AFB Carb Vacuum Restrictor

[Jeff B.]

I'm having a hard time gitting my 1964 327-300hp to idle below 700. I'm preparing for PV and want to achive 500-600. I'm sure this is a vacuum leak problem somewhere. I have checked the adjusted the seating on the primary and secondary throttle valves and they appear to fit tight. But unless I hold the throttle firmly closed by hand it just wants to find it's way back to 700 RPM. The idle screw is backed full off and the choke linkage is fully disengaged The engined idles very smooth and the air screws affect the idle conditions correctly. Timing set to 6 deg before with vacuum advance disconnected (and yes it's reconnected). If I disconnect and plug the vacume hose that runs from carb to the oil filler neck, I get a drop of almost 200 RMP. So I'm wondering if the restrictor is the correct size. It is a replacemet from CC. Does anyone know the correct bore size for the restrictor? If this is not the problem I'm thinking I have to start looking at the throttle shafts. The carb was restored by Holley's custom shop and I requested new shaft bushings but who knows if I got them. Lastly, this carb is mounted with the plastic spacer plate below it and while there is a gasket on the bottom side of the plate, there is only a think metal plate between the spacer and the carb. This looks like a prime place for a vacuum leak - has this been a problem area for these cars?

[Timothy B.]

Re: AFB Carb Vacuum Restrictor

Jeff,

Do you have the correct PCV valve installed. This will provide extra air for the engine. If the bottom of the carburetor is flat I don't think you will have any problem with vacuum leaks between base and heat shield.

I have same problem with my AFB but the car will idle to approx 500 rpm, idle adjustment screw is all the way out just touching the throttle shaft.

[Jeff B.]

Re: AFB Carb Vacuum Restrictor

There is no PCV valve, there is just a fitting on the base of the back of the carb where the PCV hose connects. This fitting has only a small hole that is supposed to restrict (meter) the vacuum flow. My fitting is aftermarket so I was wondering in the hole was the correct size (or maybe to big). From there the hose routes to a nipple high up on the oil filler tube. So the only thing that meters air being pulled into the carb vacuum inlet is the size of the hole in the fitting. Simple, but if the hole is the wrong size I want to start there. And being it's an after market part, I find it suspect.

[John H.]

Re: AFB Carb Vacuum Restrictor

Does anyone know the correct bore size for the restrictor?

Jeff -

The similar (but different) restrictor fitting that goes in the side of the '64-'65 Holleys has an .090" orifice; I can't find the spec for the orifice in the AFB fitting - maybe someone else has it noted.

[Jeff B.]

Re: AFB Carb Vacuum Restrictor

Well, mine is .096". Seems pretty close. I took the carb back off and found that the primary throttle shaft was a bit loose. It was recently rebuilt at Holley's custom shop. I would have rebuilt the carb myself but I could not get shaft bushings for the AFB so I sent it out to Holley's custom shop because they said they would make bushing for the shaft. Guess what, they didn't. It came back looking real pretty but the further I get into it the more adjustments I find wrong an now this bushing thing. I was able to find some teflon bushings to seal the ends of the shaft. And once back on the car, I was able to get it idling down to 550-600. Good enough. There are no bushing kits for the AFB that I have been able to find, probably because they were not bushed from the factory. Still, there should be a repair for a worn AFB base plate. You would think someone would sell brass bushings and the correct size drill/reem in a repair kit. Or maybe I'll end up having to make them.

[Stuart F.]

Re: AFB Carb Vacuum Restrictor

Jeff;

I believe there are parts and equipment available to re-bush the AFB throttle shafts, in fact I'm certain others have posted about just such a fix they performed themselves. Check the archives. Unfortunately, to replace the "base Plate" as you know, you might have to replace the entire carburetor as it is of the two section design.

My 63 now has a 64+ return spring (black) as well (the 63 white springs do not work). Although mine being a L76, idles at around 850 to 900 RPM, I still must use a small booster spring to maintain a steady idle. But I can see how that can be a problem for you as you must remove any such spring in order to pass your inspection. My AFB is a 3721SB replacement that I put on new back in the early 80's and has no more that 10k miles use on it and no sign of any shaft wear, as one might assume. I think it is somewhat the nature of the beast. I also have a spare 3720SA which has seen a lot more use and it too requires the booster spring.

You say you checked the fit/seating of the throttle plates in the bores. Were they removed by the rebuilder? I have seen some where they broke a screw or two and put the carb back together that way which allowed the plates to turn slightly in service, i.e. loose plates.

Stu Fox

[Domenic T.]

Re: AFB Carb Vacuum Restrictor

I have rebushed carbs and find that the shaft also gets worn also. That's a double bush. The only kits I found were for my aircraft carb which only allows you to bush the case. I had to machine my carb bushings. If you do it correctly the carb base bushings press in and that makes the inside diameter smaller, then you ream to size. But if you allow for this and make the bushing inside diameter slightly larger it works.
Also the most important thing is to make sure you are in the center of the worn hole in the carb base and use a long piloted reamer to make sure the holes for the shaft are true to each other.
You can also order reamers to your liking and ream the base going in one side and out the otherside of the carb base.
The hardest part on some carbs can be the removal of the screws that hold the butterflies.

DOM

[Clem Z.]

Re: AFB Carb Vacuum Restrictor

you can get carb shaft bushing here. also other parts to repair carbs. http://www.carburetion.com/quadrajet.asp

[Jeff B.]

Re: AFB Carb Vacuum Restrictor

Thanks for all the great info guys. I found this kit with 3/8" bushings and a reamer that looks pretty fool-proof. Big price tag though. The busings are only 5.00 a pair on the site Clem suggested, so the trick is to find a reasonable cost reamer that fits the OD of the busings.

[Timothy B.]

Re: AFB Carb Vacuum Restrictor

Jeff,

I just looked at the website Clem provided for you. The 3/8 bushing at .375 ID and .403 OD are the same as valve guide bushings. If you go to a machine shop they are available. The reamer the machine shop lent me has a pilot that's approx. 1" at .375(3/8) then the OD of .404 for the bushing.

You still have to go in from both sides so be careful and when I did my afb I stayed off the inside edge by approx 1/16" so it's like a stop for the bushing. They also have reamers with a 3/8" pilot that increase to .376-.377 etc. to open the new bushing so the shaft does not bind.

Do a archive search on carburetor bushings and you can see how I did bushings in my carburetors. That is how I did my afb primary shaft but it still will only idle the engine to 500-600 RPM. I fixed the uneven base that was damaged from the heat slot and the car idles with 19" vacuum at 600rpm. Make sure you plug the hot slot holes, this makes a big difference in gasoline temperature.