Another AFB Question - 1963

[Joel T.]

Hi All;

I finally finished rebuilding that second AFB and got it installed on my 1963 340 car... I made a few deviations from the pure stock set up.. namely, I installed .071 secondaries (versus the .069s) and installed slightly thinner metering rods; .057/.067 (versus the .060/.069).. The results were better than I expected!! There is a noticeable improvement in driveability!!

I'm getting ready to rebuild the original 3461S basically to match the replacement and hopefully replicate its performance...

Question.... The metering rod springs... I see where you can get different springs with different "Hg ratings... Is there any guidelines as to which springs to use? I have stock springs in there now which are silver in color.

Thanks!

Joel

[Clem Z.]

Re: Another AFB Question - 1963

Hi All;

I finally finished rebuilding that second AFB and got it installed on my 1963 340 car... I made a few deviations from the pure stock set up.. namely, I installed .071 secondaries (versus the .069s) and installed slightly thinner metering rods; .057/.067 (versus the .060/.069).. The results were better than I expected!! There is a noticeable improvement in driveability!!

I'm getting ready to rebuild the original 3461S basically to match the replacement and hopefully replicate its performance...

Question.... The metering rod springs... I see where you can get different springs with different "Hg ratings... Is there any guidelines as to which springs to use? I have stock springs in there now which are silver in color.

Thanks!

Joel

the springs determine when the enrichment starts so if you have a bog when accelerating that could mean the carb is not richening up soon enough. also if it richens up too soon the MPG will suffer. when we used to have corvette club economy runs back when gulf oil and grady davis,who was in our corvette club gave us free gasoline i would remove the spring from my AFB car so it would stay on lean step all the time.

[John D.]

Re: Another AFB Question - 1963

Clem, Remember when Richard Grabiak and old JD won that economy run with a 65 250HP dead horse AC car?

[Timothy B.]

Re: Another AFB Question - 1963

Joel,

If you want to taylor enrichment to you particular engine, take vacuum readings at different cruise speeds 25, 35 etc. then adjust spring to raise metering rod approx 2" lower than the lowest cruise vacuum reading.

Make sure you account for uphill in forth gear because if the enrichment comes in to soon you will get bad gasoline mileage. If you have vacuum guage and a sorce for vacuum you can do this on the work bench provided you know the vacuum readings..

[Stuart F.]

Re: Another AFB Question - 1963

Joel;

Changing the metering rod springs is a very delicate/fine Hg. adjusment procedure that, to be done right, almost requires an exhaust gas analizer or perhaps a dyno. I never considered it as a home garage experimental item. Perhaps it is more something that one might do at a race track to compensate for temperature or humidity conditions. Otherwise, from one day to the next, you'd probably not notice the difference in driveability. The most I have ever done, on a carb that has seen a lot of service, is measure their relaxed length and then maybe stretch them a tad (1/16th) to compensate for the effects of their time in service. Silly, I know.

Sounds like your jetting is pretty close to mine now. I did find an additional improvement for stumble by using a slightly larger pump squirter. Actually, when talking about bog or stumble, a lot of difference can be found on two cars using different gearing. My carb, which now works perfect on my car with close ratio and 3.36 final, may not work well on another with 3.70's or 4.11's.

Besides the above, I use Tomco inlet valves (in place of the needle/seats to allow lower float levels), a 300 hp (L-75) phenolic spacer (to reduce carb temp by 35 to 40*), and an electric choke kit (to eliminate choke hang up from the equation). I'm lovin it!

Stu Fox

[Stuart F.]

Re: Another AFB Question - 1963

I guess Tim's procedure might work, if you're really serious about doing something. You'd probably need an assistant and a darn good vacuum gauge setup in your car. I have such a gauge and the time if only you lived just down the street. Come to think of it, Tim may have just given me a reason to put that sucker in my car - temporary of course.

Stu Fox

[Joel T.]

Re: Another AFB Question - 1963

Joel;

Changing the metering rod springs is a very delicate/fine Hg. adjusment procedure that, to be done right, almost requires an exhaust gas analizer or perhaps a dyno. I never considered it as a home garage experimental item. Perhaps it is more something that one might do at a race track to compensate for temperature or humidity conditions. Otherwise, from one day to the next, you'd probably not notice the difference in driveability. The most I have ever done, on a carb that has seen a lot of service, is measure their relaxed length and then maybe stretch them a tad (1/16th) to compensate for the effects of their time in service. Silly, I know.

Sounds like your jetting is pretty close to mine now. I did find an additional improvement for stumble by using a slightly larger pump squirter. Actually, when talking about bog or stumble, a lot of difference can be found on two cars using different gearing. My carb, which now works perfect on my car with close ratio and 3.36 final, may not work well on another with 3.70's or 4.11's.

Besides the above, I use Tomco inlet valves (in place of the needle/seats to allow lower float levels), a 300 hp (L-75) phenolic spacer (to reduce carb temp by 35 to 40*), and an electric choke kit (to eliminate choke hang up from the equation). I'm lovin it!

Stu Fox

Hi Stew;

I also followed your advice and put in the larger squirters... Stock calls for .028... I went with .033... I'm really happy with the result! I had the car out yesterday up here in NJ, the weather was great... 4th gear acceleration was just outstanding, and the little stumble I had was gone....

When I pulled the original, I found what I had hoped I would not... raw gas sitting in the intake... I pulled the carb apart... The "drop" on the floats was WAY off, I mean like 3x what they were supposed to be.. Top clearance was about right. In any event, I will rebuild the unit and put it back in....

Joel

[Joel T.]

Re: Another AFB Question - 1963

Joel,

If you want to taylor enrichment to you particular engine, take vacuum readings at different cruise speeds 25, 35 etc. then adjust spring to raise metering rod approx 2" lower than the lowest cruise vacuum reading.

Make sure you account for uphill in forth gear because if the enrichment comes in to soon you will get bad gasoline mileage. If you have vacuum guage and a sorce for vacuum you can do this on the work bench provided you know the vacuum readings..

Hi Tim,

Thanks for the suggestions.. I think I can arrange to do what you have outlined... This engine is running a cam slightly hotter than stock, and my vacuum levels at idle are fairly low... down in the 11-12" range as I recall.

I'm probably pretty late to the party on this one, but I was impressed with the AFB and can see how many different things you can do with one basic unit... My buddy knocks them based upon their relatively small fuel bowls... Back in the day when he raced, he tells of using oil burner feed tubing for fuel supply with an electric pump to keep the unit supplied!

Thanks again!

Joel

[Clem Z.]

Re: Another AFB Question - 1963

Hi Tim,

Thanks for the suggestions.. I think I can arrange to do what you have outlined... This engine is running a cam slightly hotter than stock, and my vacuum levels at idle are fairly low... down in the 11-12" range as I recall.

I'm probably pretty late to the party on this one, but I was impressed with the AFB and can see how many different things you can do with one basic unit... My buddy knocks them based upon their relatively small fuel bowls... Back in the day when he raced, he tells of using oil burner feed tubing for fuel supply with an electric pump to keep the unit supplied!

Thanks again!

Joel

no worst than a Q jet which only has 1 small float bowl to feed all 4 barrels.

[Stuart F.]

Re: Another AFB Question - 1963

Joel;

Seems we're on the same page and perhaps I have repeated myself, like old men are apt to do on occasion. Really glad you're seeing improvement with yours. Removing the hesitation or stumble really makes driving our cars a whole lot more fun to drive. Now, all we have to do is stay ahead of their dilution of our pump gas with corn products (chicken feed).

Stu Fox

[Timothy B.]

Re: Another AFB Question - 1963

I think the AFB is one of the best carburetors out there for our old cars. I am aware Q-Jet is a better carb because of the small primary venturies.

I also agree that changing springs to alter power enrichment on a stock set up is not noticable. If the engine has a high performance camshaft there is tuning that can make a difference and enrichment can also help with detonation.

If you know your engine cruise vacuum and where you want enrichment to start the springs can be taylored to allow the rods to raise provided you have access to a vacuum source and guage. On a Holley it's easier because the opening point in inches of vacuum is stamped into the power valve.

[Duke W.]

Re: Another AFB Question - 1963

Hi All;

I finally finished rebuilding that second AFB and got it installed on my 1963 340 car... I made a few deviations from the pure stock set up.. namely, I installed .071 secondaries (versus the .069s) and installed slightly thinner metering rods; .057/.067 (versus the .060/.069).. The results were better than I expected!! There is a noticeable improvement in driveability!!

I'm getting ready to rebuild the original 3461S basically to match the replacement and hopefully replicate its performance...

Question.... The metering rod springs... I see where you can get different springs with different "Hg ratings... Is there any guidelines as to which springs to use? I have stock springs in there now which are silver in color.

Thanks!

Joel

Back when my 340 HP SWC was new and had idle stability problems, as idle revs dropped I began to heard a "rattling" sound. It turned out to be the metering rod pistons "dithering" back and forth due to low and fluctutating manifold vacuum as idle speed dropped off, and unless I blipped the throttle, the engine could stall.

I corrected the idle stabililty problem by replacing the 201 VAC with a 236, so I never changed the metering rod springs, but I did install slightly larger secondary jets and different rods that richened the power side, but not the cruise side. At the time I lived in Seattle, so given sea level and mild temperatures, air density was usually high compared to most parts of the country.

If you have a cam that pulls less manifold vacuum than the Duntov or LT-1 cams (about 12" @ 900) you probably need a slightly richer mixture at idle and light load.

I never did notice a SOTP difference in the jetting. Since the engine was pulling a 3.08 gear through a CR four-speed, and I tried to minimize clutch slip on start-up, I accepted the occasional start-off stumble, but with the clutch engaged it would pull cleanly from less than 1000 revs in any gear.

Since the early AFBs were tuned to work on both the 300 and 340 HP engines, they are a little on the rich side. Later AFBs that were only used on the 300 HP engine were a little leaner on the main and power side, but had slightly larger idle/low speed jets that are built into the primary venturi clusters - .037" rather than .035" I think.

I believe the '63 Shop Manual states the manifold vacuum range that the FI system should transition to the power stop - something like 5" Hg, so metering rod piston springs in this range should be about right, but it comes down to trying the next spring on either side of OE and determining by road testing what works best.

That's pretty much how these carbs were set up back in the day - SOTP testing, and it still works today if you don't want to spent a hundred bucks or more on a dyno session.

Duke

[Stuart F.]

Re: Another AFB Question - 1963

Duke;

Well stated. Of course, I can't immagine how you could pick up the sound of those little vacuum pistons unless you were already running your valve lash tighter than the original spec, or perhaps you were using a stethoscope. Even then, that's certainly a case for "mechanically tuned in hearing"!

Good report. I'll copy that for my file.

Stu Fox

[Duke W.]

Re: Another AFB Question - 1963

Remember the 201 VAC didn't meet the Two-Inch-Rule, so it was not a good match for Duntov cam idle vacuum.

The rattle was probably in the range of 3-6 Hertz and was very distinctive, especially if the hood was open and the air cleaner off to investigate why the engine would not idle in a stable manner.

The 236 VAC solved everthing - stable idle and no metering rod piston dithering.

1963 was the first year that Chevrolet included a vacuum advance distributor with Duntov cam engines and they really screwed the pooch on this deal.

Duke