3269S AFB Carb problem

[Jonathan S.]

Need some advice"

I have a 3269S AFB Carb on my '62 340 hp car and I am having some idling problems.

1) Upon cold start the car does not cold idle (car idles at 7,500 RPMs; and

2) when car warms-up and I drive it around and get to a stop light the engine settles down and idles at 10,000 RPMs

[Wes S.]

Re: 3269S AFB Carb problem

I am confused tach doen't go to 10,000 rpm. Not only that, most 327 won't hold together at that rpm.
Wes

[Michael G.]

Re: 3269S AFB Carb problem

I thinkin' the machinest who put this engine together must be the "Mac Daddy" of engine builders. A SBC that holds together at 10k rpms.....mercy!

[Duke W.]

Re: 3269S AFB Carb problem

The biggest problem with the AFB is the choke system, which also controls the fast idle. It gets dirty and sticks.

You can remove the choke housing from the carb and give it a good cleaning. Also make sure the choke air tubes are leak free including the insert in the exhaust manifold. These can rust through and foul the choke system in nothing flat. Do all the other dimension and clearance checks that I assume are in ST-12. My 1963 Corvette Shop Manual has a very extensive section on the AFB with just about everything you need to know to get them running in proper order.

Put a little graphite lubricant on the choke piston (spherical graphite in an emulsion like Lok-Eze, not graphite grease), and make sure you renew the tiny O-ring that seals the vacuum passage to the main body than draws air through the choke housing.

I've system engineered SB mechanical lifter engines that make useable power to 7200 with all OE components before the initiation of a little valve float causes the power curve to take a significant drop. So what's your secret to getting F1 type revs from a vintage small block?

Duke

[Richard M.]

Re: 3269S AFB Carb problem

Me thinks he is reading the RPM scale as "X1000" instead of "X100".

[Jonathan S.]

Re: 3269S AFB Carb problem

You are exactly right. My fault. I am a rookie, what can I say!

[Richard M.]

Re: 3269S AFB Carb problem

You are exactly right. My fault. I am a rookie, what can I say!

That's ok we all were once too!

We have to assume there is a complete choke still on it, and it has a tube running to the exhaust manifold.

On the right of the carb loosen the 3 screws holding the black choke housing cover on. Turn it back and forth a little to make sure the choke butterfly moves. If not do what Duke recommended first. If it moves freely, turn it so the butterfly closes. Leave it there and tighten the 3 screws.

Then watch the left side rear of the carb. The fast idle cam is there. On a cold engine, before starting, you blip the throttle a few times and that sets the choke idle cam on fast idle. There is an adjusting screw there you can increase/decrease the idle at cold if necessary. Start the engine. You can test it by manually moving the fast idle cam clockwise a bit and it'll contact the throttle lever and increase idle. You may have to go back and forth between the choke housing and cam lever to adjust. To make it more confusing there is also a adjustment on the lever on the left side shaft of the butterfy. I'd assume that's already set properly though.

By now you're probably saying "Too much information!" What would help is a ST12 service manual or a Carter choke adjust procedure to guide you through it. I don't have a scanned copy handy, maybe someone else can post one.

Rich

[Stuart F.]

Re: 3269S AFB Carb problem

I won't even comment on the idle RPM issue, and shame on those that did.

Duke has pointed you in the right direction about the choke. I have had trouble with every model I have used on my 63, incl. 3461S, 3720SA and 3721SB. That little pull off piston in the choke housing is the main culprit, and I have tried every lubricant known to try and keep them free. I have even polished the pistons, and opened up one housing to polish the cylinder bore. They seem to, in time, distort from the heat and also tend to build up a sort of white corrosion. Every fix I have tried in the past would only buy me about one year of good service, and then I would have to free it up again.

I have tested and reported before in other threads on the subject that the vacumm pull through the hot air tube is around 0.5" to 1.0" hg which, in the grand scheme of things, is insignificant with regards to it's effect on mixture. Now I consider mine, even as an original owned car, a good driver. I'm done showing and want to enjoy it for what few years I have left. As such, I fitted mine with an electric choke conversion and have, therefore, eliminated the hot air source. The other, and biggest, advantage of the electric choke is that it "pulls" the choke closed and, as the element heats up, it "pulls" the choke open. It does not rely completely on the pull off piston to do the job. I believe that w/o the heat, I no longer have the distortion or contamination problems of before, so the pull off piston is allowed to assist the electric choke bi-metallic spring in it's job. I have the best possible warm up cycle as a result. As a side light to this, I have left the section of hot air tube in place on the exhaust manifold so I can convert it back to original configuration in about 10 minutes (could probably do it in 5 if I were younger and more NCRS motivated).

Stu Fox