Throttle Problem 63 AFB Carburetor

[Frank D.]

Re: Throttle Problem 63 AFB Carburetor

There are only a few possibilities for such symptoms. A car will not start running at a higher RPM unless its getting more air/fuel mixture. So, your carb is calling for more gas down its throat in these circumstances... The throttle plates must be opening for that to happen. They open because of the throttle linkage. Hence, your linkage is hanging up, your throttle plates are binding, or, your carb is staying up on the fast idle cam intermittently. Carefully operate the linkage by hand through its entire stroke while watching closely. Is the linkage hanging up on the ignition shielding, binding in any way, or, stuck on the fast idle cam ? Watch the action of the accelerator pump plunger and make sure it operates straight and smoothly too. Conversly, the next time the problem occurs remove the air cleaner and nudge the linkage forward and if the idle settles down you may see where its binding.

If you're tech savvy take a video of the carb when the problem is occurring and post in on youtube and provide a link here.

[David F.]

Re: Throttle Problem 63 AFB Carburetor

Carburetor.jpg
Hi, I finally got to try looking at the throttle valve plates and linkage. The choke plates operate normally and no hang up on fast idle cam. The valve plates appear to be closing (by eye observance), but the throttle linkage sometimes does not return fully. I not sure if the spring return is not strong enough or there is some binding on the plate shaft. I took a picture of linkage and spring. Is there someway to check and eliminate shaft binding or check for?

[Frank D.]

Re: Throttle Problem 63 AFB Carburetor

With the linkage disconnected along with the return spring operating the throttle by hand should indicate if their is any binding; however, I would review post #3 above once more. My own AFB had movement between the two plates and would inconsistently return to idle....I ran two return springs for a while as a stopgap - but they can present their own problems (excessive wear on the throttle ass'y). Look for looseness in the throttle plates at the red circle (first picture) and wear on the accelerator pump shaft guide lugs as shown (red arrows in second picture).

[Bob R.]

Re: Throttle Problem 63 AFB Carburetor

David,
The picture you posted above indicates an incorrect connection from your accelerator linkage to the carb. I'm not sure what you did there but it is not correct and you are missing a bushing where the linkage attaches to the carb. I don't know if this is causing your problem but it could be. Take a look at the picture below from Frank Dreano.

[Leif A.]

Re: Throttle Problem 63 AFB Carburetor

David,
As mentioned by Bob, you're missing the rubber bushing and nylon insert where your accelerator rod attaches to the carb. Also, your spring does not look correct (too long, no tension) nor is it connected to the correct location. It goes thru the hole on the end of the accelerator rod that goes thru the carb linkage (see Frank's pic). Be sure the nylon insert is inserted from the carb side of the linkage NOT the accelerator rod side. Hope this helps.

[Stuart F.]

Re: Throttle Problem 63 AFB Carburetor

David;

You have an incorrect AFB for a 63. No 63 has a throttle arm like the one in your picture, and that is why your throttle linkage is wrong.

Stu Fox

[Frank D.]

Re: Throttle Problem 63 AFB Carburetor

Yes....the Carter AFB 3721SB in my picture above is a service replacement carb (not original) but the throttle plate holds the linkage as original...

[Stuart F.]

Re: Throttle Problem 63 AFB Carburetor

Frank;

I was referring to David's picture, not yours. Your 3721sb looks entirely correct. David should look at your picture to see the differences. I have no idea what model he has, but it sure isn't for a 63 or any mid-year with an AFB be it a 3461s or any of the 3721 series.

Stu Fox

[Frank D.]

Re: Throttle Problem 63 AFB Carburetor

Its all good Stu - I got your intent....

[David F.]

Re: Throttle Problem 63 AFB Carburetor

You are correct my carburetor can't be the original. I looked in the Noland Adame Technical Guide Vol 2 and NCRS specification guide and the numbers don't match up. A '63 300hp build 2nd week of June should have numbers 3461S 3826004. I have on my brass tag MO DD 3721SB 3851716 which should be for a '64 300hp. What is strange the throttle valve lever (my picture #17 above) doesn't look like ones in books or your (picture #8 & #18). You stated I need a bushing for connecting throttle rod to lever, but there is little clearance between lever hole (15/64") and throttle rod diameter (1/4") for a bushing to fit.
Do I need a new smaller diameter connecting rod as well? I am really lost with nothing to compare to except books and your pictures. Any help would be appreciated.

[Bob R.]

Re: Throttle Problem 63 AFB Carburetor

I'm not sure what your carb is but a 3721 is visually identical to the 3461. Tags can be easily changed the carb number is stamped on the passengers side front base of the carb along with the date. At any rate the throttle arm on your carb is not set up for proper operation. The rod you have attaching to the carb doesn't look correct either. The photo posted earlier is the correct setup. You may want to take the carb to a vendor specializing in repairs and possibly he can locate a correct carb for you or modify your carb to work correctly. Original 3461 carbs are pretty expensive.3720 Carburator 2.jpgHere is a photo of an old 3720 carb I have and you can see the hole for the accelerator rod is large a bushing inserts into this hole and the shape of the throttle connection is different than the one on your car.

[David F.]

Re: Throttle Problem 63 AFB Carburetor

OK. You are correct this is not correct carburetor. Learning where to look (thanks to you guys) it is a mix of carter parts plus some other items.
The Tag reads 3721SB - 3851716 (probably just tacked on). Lower casting; has no stamping on flange with serial or date code, on back cast in is CARTER AFB, on left side is 0-1585. Top casting on right side is 6-1518. At this point I am considering finding a carburetor shop to see if it can be salvaged. If not and I will have to look for replacement (i.e. 3461S or 3721S). I jotted down the intake manifold number 3844457 GM1 T. Think this is not original either and should be 3844459.
Since this VETTE changed hands multiple times before me I don't know who did the rebuild. This and I'm sure many other short cuts are my problem now. I am thankful for all your input.

[Frank D.]

Re: Throttle Problem 63 AFB Carburetor

OK. You are correct this is not correct carburetor. Learning where to look (thanks to you guys) it is a mix of carter parts plus some other items.
The Tag reads 3721SB - 3851716 (probably just tacked on). Lower casting; has no stamping on flange with serial or date code, on back cast in is CARTER AFB, on left side is 0-1585. Top casting on right side is 6-1518. At this point I am considering finding a carburetor shop to see if it can be salvaged. If not and I will have to look for replacement (i.e. 3461S or 3721S). I jotted down the intake manifold number 3844457 GM1 T. Think this is not original either and should be 3844459.
Since this VETTE changed hands multiple times before me I don't know who did the rebuild. This and I'm sure many other short cuts are my problem now. I am thankful for all your input.

A Corvette Forum member is selling a freshly rebuilt 3721SB if that's what you are gunning for. Usually this carb is on a 300hp, manual tranny: http://www.corvetteforum.com/forums/...1-sb-carb.html

[David F.]

Re: Throttle Problem 63 AFB Carburetor

Winter is over and I have taken my 63 Vette out of the garage. Carburetor was still a problem. So I had it rebuilt by the I guy who only does carburetors. It was all cruded up from not running frequently. Cost $460 but it is now working great. So it is on to the next update project. Thanks for all your advise last fall.

[Chester C.]

Re: Throttle Problem 63 AFB Carburetor

"SB" is for an automatic trans car, thus the odd looking linkage.