Shaft Wear Limits on Carter AFB

[Stuart F.]

Does anyone know a good measurement or "rule of thumb" to follow when deciding whether to re-sleeve the primary shaft (bushings) on a Carter AFB - in this case a 3461S from my 63.

I've tried everything I can think of to get a low speed/high vacuum lean surge (herky jerky) corrected, including; any/all jetting, sealing every other "peculiar 3461S" vacuum leak possibility (vapor vents, secondary shaft holes in body under air valve weights, etc), different primary clusters, PCV valve, choke port in main body, different top plate, thorough inspection for porosity cracks - all to no avail (you name it, I've tried it).

This carb does not seem too bad (shaft play), but it certainly has more than my other (2) 3720 series AFB's.

STU FOX

[Clem Z.]

Re: Shaft Wear Limits on Carter AFB

Does anyone know a good measurement or "rule of thumb" to follow when deciding whether to re-sleeve the primary shaft (bushings) on a Carter AFB - in this case a 3461S from my 63.

I've tried everything I can think of to get a low speed/high vacuum lean surge (herky jerky) corrected, including; any/all jetting, sealing every other "peculiar 3461S" vacuum leak possibility (vapor vents, secondary shaft holes in body under air valve weights, etc), different primary clusters, PCV valve, choke port in main body, different top plate, thorough inspection for porosity cracks - all to no avail (you name it, I've tried it).

This carb does not seem too bad (shaft play), but it certainly has more than my other (2) 3720 series AFB's.

STU FOX

new holley shafts are .368 with a .375 bore in the base plate

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Clem;

Thanks for your response - so some where in the neighborhood of .007" clearance. Sounds reasonable.

Stu Fox

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Guess you and I are the only ones interested in old carburetors. I've decided to re-bush my primary shaft on this 3461S, even though it does not seem too bad. I've tried everything else and I hate to give up on it now. I'm more concerned with the end play than the slight shaft looseness.

Thanks again for your comments.

Stu Fox

[John H.]

Re: Shaft Wear Limits on Carter AFB

Add me to the list.Let us know how it goes with the bushing of the carb , as I will have to do the same with mine in the near future.

[Clem Z.]

Re: Shaft Wear Limits on Carter AFB

Does anyone know a good measurement or "rule of thumb" to follow when deciding whether to re-sleeve the primary shaft (bushings) on a Carter AFB - in this case a 3461S from my 63.

I've tried everything I can think of to get a low speed/high vacuum lean surge (herky jerky) corrected, including; any/all jetting, sealing every other "peculiar 3461S" vacuum leak possibility (vapor vents, secondary shaft holes in body under air valve weights, etc), different primary clusters, PCV valve, choke port in main body, different top plate, thorough inspection for porosity cracks - all to no avail (you name it, I've tried it).

This carb does not seem too bad (shaft play), but it certainly has more than my other (2) 3720 series AFB's.

STU FOX

did you try richening up the idle fuel restriction a few thousands ??

[Domenic T.]

Re: Shaft Wear Limits on Carter AFB

Stu,
Me too.
I did some carbs and found it most accurate using a reamer long enough to go thru both holes at the same time.

I also made a sleeve and put it on the reamer shaft after it passed thru the first hole to keep it exactly centered for the oppisite hole.

Piloted reamers work the best and I ordered one and had it made to size and it works great.

I also had to make the bushings for my carbs. It improved the idle.

The bummer was that the carb shaft also wore so I had to make a thin sleeve and slide it over the shaft with JB weld under it to fill the worn area.

It was sleeve on sleeve.

DOM

[Clem Z.]

Re: Shaft Wear Limits on Carter AFB

use a 3/8" diameter rod in thru the holes in the base plate and put the rod into the drill press chuck. then i put the drill press vise on the drill press table and tighten the vise around the rod supported base plate. this aligns the bores in the base plate and just use the proper size reamer for your bushings. you have to do this for both sides. get the bushings here. http://www.carburetion.com/Carter4TQ.htm

[Timothy B.]

Re: Shaft Wear Limits on Carter AFB

Stu,

Before you do that give some thought to Clem's suggestion about enlarging the idle jet two or three thousands. Why don't you call Bob Kunz just to ask him before more $$$$, his # is somewhere in the archives.

Sounds like you are experencing a lean surge and I remember your not the first with that engine combo to report a surge. At that low speed high vacuum condition the engine is running mostly on the idle circuit/transfer slot which is controlled by the idle feed restriction. If retarding the timing some does not help it's the only other thing I can think of.

[Clem Z.]

Re: Shaft Wear Limits on Carter AFB

i remember my 63 340 HP with the AFB had power piston rattle as they would pulsate up and down at idle

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Dom and Clem;

Thanks for the advice and reaming method information.

On the idle fuel restriction, I assume you mean the idle/low speed jet tubes in the primary clusters. I tried 3 different clusters including those that work so well in my 3721SB, i.e. w/.037" idle tubes. The standard 3461S has .035", and I even tried those from my 3720SA which have .033". With each set, of course, you have different by-pass air bleeds which usually are .048" (3720SA), .050" (3461S) and .052" (3721SB). None of the cluster changes made any improvement.

This carb was restored (reconditioned) by a well known NE vendor, and if I listed all the things I found wrong with it, it would blow your mind. Suffice it to say that I never thought I'd have this much trouble replacing my original correct model and dated carb. Just because it looks good and new from the outside does not make it right on the inside. I'm also very disappointed with the carb parts vendors.

Thanks again for your help. I'm not too well set up to do this kind of work at my home here in Florida, so I guess I'll have to pop for it. I've had good luck with one rebuilder here in State so I'll probably have to pay the going rate to get it done right.

Stu Fox

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Tim;

I spoke with Bob Kunz at some length here this past January during the winter meet, and have followed some of his pointers to troubleshoot this problem. So far, I have not found the answer.

One other theory I'm working on is with regards the timing in that there is a difference in the idle transfer slots, but I have no way to measure it. In effect, this carb idles with the butterflys closed slightly more than those of my 3721SB, and I need to open the idle screws more to achieve the same idle speed and vacuum readings. Now then, I am using a B28 VAC instead of a standard B26 which gives me more vacuum advance timing at low speed/off idle. Perhaps it is too much - hard to believe, I know. I did back down my mechanical advance slightly, but it did not seem to help. So, is this situation due to shaft vacuum leak or the design of the idle/low speed circuit of this carb??

The shame of it all is this 3461S has very good POT and WOT, better than my 3721SB (noticeably so), but is not liveable for cruising. One other clue might be that the condition is most noticeable after the engine is fully warmed and gone down the road a piece. The first 10 minutes after choke off it seems fine.

With regards the power pistons Clem, I have tried every spring combo, main jets and metering rods in my quiver to no avail.

Thanks again fellows.

Stu Fox

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Just an update to those who might be interested: I have prepared my AFB to send to a shop for the re-bushing. I called two of the more prominent carburetor rebuilders/restorers and pretty much got the same opinion, based on their concern for saving me money. They say that my description of a high vacuum "herky jerky" surge, typically in first, second or third gear does not sound like a shaft vacuum leak. Rather, they both say it (shaft leak) will usually show up as a surging (unsteady) idle with the engine fully warmed. One went so far as to quote me the theory of how air will follow the path of least resistance, i.e. through the air horn and not past a loose throttle shaft. As such, they say they rarely re-bush the shaft in their rebuild processes. That explains why my carb was re-conditioned w/o correcting what I believe is an excessively loose primary shaft. My reasoning was based on the other AFB carbs I have, one which I know has a great deal of mileage on it and it is very snug, both in shaft fit in the bores and the end play. One shop says they will do it for me, but rather prefer I remove both shafts and all linkage (around $25.00 + taxes and shipping). To have them disassemble and re-assemble they want between $50.00 and $75.00 + tax and shipping. Obviously they would prefer to spend their time on full rebuilds where the money is. I just wish I had the equipment to do it myself, specifically a good drill press, etc., but I don't and I can't see doing that level of investment at my age.

Stu Fox

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Hope some of you who showed interest are still on board. If not, hope you have a Happy Father's day.

Stu Fox

[Timothy B.]

Re: Shaft Wear Limits on Carter AFB

Hi Stu,

I don't want to discourage you from throttle shaft bushings but I tend to agree with the carburetor experts you spoke with. I think the lean surge condition you are experencing may also be corrected by reducing the air bleed size (sq in). This will change the A/F ratio just like opening the idle feed restriction. You may want to try .006 smaller dia. using something that can be attached over the idle circuit air bleed hole just as a test. J&B weld will do the job but I understand about those original venturie clusters.

From your description of the problem I don't think power enrichment springs or main jet size will correct the problem because at that throttle plate position the main fuel contributor is from the idle circuit which is curb idle discharge ports and transfer slot.