No heat at the automatic choke

[Bob S.]

I don't get adequate heat out of my choke tube at the choke. This is a base 230hp 283ci with 2818-S WCFB. The butterfly in the heat riser has been removed and I have checked the tube inside the exhaust manifold and it is clear and intact. The choke tube itself is new and clear.
With the car up to operating temerature for several minutes you can grab the choke tube with your hand without discomfort. The heat gun shows 84 degrees on the outside of the tube. I have taken the choke cover with spring and placed it in the oven and confirmed that the spring moves with heat. With the spring removed from the choke the choke butterfly opens fully with vacuum. I just can't get enough heat through the choke tube to heat the spring to let the butterfly open properly.
Any thoughts would be appreciated. Sully

[John H.]

Re: No heat at the automatic choke

I just can't get enough heat through the choke tube to heat the spring to let the butterfly open properly.
Any thoughts would be appreciated. Sully

Sully -

Is the vacuum passage fully open from the bottom of the baseplate (just forward of the primaries, adjacent to the baseplate-to-main body screw) to the choke housing? If that passage is plugged or restricted, the choke housing won't see any vacuum, and won't pull any heated air from the tube in the manifold through the choke hot air tube into the choke housing.

[Bob S.]

Re: No heat at the automatic choke

John,
Can I check this without pulling the carburator? There seems to be some vacuum because when the spring was removed I would manually move the choke butterfly to close and it would go back to open. There didn't seem to be much force but it would open fully. Sully

[Timothy B.]

Re: No heat at the automatic choke

Bob,

That passage should see full manifold vacuum as it's tap is under the throttle blades, on my afb I think the hole is approx 3/32" diameter.

Make sure there is no vacuum leak at the gasket where it attaches to the main body. You may want to remove and inspect, the hole could be stoped up.

[Stuart F.]

Re: No heat at the automatic choke

You can use a vacuum gauge on the choke tube (removing it from the exhaust manifold, but connecting it to the choke). My AFB (63) has only a small draw through the choke - less than 2"Hg w/choke open or closed. I checked it some time back to see it's effect on the mixture before I went to an electric choke. I took my reading upstream of the manifold "x" tube via the clean air hose to the carb. An earlier C-1 arrangement, of course, will not have the closed loop. The AFB definitely has restriction (porting) in this path.

One question I have is whether your heat tube is steel or did someone change it to aluminum? We used to use aluminum with headers and found it cooled off much quicker. To compensate, we would pinch the tube with a vice grips in several places (similar to a manifold "x" tube) to restrict (slow down the flow) so it would retain the heat. You can also add a insulating sleeve. I don't recommend these steps for an original show car.

You may want to consider replacing your heat riser valve with one that functions properly. You can always wire it open for the summer.

Stu Fox

[Timothy B.]

Re: No heat at the automatic choke

Stu,

On my 63 300hp, the engine draws 18-19" idle vacuum and the choke tube pulls approx 15" vacuum at the carburetor air horn connection for the clean air tube. Pretty sure it's full manifold vacuum at the choke housing but the fittings are slip fit for the clean air tube so there are some leaks.

Where/why would there be a restriction for this vacuum choke pull off. The manifold vacuum along with the choke pull off piston opens the choke on start up, balances the piston against the choke spring tension and also pulls warm air to control the opening speed.

Unless my afb is missing something to reduce the vacuum at this port all I ever found is a brass tube in the drilled opening of the main body where the choke housing bolts to the side.

Believe it or not, my afb lets more air in from this port at idle than the PCV valve at idle and I have always wondered about that.

[Bob S.]

Re: No heat at the automatic choke

BINGO - I think you guys have hit the nail on the head. I just hooked up the vacuum guage as Stu suggested and 0 vacuum.
I will get new carburator and choke gaskets this week and pull everything to get to the bottom (sic) of the problem.
Thanks everyone for the great input. Sully

[Stuart F.]

Re: No heat at the automatic choke

Tim;

This is an interesting situation. I checked mine about a year ago in the course of determining if I needed to compensate in my jetting based on the elimination of any vacuum draw at the choke housing by going to a sealed electric unit. I connected to the hot air hose to the carb so the "X" tube in the manifold, which acts as a restrictor, could be included as a factor. From initial startup, my notes showed these readings;

- initial cold start, high idle @ 1500 rpm = 1.5" Hg.

- first kick-down to 1200 rpm = 1.55" Hg.

- second kick-down to 1000 rpm = 1.3" Hg.

- third kick-down to 900 rpm, temp @ 140* = 1.2" Hg.

- last kick-down to 750 rpm (idle), temp @ 160* = 1.0" Hg.

From this test I determined the draw through the choke was insignificant as to be a concern for my jetting.

I believe, as you say, there is a brass tube in the carb vacuum tap to the choke, and I recall mine had a small hole in it, sort of like a jet size. My carb, BTW, is a 3721SB replacement unit made in 1979.

I will research this further in the Carter books I have as well as other sources. Personally, I don't think I'd want a full vacuum (read leak) through my choke, and that is what you'd have with an open hot air tube - don't you think?

My electric choke spring captures the choke pulloff link in a manner that both opens and closes the choke, with some possible assist from the pulloff piston. It works great.

I don't know whether our discussion helps or confuses the situation on his C-1 w/WCFB.

Stu Fox

[Timothy B.]

Re: No heat at the automatic choke

Stu,

I am not aware there is any restriction in the tube running through the exhaust manifold. I am curious if there could be leaks in the clean air choke tubing where it's a slip fit causing this low vacuum your seeing. I recall reading that you had problems with the factory hot air system which is why the change to electric choke.

If you feel like it sometime in the future put a vacuum gauge on the tube that connects to the choke housing and let me know if you see manifold vacuum because at that point there should not be any leaks.

The only restriction I can think of is the vacuum pulling air around the piston and through the piston slots.

[Stuart F.]

Re: No heat at the automatic choke

Tim;

The tube used in the exhaust manifold, as I'm sure you are aware, is pinched down into an "X" shape for what I assume is the need to restrict (slow down) the air flow through it in order to conduct as much heated air as possible to the choke. I used the same technique with aluminum tubing in order to create a heated air source for the choke on one of my Dual Quad equipped Vette engines. I had the old style Hedman headers and had to make a heat stove by rolling aluminum tubing around one of the header pipes, then connect it with brass junction fittings to more aluminum tubing leading to the choke housing. I insulated the tube with a sleeve and tried to get a good choke adjustment; one notch rich and it would not release, one lean and it wouldn't set completely (you get the picture). So, using the pinched tube principle, I set a vice grips to pinch a test section down to about 1/16", then used that setting to pinch the tube from the header to the choke. One pinch helped, so I pinched it again at 90* and slightly above the first pinch and; wala! it worked like a champ.

That's why I conducted my test on the 63 as I did. I wanted to see exactly how much draw there was from the carburetor choke. I believe I had good and tight hose and hot air tube connections through the exhaust manifold tube. I had not tested at the choke itself because that would not show me what I wanted.

The problem I had with the original hot air choke system on my 63 was the high maintenance required to keep it working properly, i.e. white corrosion binding up the pull off piston. I also was looking for ways to remove heat from my carburetor, i.e. the phenolic spacer under the carb, fuel hose connection from the filter to the carb rather than steel (also needed for spacer), due to the fuel percolation problems I had. It helped by dropping the temperature I saw at the carb (with an IFR gun) close to 45*F. That, along with my use of Tomco fuel inlet valves in place of needle/seats, has practically eliminated the problem (no more stinky garage).

As mentioned before, I will research this further to confirm (or not) my theory. Regardless, I would not change a thing now for my "driver" use of the car. It has never run better since I took delivery.

Stu Fox

[Timothy B.]

Re: No heat at the automatic choke

Stu,

I replaced this exhaust manifold heat tube a few years ago and don't remember the old tube being restricted when it was removed. I hope others comment because I would like to know for sure.

I can tell you for sure the new stainless tube I purchased has no restriction, it's straight through. As I said earlier, this draw at idle pulls more air then the PCV.

[Stuart F.]

Re: No heat at the automatic choke

Tim;

As a starting point, I went out and checked my spare (backup) 3720SA. I removed the choke housing to get to the vacuum port in the carb body and it does, indeed, connect to the same direct port as the vacuum advance port at the front/center base of the carb. That port divides with a curved channel to each primary bore directly above the base gasket surface. As such, the only restriction in the carb would be the size of the port which is about 3/32" or so (I didn't measure it. Therefore, if you did measure at the choke housing as you did, you should get something approaching normal vacuum as restricted by the pulloff piston, etc. Again, I never tested this way as it would be irrelevant to what I wanted to determine.

The author of the thread said he was dealing with an earlier WCFB, as I recall, so he may have different porting. Still, if he was not getting any vacuum signal at all, I suspect something is amiss either with his base to center section gasket or some restriction in the port path.

The only manifold heat tubes I have changed were from earlier Chevy V8's, like late 50's with ram horn manifolds and they each had the "X" tube configuration.

Before I ran my vacuum test, I applied 125 psi compressed air through the tube to make sure it was clear and it was (into the manifold and up/out of the hot air tube, and a back flow as well).

We can probably see one of those tubes on-line. I will check.

Stu fox

[Christopher R.]

Re: No heat at the automatic choke

Tim;

The tube used in the exhaust manifold, as I'm sure you are aware, is pinched down into an "X" shape for what I assume is the need to restrict (slow down) the air flow through it in order to conduct as much heated air as possible to the choke.

Stu Fox

The corroded old tube I took out of an old exhaust manifold was not pinched or restricted in any way. Nor was the new one I put in.

[Clem Z.]

Re: No heat at the automatic choke

Tim;

As a starting point, I went out and checked my spare (backup) 3720SA. I removed the choke housing to get to the vacuum port in the carb body and it does, indeed, connect to the same direct port as the vacuum advance port at the front/center base of the carb. That port divides with a curved channel to each primary bore directly above the base gasket surface. As such, the only restriction in the carb would be the size of the port which is about 3/32" or so (I didn't measure it. Therefore, if you did measure at the choke housing as you did, you should get something approaching normal vacuum as restricted by the pulloff piston, etc. Again, I never tested this way as it would be irrelevant to what I wanted to determine.

The author of the thread said he was dealing with an earlier WCFB, as I recall, so he may have different porting. Still, if he was not getting any vacuum signal at all, I suspect something is amiss either with his base to center section gasket or some restriction in the port path.

The only manifold heat tubes I have changed were from earlier Chevy V8's, like late 50's with ram horn manifolds and they each had the "X" tube configuration.

Before I ran my vacuum test, I applied 125 psi compressed air through the tube to make sure it was clear and it was (into the manifold and up/out of the hot air tube, and a back flow as well).

We can probably see one of those tubes on-line. I will check.

Stu fox

that choke vacuum passage in those early WCFBs was the cause of the carb flooding in the corners because the passage ran up passed the float bowl and it would suck in fuel if the gasket between the top and the main body did not seal. they later put a little brass extension pipe in the main body that fit up into the carb top to seal this area from the fuel. if this carb does not have the extension tube and the gasket is not sealing well the vacuum could be lost there

[Stuart F.]

Re: No heat at the automatic choke

The picture shown in the Paragon catalog appears to have the ends rolled out to a larger size (where it would press fit into the manifold) and a slight pinched area around 1/2" to 3/4" from the rolled area on one end.

Regardless, I may have been dealing with an earlier model, which may have been the reason they changed the design to what you guys are reporting.

Clem; I thought the main cause of the WCFB flooding out on corners was the float drag on the sides of the chambers. I used to set up dual quads for 1/4 mile tracks with much success paying close attention to the float alignment and pivots. I also dropped the floats down some and, of course, used larger needle/seats. My favorites were Caddy El Dorado sets with the four equal large venturis, which ran both primaries together and used air idle valves. We mounted them on 1/2" adapters. The set I had on my car allowed great drifting on paved streets w/o missing a beat. In a 1/4 mile I could pick up a couple tenths and 2 to 3 mph (but they weren't legal).

Stu Fox