Mark71

I just had to change carbs on my 71 Cutlass 350. In the process of removing my old (71 Q-Jet) and putting on my "new" (rebuilt 73 Q-Jet), I realized that the tube going from the stock intake manifold up to the "heat stove" on the choke is basically just sitting there... It fell right out when I lifted the old carb off. That was the same for the second tube that goes up to the back of the air horn in the carb.

Three questions:
1) Since they are just "sitting there", not sealed or threaded in any way, is that technically an intake leak? I mean, could it be causing other issues?
2) What is that other tube coming up to the air horn on the carb?

For one thing, those are both exposed to heat from the exhaust cross-over, right? I am not sure how that stock manifold is setup, but I think I read that in my searches here...

Thanks!

TripDeuces

One goes to the choke and the other goes to the back of the carb to help stop icing I believe. The tube you can't see within the manifold should be one solid tube. You can unbolt that assembly and visually inspect the tube for damage. Since it's a sealed tube there should be no intake leak unless it's damaged. I hope that answers the questions.

2blu442

Hi Mark
There is also a U shaped tube inside the manifold that if its not rusted away would keep there from being a vacuum leak at the manifold. Air is drawn through this U shaped tube to heat the choke coil, so for your choke to work you'll need to replace the two tubes and maybe unbolt the one in your manifold and check it too. Problem being the two small bolts that hold the U shaped tube in will often be brittle and break when you try to remove them. You can get these tubes repop or post a parts wanted thread to see if anybody has good used ones for sale. John

Trip, looks like you can type faster than me!

MDchanic

Or, to put it a different way:

There is only one tube.

It attaches to the back of the carb with a small piece of hose, and to the choke unit on the side of the carb with a screw fitting.

There is a vacuum port between the carb body and the round choke unit which sucks air in through the inside of the choke unit. Since the inside of the choke unit is under vacuum, it wants to suck air in through the screw fitting. It sucks air through the tube, from just inside the back of the carburetor, through the heat recess in the manifold (where it gets warmer), and into the choke unit, where it heats up the choke spring and moves the choke butterfly.

The reason the tube is connected to the back of the carburetor is so that the mechanism is sucking in filtered air from the air cleaner, and not dust and crud, which would foul the choke mechanism.

Bottom line: it's one tube. It sounds like you've got three tubes. One way or another, you need to make all of those tubes back into one if you want the choke to work right .

- Eric

compedgemarine

not to dissagree Eric but they are actually three tubes. the one in the manifold, the one to the rear of the carb to the manifold tube and the one from the manifold tube to the choke. the two outer ones are swedged down in size on the end and should be pushed in pretty tight to the manifold tube. usually they rust together from the heat and so they seem to be one. looks like his are too loose to stay pressed in. some where are here I have a few sets of the outer two tubes.

MDchanic

Yes, okay, I can't argue with that, but the three segments are supposed to be more or less permanently attached to each other.

Odds are you can seal them all up with high-temp. silicone sealant if you don't mind the look of it. You can confirm that everything is sealed by putting your finger over the inlet at the back of the carb - it should suck onto your finger nice and hard.

- Eric

compedgemarine

yes they should push together pretty tight so they dont leak. I have had some that after years of heating and cooling they would not stay in tight and some that had rusted together so tight that nothing would ever get them apart. as you said they should seal up so it is only pulling clean air from inside the filter area.

Mark71

Thanks for all the help guys. First, I confirmed that the choke is working. It is a newly rebuilt carb and it pulls off much faster than on my old carb. Maybe even too fast, but I'll adjust as necessary.

I want to confirm how this is supposed to work:
1) Air is sucked into the air horn and pulled down into the manifold where it is heated.
2) That same air is pulled into the choke after heating in order to cause the choke spring to pull the choke off.
3) All of this is controlled by vacuum through the choke assembly.

So is the vacuum through the choke just full manifold vacuum? Is it metered somehow so that the mixture stays right?

I did a bit of testing and spraying carb cleaner around those tubes, right where they exit the manifold results in a definitely changed idle sound. There must be a leak somewhere.

MDchanic

Far as I know, it's just a controlled vacuum leak through a metered orifice.

I had a carb apart a few days ago - could have followed the passages then, but it's all back together now. Sorry.

- Eric

joe_padavano

Correct. There's no specific vacuum level, the choke just uses vacuum to pull heated air past the choke coil.

Mark71

Ok, so let's say I wanted to eliminate the heat stove choke and use an electric one instead. Could I just fab up a simple block off plate for the choke tubes and solve the whole thing?

I assume I'd need to block off the threaded connection that allows port that used to let in the heated air, but otherwise, is there any problem with this plan?

Is an electric choke conversion on a Q-Jet a good plan?

2blu442

Mark, I've not done the swap but think it would work just fine. I've even purchased an electric choke setup for a quadrajet planning to use it on an aftermarket manifold with no choke stove setup. I think your on the right track of what needs to be done to seal up the tubes in the manifold too. John

MDchanic

+1.

An electric choke should work fine, too, Mark.
Just depends on whether you want to get (or fix) the tubing or a new choke coil and wiring.

All you need to do (other than wiring, installing, and adjusting the new electric choke coil) is to put a dab of silicone sealant over the vacuum hole inside the choke unit.

No need for block-off plates or anything else - you can leave the tubing right where it is, cur it off level with the plate on the manifold, or make (or buy, I suppose) a new plate.

- Eric