Shaft Wear Limits on Carter AFB

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Tim;

Thanks for your response. What led me to the throttle bushings was that I feel I have tried most everything else without any noticeable improvement. I tried three different sets of primary clusters; from my 3720SA, 3721SB and the 3461S. Each has different low speed/idle jet tube sizes as well as air bleeds. The ones from the 3721SB are the richest and work perfectly in that carburetor (currently on the engine for the summer). There was no difference in the lean surge with any of them in the 3461S. If you could see the 3461S side by side with my old 3720SA (which has well over 40k miles on it). The primary shaft on the 3461S has about 1/16" play side to side when open, while the 3720SA has very little play if at all. You can wobble the shaft by hand a noticeable amount on the 3461S, while the 3720SA is almost perfectly stable (hard to feel any play by hand). Also, I had to idle the 3461S with the emulsion screws 2-1/2 turns open while my 3721SB needs only 1-1/2 turns.

Thanks again.

Stu Fox

[Duke W.]

Re: Shaft Wear Limits on Carter AFB

My experience with the 3461S is that when it got to about 100K miles, the idle speed would change depending on how hard the throttle was released to the stop. I attributed this to shaft wear. If the throttle was quickly released and hit the idle stop hard it would idle at say 850. Then if you opened the throttle slightly and returned it gently the idle would be 950. I attributed this to the shaft moving around in the worn bore and leaving a different flow area depending on how hard it hit the idle stop.

As far as driveability was concerned, I never had a problem. It was calibrated fairly rich to work on the 340 HP engine. In '64 when it was only used on the 300 HP engine the main and power systems were leaned out a bit, but the idle jet increased from .035 to .037".

On thing to look at is the hot idle compensator. It should definitely be closed at room temperature. I'm not sure at what temperature it opens, but if it opens too soon it could lean thing out.

Duke

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Duke;

Thanks for your comments. Ironically you should describe the inconsistent idle characteristic. I found that to be true with each of my AFB's, even when new. I have always used a light booster spring to make my idle return more consistent as it has always been a pet peave of mine. I even drilled a small hole in the throttle arm of the 3461S for that purpose, exactly like there is on the 3721SB I have, and run the light spring to the second hole in the bracket on the water manifold (front of the intake manifold).

As for the Hot Idle Compensator, I did remove it and used a small block off plate and gasket that I made. It did not help the condition, but it made sense to try as the condition seemed worse when the engine was full warm and run down the road a piece. The one that came with the carb from the rebuilder was junk, so I cut it up to make my block off plate template. Incidently, believe we talked about this before as I was considering what flow benefit might be gained by eliminating the "wings" on the compensator, and you surmised that they were there to act as a heat sink.

Thanks again. Have a great day.

Stu Fox

[Timothy B.]

Re: Shaft Wear Limits on Carter AFB

Stu,

If there is .060 play it's probably a good idea to have it bushed, you have tried everything else. I was told by Bob Kunz that the manufacturing differences in these carburetors is great, some are better than others.

Are the emulsion screws the same on each carburetor because some have a little different shaped point and would require a different adjustment. Other than that do you think the transfer slots are the same length below the closed throttle blade as this would also contribute more fuel and a different emulsion screw adjustment.

I think at this point I would go back to stock pcv and hot air choke as these are calibrated vacuum leaks and start over in my mind. On other forums I have read, this problem would be corrected by enlarging the idle feed restriction and or reducing the idle air bleed.

Is that Petronics you have one of those mulit spark units? Just a guess but do you think that could have anything to do with the surge.

[Stuart F.]

Re: Shaft Wear Limits on Carter AFB

Tim;

I did try different Emulsion screws, but only because some looked better than others - like one had a little ridge from overtightening inward. Otherwise, they were the same screws - at least to my eye.

Interesting situation I have found with the transfer slots; when the throttle is completely closed (idle set screw backed clear out), a small amount of slot is still exposed below the notch in the choke side throttle plate - and also the same proportional additional amount of slot is exposed as the throttle is cracked open. I noticed this with the body laid bottom side up. Now, some of this is due to the shaft play, but even when I hand close it to remove the play there is still some slot showing. This may be in line with what Bob Kunz told you about manufacturing clearances as my 3720SA does not show any of the slot.

About the idle/low speed jet tubes in the clusters; I would hesitate to run a drill into these tubes to enlarge them as they are pretty small to begin with and I could see where shavings could get trapped in the camber within the cluster. No amount of air pressure would ever get them out.

Regarding my Pertronics unit; it is the II unit along with their matching high voltage coil. During the course of trying to de-bug this 3461S, I have gone back to my 3721SB several times and the engine runs like a champ with it, so the finger points to the 3461S without any doubt.

Thanks for your interesting points. I'll keep working on it and keep you posted.

Stu Fox