If it is the wrong one, could that be the reason the car ran so bad. ?

Thanks again EV1, anymore info is appreciated.

It's just 2gc X2...lol except the floats!

[dhcarguy]

I did not pay attention when I took apart the fuel filter housing. What direction does the bronze filter go. Cone towards the carb? or Flat end towards the carb?

thanks again, Dave

Bet you all will be glad when I am done with this. (LOL)