Help with an AFB Disassembly

[Thomas R.]

[Timothy B.]

Re: Help with an AFB Disassembly

Thomas,

Make sure you remove the air cleaner stud as that threads into the lower casting.

[Thomas R.]

Re: Help with an AFB Disassembly

TimothyThat's it! Thanks

[Timothy B.]

Re: Help with an AFB Disassembly

Thomas,

I am curious if you have the original 3461 afb carburetor on the engine and if you have ever experienced this trailer hitching effect at other times while driving. It's a lean surge and some others here have reported issues like that in the past.

[Thomas R.]

Re: Help with an AFB Disassembly

TimothyNo The car has been running fine. The problem started when I ran it hard through the gears recently. The car sits in Florida a lot while I am up North. When I do start it I have to crank it until gas reaches the carb. I think that there is residue left in the float bowels because of this. After following all the threads on this problem I tried all the recommended fixes. Nothing has worked. I have use of a snap-on MT461 which allowed me to short individual cylinders as well as odd and even cylinders for a balance test. What shows up is a problem with the odd cylinders. There is little effect in adjusting the idle mixture on the left side. I believe this manifold splits the feed through the carb into left an right side. I think I may have clogged part of the low speed circuit on this side of the carb. I am waiting for the rebuild kit to arrive. I will post what I find and if it corrects the problem.

[Timothy B.]

Re: Help with an AFB Disassembly

Thomas,

If you have the lid off get some carburetor spray at the local auto parts store and use a wire tie like from a loaf of bread and probe all the little holes in the venturi. Use the spray to clean them but watch your eyes and car painted surfaces.

[Duke W.]

Re: Help with an AFB Disassembly

I believe this manifold splits the feed through the carb into left an right side. I

Each side of the carb on a 180 degree manifold feeds the two inside and two outside cylinders from opposite banks. Firing interval of these combinations is every 180 degrees, so it's the equivalent of a progressive two-barrel feeding two four-cylinder engines that are 90 degrees out of phase.

1 4 6 7
..8 3 5 2


You might be able to mitigate the problem by removing the idle mixture adjustment screw and spraying in carb cleaner. If that doesn't help pay special attention to the primary venturi clusters, which contain the idle jets, air bleeds, and emulsion tubes.

The nominal setting on AFB idle mixture adjustment screws is 1.5 turns out from the seat, and best idle should be somewhere between 1.25 and 1.75 turns out from the seat.

The 3461S is set up pretty rich in order to work on both the 340 and 300 HP engines. The follow-on 3721S/SA/SB, which were only used on the 300 HP config. had leaner setups on the jets and rods, but the idle jet size was increased from .035 to .037".

Duke

[Stuart F.]

Re: Help with an AFB Disassembly

As Tim knows, I have pretty well run the course on trying to cure a lean surge on my 63 L-76 when I have my 3461S on the engine. My slightly modified and sweetened 3721SB works perfectly fine and is my daily driver carb, so I know it is not ignition related.

The last change I am doing to the 3461S now is to have the primary throttle shaft bores re-bushed. The reason being, even though I was not experiencing any vacuum leakage around the shaft, I noticed on the bench that with a full return spring pull back on the throttle shaft at simulated idle speed setting the exposure of the transition slot was less in the left side throttle bore than in the right. It may or may not mean much, but that is what I am down to.

If this latest change doesn't solve the lean surge, that carb is going into a box for the rest of my life and it can accompany the car to the next owner as "the original carb".

Stu Fox

[Timothy B.]

Re: Help with an AFB Disassembly

Stu,

I hope you are feeling well, it's been a long cold winter here in MD.

I wanted to suggest something about the lean surge condition you are experiencing with your 3461 AFB carburetor. In the idle circuit there are two air bleeds, one is referred to as the by-pass which is part of the economizer. This economizer is a small restriction that acts like a transfer slot jet on a Holley. It's purpose at high vacuum conditions like cruise or deceleration it is to become choked so the engine will not get a over rich mixture of fuel.

The next idle air bleed is the one drilled into the top of the venturi cluster. That's the one you need to make smaller, measure the bleed diameter with a small drill bit and reduce it by .004-.005 and I think your problem will go away. If you drill the hole and tap for 6-32 brass allen set screws the proper size hole can be drilled into the set screws. It's simply a tuning measure and is not noticeable but I bet you engine will love it. By reducing this second bleed diameter the vacuum will pull less air and the transfer slot will remain active just a little longer as the blade opens.

[Stuart F.]

Re: Help with an AFB Disassembly

Tim;

Thanks for your expresion of concern. Like you folks up in winter land, I too missed out on a lot of great cool weather here during which I could have done a lot of project work - including my on going sorting out of my 3461S. Thanks for your interesting information about the air bleeds in the primary venturi clusters. You can understand if I tell you I'm reluctant to alter the clusters as they are unique to the 3461S. If possible, I'd prefer to do any such modifications on my donor 3720SA parts and test with them. The re-bushing of the primary shaft on the 3461S is not an alteration so much as it is a maintenance (rehab) function that should not affect the value of the unit as an OE. I have an will change main jets and metering rods because they are easily returned to stock OE.

Once I get the main body back with the new bushings I'll begin to test and re-test as needed to try and fix the lean surge.

Thanks again for you valuable ideas. I really appreciate them and will re-post on my efforts.

Stu Fox

[Thomas R.]

Re: Help with an AFB Disassembly

Thanks for all the help and advice everyone! The problem has been solved with a complete rebuild of the carb. Upon disassemble I found a small amount of crud in the left bowel. The crud was drawn into the car and affected the engine as described. It looked for all the world like an electrical problem. I used carb cleaner and blew out all the passages. I tried to blowing carb cleaner through the idle mixture screw holes but that didn't work. The car is back to running great again. Anyone have a suggestion to prevent this from happening while the car is in storage? I do use stable in the gas. Whatever gas is left in the carb evaporates while it sits.

[Stuart F.]

Re: Help with an AFB Disassembly

Tom;

I'm here in Florida as well, but my car doesn't sit as long as yours. However, I too have float bowl evaporation and if my car sits longer than a week, I always prime it using a turkey baster. I just pop opin the gas cap and draw out a small portion of gas and inject most of it into the left primary vent tube and let the rest spill into the throttle bores. Then, the engine hits off right away. I also use non-ethanol gas which is available at stations near a lake that has power boats. I've found it holds up better than straight pump gas as far as residue is concerned.

Once I get my 3461S back I will be testing it for lean surge again and will post the results.

Stu Fox