Carter AFB Choke Question

[Bruce Boatner]

The automatic choke on my 1964 327/300 is not opening after the engine reaches operating temperature. I am thinking that either insufficient vacuum pressure or too much resistance between the choke piston and cylinder (or both) are preventing the choke from opening properly.

I recently unfroze/degunked the choke piston after it had been frozen in place for who knows how long (frozen when I bought the car). After removing the choke piston and thoroughly cleaning the piston and cylinder bore, I still encounter a fair amount of resistance when manually moving the piston up and down in the bore. I liberally applied sealant to the bottom of the cylinder bore on and around the cap at the bottom thereof. I also made sure that the O-ring between the choke assembly and carb is in place.

Does anyone know how tightly the choke piston should fit in the cylinder? I am tempted to attack the cylinder bore and/or piston with some emery cloth to produce a looser fit, but worry that I may lose all vacuum pressure if a tighter fit is called for.

Any advice or insights would be GREATLY appreciated!

[Duke W.]

Re: Carter AFB Choke Question

I recommend you remove the choke housing from the carb., disassemble it, and thoroughly clean it. Also clean the external linkages on the carb as required to ensure that the choke valve is not sticky.

When you reinstall the choke housing on the carb, you will need a new small O-ring to seal the choke vacuum passage to the main body. These should be included in AFB zip kits.

The AFB is a very trouble free carb., but it's biggest problem is the choke system. At least the entire carb does not have to be removed and disassembled to correct most choke system problems.

My hunch is that the thermostatic springs go out of calibration over the years, so you might have to set the choke housing preload one or two notches richer than spec.

The '63 Corvette Shop Manual has a complete section on the AFB, which is an excellent reference with all the linkage adjustment specs and complete overhaul instructions. The '64 and '65 Supplements will have the updated data for later years.

Duke

[Clem Z.]

Re: Carter AFB Choke Question

make sure the vacuum passage in the carb body is open. the piston should slide easy.

[Bruce Boatner]

Re: Carter AFB Choke Question

Duke,

Thanks for your input. Can you recall if the choke piston should move relatively smoothly in the bore? Mine currently seems to be a little tight.

Thanks!

[Duke W.]

Re: Carter AFB Choke Question

Yes, It should slide with virtually no friction. Check the piston and bore for scoring/galling and dress it down with sandpaper, if necessary.

Clean the metal parts in strong carb cleaner and thoroughly rinse them, so everything is squeaky clean.

Make sure the air tubes are clean and intact, including the internal tube in the exhaust manifold.

Duke

[Bruce Boatner]

Re: Carter AFB Choke Question

Thanks Duke and Clem. I think you may have helped me solve the problem because the piston does not currently slide easily. I'm going to try a little honing over the weekend and hope that does the trick.

[William C.]

Re: Carter AFB Choke Question

Try a good cleaning before honing. The fit hast to be tight enough for the vacuum not to escape without pulling the piston into place. Remove all the dirt, carbon, etc before you start more agressive tactics, and if anything start with the piston. They are more plentiful than the housings

[Bruce Boatner]

Re: Carter AFB Choke--One Step Closer Now!

I ended up cleaning the choke piston VERY well by using a wire wheel mounted on a bench grinder. I then kept working the piston back and forth manually in the choke housing while applying liberal doses of carb cleaner. Eventually, the piston began moving more freely in the choke housing. That did the trick. The choke closes just fine now with vacuum pressure.

I think I need to play with choke setting a little because the choke still does not open fully with a hot engine. Could be that the replacement spring is not "calibrated" to factory spec's--if so, I will keep playing with the spring load until a find a position that works. Or, maybe I just mounted the spring wrong. Oh well, I can see the light at the end of the tunnel now.

Thanks for everyone's help.

Bruce