No heat at the automatic choke

[Clem Z.]

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

a lot of the carb kits still tell you about this problem and how to fix it.

[Clem Z.]

Re: No heat at the automatic choke

here is a scan of the WCFB and the choke vacuum port is circled. it is next to the float bowl and if there is not a good seal there it will suck gasoline into the engine

[Stuart F.]

Re: No heat at the automatic choke

FWIW, early Carter "Power Center" Form #3703 information on the AFB-Type Carbureter diagrams show the "Climatic Control Choke Circuit" with a Choke Restriction. This data is from the late 50's/early 60's, and many variations occurred after then. As stated in one of my earlier posts, my 3720SA circuit does not appear to have a restriction except the passage size itself.

Stu Fox

[Timothy B.]

Re: No heat at the automatic choke

Stu,

Here are pictures of a 3720S and a 3721 SB showing the only restriction I can see in the complete vacuum choke pull off.

It's the vacuum pulling on the piston that holds the choke open, it's the spring tension that closes the choke.

[Clem Z.]

Re: No heat at the automatic choke

Stu,

Here are pictures of a 3720S and a 3721 SB showing the only restriction I can see in the complete vacuum choke pull off.

It's the vacuum pulling on the piston that holds the choke open, it's the spring tension that closes the choke.

easy way to clean the carbon out is to start the engine and squirt carb cleaner into the passage and the vacuum will suck it thru

[Clem Z.]

Re: No heat at the automatic choke

easy way to clean the carbon out is to start the engine and squirt carb cleaner into the passage and the vacuum will suck it thru

you can pull the piston out of the choke housing and squirt the carb cleaner in the piston bore to clean out the vacuum passage with the carb on the engine with the engine running

[Stuart F.]

Re: No heat at the automatic choke

Tim;

Your pictures show exactly of what I speak. I am in complete agreement that the tube in the passage is the only restriction in the choke circuit for the AFB models we are familiar with. My point was simply that the earlier models were designed to have a restriction, and that it was called out as such on their circuit diagram.

BTW, I am familiar with the basic design/operation of the choke pull off and the bimetallic thermostatic spring. The electric choke kit I have installed enhances the operation by both closing and opening the choke. The thermostatic spring captures the piston link (instead of just pushing against it) to close it when cold and pull it to open when warm. In effect, it assists the vacuum operated piston to open the choke. This is the same system used in most aftermarket electric choke kits as well as the Edelbrock versions of the AFB, with the exception that my kit uses a thermal temperature sensor mounted on the intake manifold to control the electric power to the choke. That way it is much more sensitive to the actual engine temperature conditions.

Not much of what we've discussed about our AFB's helps Bob with his problem with a WCFB. Clem probably has addressed that better based on our earlier discussion of the vacuum pull at his choke. He reported that he had no vacuum reading there, but was then going to look at the cause being perhaps gaskets. As Clem pointed out, it is easy for that circuit to become plugged with carbon which would negate the choke function. Bob needs to look at that as a possible cause and not just due to a gasket problem.

Stu Fox

[Timothy B.]

Re: No heat at the automatic choke

I wanted to follow up on this post showing the vacuum pull I spoke about on the choke circuit of my AFB 3461.

The restriction Stu and I talked about is the brass bushing in the carburetor, this circuit seems to be a calibrated vacuum leak and it pulls filtered air from the carburetor air horn.

I installed a new heat tube in the exhaust manifold and did not notice any restriction other than a tooling mark on the old tube. I am not sure if the pull off piston acts as a restriction to air flow in any way but this car has a noticable air flow in this circuit.

[John F.]

Re: No heat at the automatic choke

Doesn't a little rubber O-ring gasket go between the choke housing & carb where that little bushing is?

[Timothy B.]

Re: No heat at the automatic choke

John,

The small gasket seals the carburetor passage to the choke housing and vacuum pulls the little piston balancing it against the bi metal spring tension allowing the choke operation.

The vacuum also escapes around the piston and the passage pulls hot air from the exhaust manifold through the choke housing to relax the bimetal spring. I have no restriction in this passage other than the bushing in the carburetor.

[Stuart F.]

Re: No heat at the automatic choke

That's a nice steady 18.75 Hg vacuum reading Tim. Nice photo. I wish mine were as steady, but I guess the Duntov causes a lot of my needle wiggles. Someday I may run another test on mine to confirm my original data, but it's running so sweet now I don't want to mess with it. It has never run better in the nearly 48 years that I've owned it; best start ups after long sits (weeks), best warm ups w/o hesitation or stumble, best pulls to red line, best restarts after heat soaks, and very little garage stink after shut downs. My experimenting days are over - almost sad to have to just drive it and enjoy it now.

Stu Fox

[Timothy B.]

Re: No heat at the automatic choke

Hi Stu,

I am glad you replied to this thread. After reading what you reported about the vacuum pull on your choke circuit I wanted to show what this car is reading.

I am still trying to figure out why my 300hp engine with the 3461 Carter idles about 600 rpm with the throttle blades closed completely. These calibrated vacuum leaks could make the difference which is why I wanted to post my results.

I have installed throttle shaft bushings and trued the bottom along with the spacer and intake surfaces. Approx .020 (maybe less) transfer slot exposed with the blades shut. Timing at 8BTDC, I am going to remove the top and use my mighty vac to double check the needle and seat for any gasket seepage along with submerging the floats into hot water to check for bubbles. Car runs fine except this issue, I don't know if it's a problem or not..

[Stuart F.]

Re: No heat at the automatic choke

Tim;

Very strange situation you have there. Keep us posted on your progress and, in the mean time, perhaps we can come up with some other suggestions/ideas for you.

One thing I find puzzling is why the vacuum test with my engine is so different from yours.

Stu Fox

[Timothy B.]

Re: No heat at the automatic choke

Stu,

The only thing I can think of is there are leaks where these choke tubes slide together on your car because vacuum (manifold vacuum) would be the same even if the bushing in the carburetor is a smaller size, the air flow would be different though.

This 3461 carburetor has me stumped, the engine runs fine just this idle issue. I keep thinking someone did a hotrod trick on it but I have been over every inch of it. Do you know if throttle blades are different thickness for some of the carters?

[Stuart F.]

Re: No heat at the automatic choke

Tim;

As to there being different thickness butterflys; Id have to say "yes", as well as different materials. But, I could not begin to tell you which type you'll find on which model carb as I was never too concerned about the thickness so much as the diameters. I have only run across one carb that someone tried to remove the butterflys, broke several screws, then gave up and covered up their mess with epoxy. I have seen both steel and brass appearing butterflys, as well as vent tubes. Your 3461 should have brass tubes, but I have one 3721 with rolled steel vent tubes, and a 3720 with brass.

I feel confident that my test did not have any vacuum leaks. The tube in the bottom of the manifold has a very snug fit and the tube from the top to the carb was also a snug fit. Besides, eliminating the hot air circuit completely by using the electric choke did not alter the idle speed or manifold vacuum reading. As I stated before, that vacuum draw on mine was insignificant and as a result I did not consider it in my jetting choices.

Tim, perhaps you might want to start a more specific thread on this issue as it is apparent no one else is following this one. It would be interesting if we could get a few others to conduct similar tests on their Vettes, or perhaps can relate some of their experiences. There certainly are a lot of other 63's out there, both L-75 and L-76's that have had to have gone through similar situations as ours in order to make their engines run properly.

Stu Fox