any4xx

‘73 454/TH400.

Brief history:

I recently bought this car back after selling it to a friend about 8 years ago. I got it back with a lot of it in boxes. The Quadrajet that had been on it when I sold it was in a box along with a second one. Both were mostly disassembled. A third one is now on the car. My friend told me he had bought this one because it was “correct” for the car but hadn’t driven it since putting it on. He said he had rebuilt it. I had the engine running last week, but not well.

Today I pulled it off and opened it up. Internally everything looked to be clean and assembled properly. But when I was reassembling the choke components I realized that the little “bar” that hangs down and stops the secondary throttle plates from opening moves freely, but nothing in the choke linkage actually connects to it or touches it to push it away from the throttle “pin.”

I sorted through the pile of carburetor parts and found that one of the other two little bars was different. When I reassembled the choke components with that bar installed, the “weight” that hangs in the forward position would now push the top of that bar so that the secondary throttle plate pin was now unblocked and they could open freely. So I THINK I now have this right.

Can someone here who knows Quadrajets confirm for me that the secondaries are supposed to be blocked when the choke is on then open regularly when warm? I’ve attached a couple of pictures of my choke linkage. Does it look like I got everything in the right places?

Thank you for any help. I hate fixing things that someone else has already fixed.

any4xx

Here’s two more overall pictures of the choke linkage. The first one is with the choke on. The second is with the choke off.

7T1vette

Yes. The secondaries are mechanically prevented from opening when the choke is operational. Once the choke is 'off', that link is released and the secondaries are free to open.

any4xx

Thank you. That makes sense. The incorrect lever obviously came on one of the other two carburetors. Weird to me why it would be different. I’m used to Holley’s where everything is pretty much the same from model to model. I’m going to have to educate myself on Quadrajets.

lars

The 4MV Q-Jets use a different design secondary lockout lever than the M4M carbs.You can physically install an M4M lockout on your 4MV, but it won't function. You have the correct 4MV lever on the carb now, and it should work as intended. Very few parts will operationally interchange between the 2 major Q-Jet models.

Lars

any4xx

Thank you. The lever that was there before made no sense to me. I am just glad that there were piles of parts from two other carburetors there so I could find the correct lever. Again, I’ve got a lot to learn if I’m going to make this thing sing. I hate to say it, but there are also packages of new metering rods among the pile of parts. I hope he left well enough alone there, but I guess I’ll know when I get it on the road. Like I said before, I’m used to tuning Holleys. These look to be even easier to tune. I just need to get familiar with the systems.

Thank you again.

Bikespace

You can't do better than having @lars answer your questions, but it wouldn't hurt to also get a copy of Cliff Ruggles' Book to use as a reference.

any4xx

Thank you. I was looking on the AtoZ site and saw a few Quadrajet books. I wasn’t sure which one to order. I’ll get on that one.

lars

For reference, these are the 2 major secondary lockout lever designs - there are some "sub-species" variants. The lever on the left is for the 4MV (divorced choke) Chevy carbs. On the right is the 1975+ M4M integral choke carb lockout lever. The two cannot be interchanged.



Lars

any4xx

My carburetor did have the one on the right. It now has the one on the left. Who knows why, but I’m happy to have accidentally gotten it right. ;-)

Thank you again.

leadguy

I just learned a few minutes ago that my 72 has a 75 Quadrajet carb. Is there a performance or other important difference that I need to be aware of?

lars

The 72 and 75 Q-Jets are two completely different model/series carbs. The 1966 - 1974 carbs are 4MV carbs (divorced choke). The 1975+ carbs are M4M (integral choke) carbs. They all flow 750 cfm. That's where the similarity ends. Your question is a completely different subject than the OP is discussing here - you might want to open a new thread about your issue - it doesn't have relevance in this thread.
Lars