I have a 1971 four barrel rochester on my 1968 Cutlass S. 350 engine. I believe the intake is a 1971 or 1972.
Question 1 - Can anyone tell me purpose of the secondary choke pull off?

Question 2 - I have no primary choke pull off and yet no little notches in my primary choke butterfly. Is the choke plate wrong? Do I need to put notches in it for it to function properly on cold start ups?

Question 3 - I need a idle solenoid because my car has A/C, is the solenoid different for a 1971 or 1972 intake as far as the mounting goes?

Any help is appreciated, thanks in advance........Todd

 

72 350s (and I think 71 is the same), do not use a solenoid on A/C cars. They use a dashpot that serves no purpose that I can see, but is supposed to stop the throttle lever from closing too far if you suddenly jump off the gas.

The choke pull off pulls the choke off the high step once the engine starts. The notches you refer to are on the linkage on the right side. As the choke coil heats up, the choke plate opens further and it drops down a step on the linkage (and engine speed reduces) until it is fully open (choke off) and on the bottom step and at low idle.

 

the "choke pull-off" is properly called the vacuum break. it's primary purpose is to open the choke valve a specified amount, after the engine has started running, when the engine temperature requires enrichment(cold engine).

that vintage of engine and intake only used a primary vacuum break, the secondary vacuum break was added in later years to help reduce emissions on a cold(choked) engine. it sounds to me like somebody put a later carb. on your engine.

if i'm understanding question 2 correctly, the choke cover you have doesn't have notches for setting the choke per factory procedure. you'll have to figure out where to set it, trial and error method.

you may not need an a/c throttle-up solenoid, try it without first. these were more for the anemic motors made in the mid '70's and later.

pix of your carb. would be helpful.


bill

 

Hey Bill I put this carb on my car as it was originally a 2 barrel. It ONLY has a secondary vacuum break as you call it. (20 years as a licensed mechanic in Canada and we called it a choke pull off).for the secondary choke plate. The primary choke plate does not have a vacuum break.

 

There is only one choke plate on a QuadraJet, and it had better have some sort of vacuum break or pull-off, or the car will load up and stall when started cold.

I'm a bit confused about what you're talking about.
Maybe pictures would help to make it clear.

- Eric

 

I believe he has a late 70s chevy q-jet,ive seen these with only a rear mounted pull-off that has two slides on it;one is for the choke plate and the other connects to the secondary top plates.the choke thermostat probably has a tab under one screw that goes into a notch in the plastic that doesn't allow movement unless you renotch it,pics and or carb #would help,jc

 

Yes jc I only have 1 pull off going to the secondary butterfly. My apologies as I called it a choke plate and it is the secondary butterfly. The primary do not have a choke pull off going to it. My understanding is carbs with no choke pull of had small notches in the choke plate to assist in cold start up, but I do not see any in the choke plate. The choke stat is fine, adjusted as it should be. I'm not talking about that at all. Carb number is 7041250

 

based on the number its a 1971,ive never seen that vintage with just a rear mounted pull off.any pics?

 

Based on your last post, you mean the rod going from the choke vacuum break to the secondary air plates (butterfly)? It's to ensure the right amount of vacuum before the secondary air plates open, at WOT. Namely, no vacuum, IIRC. You'll notice that revving the engine to WOT barely opens the secondary air plates. That's normal.


The only 'pull off' the (primary) choke plate has is the choke housing on the side of the carb, with the round plastic cap and metal housing, and hot air tube screwed onto the side. Unless you don't have a choke housing or a divorced choke, your choke has all the parts it needs and SHOULD work, if adjusted correctly.

 

As stated above, the '71 vintage 7041250 carb has only one vacuum vacuum break. (edit: called the primary vacuum break or choke pull-off). When the choke is adjusted properly, the primary butterfly is open just slightly to allow air flow.

edit: I just noticed when looking at the picture I posted that the bottom part of the vacuum break linkage appears that it will contact the tab on the choke linkage, thereby pulling the choke butterfly open slightly when the vacuum break pulls the linkage fully forward. This would achieve the operation you described.


I have never seen "notches" in the butterfly of this era Olds carb to assist in cold weather startup.

Below is the only picture I could find - it has an electric choke, so no help with that part of the question.

P4270451_zps86a0c28f.jpg

 

Thank you guys, you have answered my question about what this vacuum pull off does. It is what I thought but didnt really understand why it was needed.
You have also answered my question about the choke for the primary choke plate. I have adjusted it correctly but I was always under the impression that it needed a choke pull off for the primary or as some carbs without one have little slits in the choke plate to assist in cold starting and mine does not. You say you have never seen this so I am happy with that answer and feel it is a stock carb for a 1971 intake.
Question 1 and 2 are down anyone know about question 3 and if anyone has a idle solenoid for a 1971 or 1972 intake?

 

My '71 with AC did not have that solenoid. With everything tuned properly I have never felt the need for one, and I have been driving this car for over 25 years.

See my edit above for an explanation of how the vacuum break also functions as a choke pull-off.

 

So you just adjust the idle with the car in drive and A/C on?

 

I set the idle to ~700 RPM in Park. Fairly much what the manual says (I think it says 650 or so).

Also, having the ignition advance set correctly is crucial. I once installed an advance curve kit with the lightest springs and I didn't set the total timing properly - I just set the initial to 12 and thought everything was OK. I didn't realize that the light springs allowed some mechanical advance to come in at idle. When I turned the AC on, the idle speed decreased, which caused the advance to decrease, which caused the idle to decrease - you see where this is going. It created a negative feedback loop where the idle decreased until the engine died. Installing slightly stiffer springs and setting the timing properly cured this.

After that experience, I set the distributor (HEI) so that it had ~18º initial advance. This created higher vacuum, a smoother idle, and more off-idle torque. Now the RPM does not change any at all when I'm idling and turn the AC on.

 

I believe I set my total advance around 36 degrees. Thanks for that note I've done it but it all makes sense.

 

This thread has been very informative for me. I also wondered if my 71 had a idle kicker for a/c use, because the air has been inop since i've owned it. I am just now getting the a/c working. I will keep in mind about the advance.

 

op, what you are holding in the 4 pix you posted is the primary vacuum break. it serves 2 purposes: 1. to open the choke upon start-up, a specified amount. 2. to dampen secondary air valve opening. there is nothing inside the choke housing on this series of carbs. (4M) to provide a vacuum break function.


bill

 

Bill this vacuum break is not hooked up to the choke plate in any way what so ever.

 

Tcaroll: Correct, but the front-most end of the rod that connects from the vacuum break to the secondary air valves interacts with the choke mechanism by hitting the tab on the choke mechanism and pulling it open during initial startup, as vacuum increases. They don't touch when the car is cold, but it definitely has an effect. The warm air from the exhaust crossover will eventually heat up the bi-metal spring in the choke housing enough that it'll open the choke plate completely. Lightly hitting the throttle at this point will allow the something-arm to drop off the steps of the high idle cam, resulting in low idle.

 

From the 1971 Chassis Service Manual, page 6M-39:
"The choke system consists of a choke valve, a vacuum beak diaphragm, a choke housing and coil located on the side of the float bowl, fast idle cam, connecting linkage, and air valve lockout lever...

"During engine cranking, the choke valve is held closed by the tension of the thermostatic coil. This restricts airflow through the carburetor to provide a richer starting mixture. When the engine starts, manifold vacuum applied to the vacuum diaphragm opens the choke valve to a point where the engine wil run without loading or stalling..." [itaics mine]

So, as I said, no matter what it may look like to you, there IS a vacuum break on the choke.

- Eric

 

I edited my posts above to reflect the correct terminology for the vacuum diaphragm. Sorry for calling it the wrong name.

 

Seff thank you for that explanation as I get it now. I did not realize the linkage itself contacted the choke/step mechanism as It is not visibly touching it when the engine is cold.
MDchanic thank you as well for the reference to the manual. I need to understand how things work and I am relentless until I get it. Seff's explanation put the light bulb on for me. Thank you again to all who have shared in this thread. I need a vacuum break as mine is not working. If anyone has a good working one for sale or they do not need please contact me asap at toddvcarroll@hotmail.com ~Cheers~ Todd

 

You're welcome. I'm afraid I was a bit hazy when I replied earlier, and failed to search for and include a proper illustration. This one is from the 1971 Chassis Service Manual, page 6M-48:



- Eric

 

Excellent diagram thank you. I will have to look closely again as it really does not appear to be touching this linkage. I may have the wrong rod or may just need to adjust it. Thanks again I am enlightened lol.

 

The 71 carb I had needed a lot of 'convincing' for the rod to get close enough to the tang, and not merely push past it.

 

Thanks for that Seff appreciate it. Probably why I did not realize it was actually functioning as the choke pull off.

 

As for what the vacuum break does for the secondary air valve (aka butterfly), it controls the RATE the air valve opens.

It doesn't limit how far it can open, and it doesn't make sure there's enough vacuum first. It simply keeps the air valve from slamming open when you floor it. This is done to prevent a lean condition from developing in the secondaries. A "double-pumper" carb achievs the same thing through brute force by shooting extra gas into the secondaries. Rochester's solution is more elegant (and more effective, in my opinion) in that it essentially makes the carb smarter than the driver. The vacuum break "knows" that the secondaries don't need to be slammed open, because it will take a second or two for engine RPM to climb high enough actually need that much air and gas.

The vacuum break becomes a valuable tuning aid (strip or street) once you understand how it works. You can use vacuum breaks with a smaller or larger orifice (in their feed tube), or insert a precision-drilled plug of your own making, to control the rate the seconday air valve opens.

 

Awesome Brian thank you for this....Todd

 

You can get one from your local NAPA store:

http://www.napaonline.com/Catalog/Ca...807_0064578445

Part Number: CRB 24807
Product Line: Echlin Fuel System

 

No I ordered one from Walker online they were a little cheaper than quadrajets.com