AFB Concerns

[Stuart F.]

I've been playing around with a rebuilt 3461S on my L-76 63 that I got from one of our great members. From day one it has had a lean surge at high vacuum cruising in 2nd and 3rd. Even after every measure I have taken to correct this, I now instead have a "herky jerky" under those conditions. I also have a 3721SB which, when installed, does not exhibit these faults. Therefore, I've ruled out any other source including ignition.

The carb first had primary clusters with .033" low speed/idle jet tubes (incorrect), so I enlarged them to the stated original size of .035" (no change). I then changed to earlier model clusters with .037" jet tubes (no change). The primary jets were .098" (not correct), so I changed them to .104" (std.) (still no change). My good friend Tim B. suggested I might have too much timing and still too lean. I'm running a corrected 12 degrees initial with a B28 VAC. I then changed to richer metering rods: .065" x .057" (from .069" x .060" std.) (still no change).

I took the car out again for test drive yesterday. It ran well for the first 10 to 15 minutes, i.e. started well with good warm up, very responsive off idle, and super POT and WOT. Then again it started the "herky jerky" at high vacuum in every gear. I considered turning around and heading for home thinking I may be losing a coil (and me w/o my cell phone), but it settled down and was less noticeable on my return trip.

What I can't figure is why this is occurring with the 3461S and not when I'm running the 3721SB. I'm ready to just put the 3461S on the shelf as a correct carb for the car - a trophy!

Stu Fox

[Timothy B.]

Re: AFB Concerns

Hi Stu,

I received your email yesterday and did not get a chance to reply yet. Did you perform the test I suggested plugging the vacuum advance with a golf tee to take that timing out just to see if making the engine less efficient stops the jerky feeling. IMO, at those high vacuum situations you are more that likely running on the idle circuit so metering rod changes etc are probably not going to help much.

As you know, the idle circuit is controlled by the idle jet and air bleeds, this circuit also controls A/F emulsion for the transfer slot and that's where I think your problem may be. With too much timing the point of peak combustion may be occuring to soon with the burn ending causing the engine to surge. Lets see what others have to say.

[Stuart F.]

Re: AFB Concerns

Thanks for the reply Tim. Yes, I sort of did as you suggested. But I made up a hose section with a fish tank valve so I could adjust the vacuum full closed or partial with out any change except a little less responsive at low speeds. My primary clusters are the modified ones with clipped tabs, smoothed and polished, etc. They have the .037" jet tubes with the .052" by-pass jets.

Why does this carb work so well during warm up, then go bonkers a while later down the road after a period of operation at full warm?? Beats me! (BTW, I've changed the fuel line hoses and filter, and found no sign of debris).

Stu Fox

[Clem Z.]

Re: AFB Concerns

Thanks for the reply Tim. Yes, I sort of did as you suggested. But I made up a hose section with a fish tank valve so I could adjust the vacuum full closed or partial with out any change except a little less responsive at low speeds. My primary clusters are the modified ones with clipped tabs, smoothed and polished, etc. They have the .037" jet tubes with the .052" by-pass jets.

Why does this carb work so well during warm up, then go bonkers a while later down the road after a period of operation at full warm?? Beats me! (BTW, I've changed the fuel line hoses and filter, and found no sign of debris).

Stu Fox

do you have a aluminum intake and if so are the heat riser crossover blocked to prevent the gas from boiling in the carb. air bubbles will cause a lean miss. try lighter springs in the power pistons. i remember my 62 340 HP corvette you could hear the power pistons going up and down,clicking at idle because of the vacuum pulsating.

[Duke W.]

Re: AFB Concerns

I had the same problem with my 340 HP engine - metering rod piston rattle, but that got fixed when I set up the spark advance map to '64-'65 SHP/FI specs with as much as 18 deg. initial on the tab, which is about 16, true. These changes also solved the idle instabililty problem and offered noticeably better low end torque and fuel economy.

Stuart - as you know the 3721SB has larger idle jets in the primary cluster, but I'm not sure if the air bleeds were changed. In any event, the 3721SB appears to be calibrated a little richer off idle, but a little leaner at cruise. This somewhat makes sense - especially the leaner cruise - since this model AFB only had to work on the 300 HP engine.

You didn't say where your idle adjustment screws are set, but the nominal initial setting is 1.5 turns out from the seat, and this setting was typically okay on my engine, but sometimes I was at 1 3/4 turn out.

The idle mixture screws also have an effect on the off idle circuit, so I suggest you go through the idle speed mixture adjustment procedure in the shop manual, and if the off idle leaness is still there, turn out the idle speed adjustment screws in 1/8-turn increments and keep testing until you get to at least 2 or 2.5 turns out and let us know if this improves the situation.

One of the most difficult carburetor calibration jobs is the transition from the idle/off idle circuit to the main circuit.

A test I use for off-idle leanness diagnosis is: with either my foot on the throttle pedal or operating the linkage by hand in the engine compartment - starting at idle increase speed in 100 RPM increments to about 2000, and then back down again. If the engine hesistates or won't hold a given RPM without constantly adjusting the throttle setting, it's lean.

Another thing I have observed is that some owners drive around one or two gears lower than necessary. This has a significant negative effect on fuel economy, and may also exhibit some driveabililty issues. Once I got my 340 set up properly, I had no problem cruising or accelerating from 1000 revs in top gear, and with the 3.08 axle ratio 1000 revs is about 26 MPH.

I did notice some throttle response issues when I was racing and had to modulate the throttle while in a steady state corner at high revs - say in the 4500 to 5500 range.

Duke

[Stuart F.]

Re: AFB Concerns

Clem;

Thanks for your input. Due to previous problems (posted previously) with perculation and a "stinky" garage, I raised my carb up with an L-75 phenolic spacer and 2-340 hp base plate gaskets on my aluminum intake manifold (a sandwich)a . I found a drop of some 40 degrees as a result. I also isolated it from any steel tubes (that could conduct heat) by going to an electric choke and fuel hose between the filter and carb inlet (also needed due to carb elevation). I am also using non-ethanol gas with MAX2000 octane booster (TEL). I do not, however, have my heat riser passages blocked, and I still use my heat riser valve (not wired open for summer yet). This carb also has Tomco Inlet Valves instead of standar needle/seats, and I run the floats down to a good 5/16". I do have the hot idle compensator valve removed and passage blocked. At full warm idle, it pulls a nice steady 15" Hg on the gauge. Regarding the metering rods, I currently are using the "pink" (7" Hg), and have tried "Orange" (5" Hg) w/o any noticeable difference.

Thanks again for your ideas. I may just wire open the riser valve early this year and, if convinced, change to an FI spacer that I've rounded out the inside on.

Stu Fox

[Clem Z.]

Re: AFB Concerns

Clem;

Thanks for your input. Due to previous problems (posted previously) with perculation and a "stinky" garage, I raised my carb up with an L-75 phenolic spacer and 2-340 hp base plate gaskets on my aluminum intake manifold (a sandwich)a . I found a drop of some 40 degrees as a result. I also isolated it from any steel tubes (that could conduct heat) by going to an electric choke and fuel hose between the filter and carb inlet (also needed due to carb elevation). I am also using non-ethanol gas with MAX2000 octane booster (TEL). I do not, however, have my heat riser passages blocked, and I still use my heat riser valve (not wired open for summer yet). This carb also has Tomco Inlet Valves instead of standar needle/seats, and I run the floats down to a good 5/16". I do have the hot idle compensator valve removed and passage blocked. At full warm idle, it pulls a nice steady 15" Hg on the gauge. Regarding the metering rods, I currently are using the "pink" (7" Hg), and have tried "Orange" (5" Hg) w/o any noticeable difference.

Thanks again for your ideas. I may just wire open the riser valve early this year and, if convinced, change to an FI spacer that I've rounded out the inside on.

Stu Fox

with the floats lower than the stock setting that would concern me with the problem you are having. i would raise the floats before trying anything.

[Stuart F.]

Re: AFB Concerns

Duke;

I too have a reworked distributor map, but I will confess, I didn't know about the L-76 balancer vs. timing tab varience until just recently when you fellows brought it up. I had been running the equivalent of only 8 degrees initial for 48 years! I boosred it up to the edge of the tab (about 13.5), but have since reduced it to a corrected 12 degrees. As to the mixture screws, I did test with them out about an extra 1/2 turn or about 1 to 1-1/2" hg less idle vacuum, but did not notice any improvement. The currently installed primary clusters mirror my 3721SB jetting wise, but they had the small tabs which I ground to a pointed triangle (why? I don't know).

I should mention that this carb came to me sort of messed up by the rebuilder. It had ywo different primary clusters (one correct and one not), the wrong top plate with rolled SS vent tubes from a 3721SB instead of brass (understand SS were in replacement 3721S models only), a folded over accelerator pump with no override spring, a leaking choke housing to main body gasket, .033" low/idle jet tubes in the clusters, and a number of other issues. Each time I discovered a fault, I thought I was solving the problem, but alas it was not to be. The clusters I'm using now are from my old 3720SA parts carb, so I have reworked them. Now that I have the correct 3461S clusters, I don't want to permanently alter them (even though I know the tools exist to remove and replace these jet tubes).

Thanks for your "leaness test" instructions. I'll give that a try. I must admit that I am a gear changer. After so many years with the Sintered Metallic brakes I got used to using the engine a lot for braking and staying in a lower gear gave me better slow down characteristics. With my new linings, that isn't needed so much, but I'm spoiled. (Yesterday I had to panic stop to miss hitting two adult and two baby Sand Hill Cranes crossing slowly over the road. My view was blocked by a truck in the other lane. I saw his brake lights go on which caused me to lift, then I saw them and climbed on the brakes. As you may identify with, the passenger seat back flew forward and a bunch of stuff came forward out from behind the seats, but I stopped in time and got a thumbs up from the trucker. Love those Matrix Ceramics.).

Clem;

You might be right. My plan is to pull the 3461S again and change out the Inlet Valves for regular needle/seats, and adjust the floats more to normal. As noted before though, I have great POT and WOT.

Stu Fox

[Clem Z.]

Re: AFB Concerns

i would guess you all have seen this.

1964: Service Bulletin: Recalibration of Carter AFB Carburetors Very late into the 1964 model year, Chevrolet advised dealers of a change in carburetors earlier on the 1964 Corvettes. The date given of 11/26/63 was at the Carter carburetor plant. By the date of 11/26/63, almost 6,000 1964 Corvettes had left the St. Louis plant with the first design AFB carburetors. ------------------------------------------------------------------- Subject: Recalibration of Carter AFB Carburetors Model and Year: 1964 800 & 1000 Series with 327" Engines Source: Chevrolet Central Office Technical Service Bulletin Bulletin No: DR #674 Section: VIm Date: May 28, 1964 TO: ALL CHEVROLET DEALERS To minimize hesitation on acceleration and improve overall engine performance, the Carter AFB Carburetor used on 800 and 1000 Series vehicles wtih 327" engines was recalibrated effective at the source approximately November 26, 1963. Carburetors incorporating the latest calibration may be identified by the following numbers which are stamped on the metal identification tag. 3720SA - Units with Automatic Transmissions 3721SA - Units with Manual Transmissions If hesitation on acceleration is encountered on early production vehicles, the carburetor should be rebuilt using the parts listed on Page 2 of this bulletin. 3862088 1 Primary Cluster (choke side) 3862089 1 Primary Cluster (pump side) 7009575 2 Primary Metering Jet 1460475 2 Secondary Metering Jet 3862090 1 Primary Cluster (choke side) 3862091 1 Primary Cluster (pump side) 7009575 2 Primary Metering Jet 1460475 2 Secondary Metering Jet 3821716 2 Metering Rod

[Duke W.]

Re: AFB Concerns

It was recalibrated again, not long after the SA versions were released, during the '64 model year and these 3720SB/3721SB models became the final production version. These versions were also sold OTC as service replacements for all previous models at least through the late seventies, and they are probably more commonly found that earlier versions.

Duke

[Stuart F.]

Re: AFB Concerns

Just to update my plea for help and ideas on how to fix my AFB 3461S;

Today I swapped out my 3461S for my spare 3721SB just to make absolutely certain that the symptoms I'd described were due to the carb and not anything else such a ignition, etc.

As expected, the car ran perfectly fine. My 3721SB is a replacement carb I've had on my car since about 1980. It has the following specs, most are original, but I have updated it some:

- .104" primary main jets
- #16-389 metering rods (.069" x .060")
- Std. transition springs (8"Hg)
- Std. Primary Clusters (#793 & #794) w/tabs
- .037" idle/low speed jet tubes
- .050" By-Pass jets
- .028" air bleed/anti perc vent tubes (short)
- #284 pump nozzle (.028" squirters) replaces std. #264 w/.0265" squirters
- Std. needle/seats (.110")
- Foats set @ 9/32" x 3/4"
- .070" secondary jets (replaces .0689")
- light weight air valve

I'm at a loss on how to bring my 3461S up to the performance of my 3721SB. The difference is night and day. What should I be looking for? a crack some where? a small vacuum leak? Why does it only go bonkers after a period at full warm up? (a "herky jerky, almost like a cut out").

Stu Fox

[Clem Z.]

Re: AFB Concerns

i would interchange the boosters from the "good" carb to the "bad" carb and give it a try

[Wayne W.]

Re: AFB Concerns

Back in the day, the biggest problem we had with AFB was the power valve sticking. It caused surge at cruise. The fix was to remove the rods and clean the pistons and the bore with light Emery paper. Would work nice until it stuck again.

[Stuart F.]

Re: AFB Concerns

Clem and Wayne;

Thanks for your responses.

Clem, I have tried to match up the original clusters (boosters) as the rebuilder had fitted it with one correct #633 and one incorrect #794. I got him to exchange the #794 for a correct #634, but that one had a .033" idle/low speed jet tube. I drilled it to .035", and ran the drill through the other #633 just to be sure they matched. I then later exchanged them for another set from a parts donor carb I have which has .037" jet tubes. None of these changes achieved any improvement.

Wayne, I have several sets of pistons and a number of different metering rods and a full set of transition springs. I have polished the pistons and have tried several different combinations of rods and springs. I will admit; however, I have not polished the bores (chambers) in the main body with, as you did, emory cloth. You might have a good point there as this carb body was reconditioned using a "glass bead" cleaning process and has a satin type finish (rough). It is conceivable that the bores are too rough for proper movement of the pistons, specially when it is full warm. I replaced the top plate with one I had reconditioned here in Florida and they used an aluminum bead process. The finish is much smoother. I recall having to polish the piston bore in the choke housing of my 3721SB anually when I used a standard hot air arrangement due to "white corrosion" build up. This may be similar, but with a different cause. Thanks for the tip. I'll see what kind of "hone" arrangement I can put together to clean them up.

Thanks again guys. I'm going to take a real close look at everything this time and see what else I can discover. As I mentioned once before; this carb may end up as a shelf trophy if I can't get it sorted out.

Stu Fox

[Timothy B.]

Re: AFB Concerns

Stu,

Remember the conversation a while back about the holes in the secondary of the 3461 main body where the engine can suck air. I was struggling with my 3461 not idling slow enough with the throttle blades closed down completely. Could it be possible that under high vacuum conditions air is being pulled into the engine through these holes plus wear in the secondary shaft causing a lean mixture thus surge.

On my 3461 I ended up plugging the holes with rtv and restricting the hot air choke from .098 to .070 which reduced air flow and slowed the idle. My next move was to restrict the pcv but I like the fact it's fully operational, the car idles very slow now, I'm happy.