'65 AFB Problems

[Mike M.]

The original Carter AFB carb on my 300hp/327 has been rebuilt and runs fine. The problem is the idle which will not stay at a set rpm (it varies from about 500rpm to 750 rpm+/-). We think that the throttle shaft is worn (it has some noticeable play in it) and either needs replacing or, if available, a bushing kit put in. Do any of you folks know if such a bushing kit is available or if a replacement throttle shaft is available and , if so, who is the vendor. Thanks for any help.
Mike

[Mike G.]

Re: '65 AFB Problems

The repro springs are weak. My 3721 suffered from the same issue. A new stiffer spring eliminated it.

[Mike M.]

Re: '65 AFB Problems

Mike: You mean the throttle return spring. Right?

[Jim D.]

Re: '65 AFB Problems

Sounds more like you have the wrong vacuum advance unit on your dist. That's a very common problem cause of the problem you have.

[Mike M.]

Re: '65 AFB Problems

The car has the original distributor. I think the throttle shaft/bushings are worn out after 45+ years of use. We will see.
Thanks for the responses.

[Mike G.]

Re: '65 AFB Problems

Mike: You mean the throttle return spring. Right?

Yep. Found many reports of weak springs causing inconsistent return to set idle speed. Doubling up with a helper spring was a fix mentioned often.

[Clem Z.]

Re: '65 AFB Problems

make sure the mechanical advance weights in the distributor are not sticking in the advanced position as this will increase the idle speed

[Stuart F.]

Re: '65 AFB Problems

I have always had to use a booster throttle return spring on my 63, whether I was running my 3461S rebuilt original spec carb or my preferred replacement 3721SB. My 3461S has a loose primary shaft too, but does not seem to be affecting the idle. I had already purchased a set of bushings from "Carbs Unlimited" and discussed doing the work with a rebuilder and they talked me out of it based on how I described the operation of the carb (a slight lean surge condition).

I use a light spring from the second hole in the standoff bracket to the lower hole in the throttle arm. It is easy to take off and put back on for show/judging vs. driving. Mine is an L76 340hp so I idle it at a steady 850 rpm.

Stu Fox

[Mike M.]

Re: '65 AFB Problems

I have always had to use a booster throttle return spring on my 63, whether I was running my 3461S rebuilt original spec carb or my preferred replacement 3721SB. My 3461S has a loose primary shaft too, but does not seem to be affecting the idle. I had already purchased a set of bushings from "Carbs Unlimited" and discussed doing the work with a rebuilder and they talked me out of it based on how I described the operation of the carb (a slight lean surge condition).

I use a light spring from the second hole in the standoff bracket to the lower hole in the throttle arm. It is easy to take off and put back on for show/judging vs. driving. Mine is an L76 340hp so I idle it at a steady 850 rpm.

Stu Fox

Stu: I had tried the second spring set up this morning and that seems to help the idle remain constant on the carb. I likely will get the bushings from the source you mention as my rebuilder is saying that the throttle shaft bushings are worn and need replacing. Thanks to you folks for the advice.
Mike

[Stuart F.]

Re: '65 AFB Problems

The AFB's, as built, do not have any removable or replaceable shaft bushings perse. The throttle shafts ride directly in the shaft holes bored into the base aluminum material. To insert the new Bronze type bushings, these shaft holes have to be reamed to a specified dimension to accept the bushings which are then press fit into place, but "only" in the outer bores, not the inner bores (those inside of the throttle butterflys).

I had tested my 3461S using starting fluid both at idle and POT, spraying it directly at the shaft ends and did not experience any increase in RPM, as might be expected if there was a significant vacuum leak present there. The only other thing that concerned me was that, with a loose primary shaft, the amount of exposed transition slot was more in one throttle bore than the other at idle and immediately off idle. What effect this may have in the low speed operation is a matter of speculation, but may be one source of the slight lean surge I've continued to experience with this carb, even after many experiments with jet sizes and air bleeds, etc.

Stu Fox

[Mike M.]

Re: '65 AFB Problems

Stu: Is the throttle shaft diameter 3/8" (.375)? Or, do you need the shaft bushings that are 3/8"? One bushing kit comes with a correct drill bit to be used to ream the existing holes. Tks,
Mike

[Stuart F.]

Re: '65 AFB Problems

Mike;

The bushing sets I got from Carbs Unimited did not come with a "proper size" drill bit. Again, I have not installed them as I was advised against doing so by a tech at "All American Carburetors" (they have done great work for me in the past), primarily because my problem with my 3461S is not a stable idle problem. I will impart some of the wisdom I gained from other members of the board who have done the procedure FWIW;

One method would be to set the carb base up for drilling/reaming in a drill pressusing a 3/8" diameter rod thru the shaft holes in the base, then chuck the rod into the drill press. Put the drill press vise on the drill press table and tighten the vise and tighten the vise around the rod supported carb base to align the bores in the base. Then, use a "proper size" reamer (?) for the bushings. this procedure would have to be done for both sides.


Use a piloted reamer so as to just ream the outer shaft holes on either side with the pilot engaged in the inner holes, but do not ream the inner holes.

Incidentally, most discussions I have had with members who have done this procedure indicate it was not necessary to do until they reached about 100k service with the carburetor, so you can see it is not a usual source for problems like some would want you to believe.

Hope this helps.

Stu Fox

[Mike M.]

Re: '65 AFB Problems

Stu: Thanks for the help. Right now doubling the throttle return spring seems to be working pretty fine. I think I will run with that fix (my carb guy says it is just a 'crutch' instead of a fix, but it was cheap and easy!) My car has 82000 miles on it. Tks again.
Mike