Very Weird Carter AFB Issue

[Paul B.]

Hi,
So I bought a correctly dated 3461S AFB for my 63 300HP, it had supposedly been sent out for rebuild many years back, but never put back on the car, apparently never had gas in it afterwards.

It had been been blasted, clean inside and out, new hardware. I blew out the passages just in case, changed the needles and seats, adjusted the floats and put it on the car.

The car ran OK, a bit more tuning needed, but after the motor gets good and hot, say 180 on a hot day in traffic, something binds in the carb and RPM's will not drop back down from whatever max RPM you attained.

To be clear; the issue is in the carb, I can disconnect the throttle linkage with the car running and the lever on the end of the primary plates shaft will stay at whatever RPM its at, even all the way up, even with the the return spring connected.
It's hard to move, you can feel the resistance.

If you however you shut the car off, the lever and throttle plates instantly move freely as they should, start it again and it binds instantly.

I also disconnected the accelerator pump rod, tapped on both ends of the throttle plates shaft to see if it would free up, no difference.

When it cools down, it works fine as it should, stays good until the next time it's really good and hot.

Ideas anyone?

Thanks,
Paul

[Duke W.]

Re: Very Weird Carter AFB Issue

Given enough miles the throttle shaft bores wear and engine vacuum at idle can move the shaft causing it to bind a bit. he first symptom is usually an inconsistent idle speed. A common "fix" is a heavier return spring, but this will ultimately increase wear and make the problem worse.

Renewing the throttle shaft bores is not part of a normal "rebuild", but the specialist carburetor restorers can probably do it by boring the main body to accept pressed in bushings and then precision reaming them to size.

It could also be a thermal problem. Assuming you don't normally drive the car in very cold weather, it's a good idea to plug the heat slot with Welsh plugs or drill/tap for pipe plugs. Also wire the heat riser open.

Disconnect the return spring and try to wiggle the throttle bore shaft with the throttle lever. If you notice play, the throttle shaft bores are worn and need to be repaired.

Duke

[Dan B.]

Re: Very Weird Carter AFB Issue

Yes, as Duke mentions the throttle shafts which are steel, will wear an oval pattern in the aluminum carb bore and produce the condition you describe of uneven idle speed on return.

I bought a kit which includes the proper reamer, alignment tool, and new hardened bushings and did both of mine. It was fairly easy to do (depending on your skill level) and made a big improvement in steady idle.

[Paul B.]

Re: Very Weird Carter AFB Issue

Hi Duke,
Thanks for the detailed reply as usual!

The heat slot is plugged with pipe plugs and the heat riser has been removed, also has the phenolic spacer with the slot blocked.

It was scary as hell when the carb jammed up the second time, it had been driving OK and I thought it was resolved, but at high RPM's as I was merging onto the highway it wouldn't come back down when I lifted, fortunately I got to the key quickly and shut it off, that caused it to immediately drop back down to idle position so that I could restart it and get it home at lower RPM, fortunately I was close by.

I just spoke with Don Oaks of The Carb Guys near Toronto, very well regarded for his carb restoration work by the Toronto NCRS chapter guys and others in this part of the world.

He can do a full rebuild including align hone and throttle shaft bushings, he is confident that the issue will be resolved.

I'll first contact the seller and ask to return the carb for refund, I paid Paypal with my Visa, or give him the option to pay for the repair, about half of what I paid for the carb. It clearly cannot be used in it's present state!

Thanks,
Paul

[Mark E.]

Re: Very Weird Carter AFB Issue

I don't know about AFB, but with Quadrajets, it can be a hunt to find a decent core that has the proper part number and date. And when you do, it can be 4 figures. If this is the case with AFBs, depending on what you paid, you may want to just fix it and keep it. It's a relatively minor wear item.

[Paul B.]

Re: Very Weird Carter AFB Issue

Thanks Dan and Mark, sorry I missed your reply Dan when I responded to Duke.

Just to clarify some, the condition is not a mild increase in resistance, it basically locks the shaft in position whenever it occurs, you have to use significant force, or kill the motor, to get it to move again. The return spring would have to be the size of a garage door spring!

If I have to pay for the repair, I will be into four digits Canadian for a carb that will hopefully work well, but will not have the plating and other cosmetics redone, I can get fully restored with cosmetics for pretty much the same amount.

The seller seemed to be a decent guy to deal with, hopefully it will all workout OK.


Thanks,
Paul

[Dan B.]

Re: Very Weird Carter AFB Issue

Here's the basic procedure:

https://www.youtube.com/watch?v=buwnm0lvGp8

Not the best video, but you get the gist of it. The kit is around $40 US. If your carb is otherwise good and correctly dated, this would be my choice vs. spending big bucks for another carb.

[Duke W.]

Re: Very Weird Carter AFB Issue

Just to clarify some, the condition is not a mild increase in resistance, it basically locks the shaft in position whenever it occurs, you have to use significant force, or kill the motor, to get it to move again. The return spring would have to be the size of a garage door spring!

Thanks,
Paul

This is not an issue I am familiar with. The inconsistent idle speed I mentioned is nothing like a jammed throttle. It's just a nuisance. What you have is a serious safety issue, and I wonder if the problem is so severe the carburetor may not be salvageable. It's especially perplexing that the problem only occurs when the engine is fully warmed up.

Duke

[Dan B.]

Re: Very Weird Carter AFB Issue

Possibly one of the butterflies hitting due to the play in the shaft when hot.