Carter AFB With No Velocity Air Valve

[mcmlvii]

My '66 383 came from the factory with a Carter AFB and doesn't have the velocity valve above the secondaries like I was used to on other AFBs I've had way back in the dark ages. The problem is that there is a noticeable bog if I floor the pedal, but if I gradually open the throttle so the secondaries come in slowly, the power comes on better (but still not great due to today's gas - still working on timing).

My question is, does any AFB without the velocity valve have a similar bog "feature" if the secondaries are slammed wide open, or is there an issue with my particular carb? I rebuilt it a few years back, so maybe it's time to do it again. I don't drive the car that much and it's rare that I romp on it, so I don't really know if this bog issue has been there all along.

Just thinking out loud here, I'm curious how Mopar/Carter got away with making some AFB's without the velocity valve, when other variations needed it. Anyone have a thought on that?
Thanks
John

 

[Snotty]

Is it an AFB or an AVS? They look similar.

 

[Davea Lux]

I would check the accelerator pump, if it has been several years since the last rebuild, the pump may be failing and not delivering full flow, that would explain the "bog" condition due to an over-lean condition when you stomp on it. Float levels that are set too low can also make a big difference on how good of throttle response you will get. If the secondary float is set too low the carb will lean out on full throttle. Debris clogging the secondary jets on an AFB will cause a similar over lean condition. The air velocity valve serves to slow down the air flow to the secondaries on the carbs that were so equipped so that less fuel needed to be added via the accelerator pump to insure smooth performance on full throttle. I think it was mainly added for emissions reasons. If your AFB is set and functioning properly, you should not be getting a serious bog condition.

Dave

 

[mcmlvii]

The carb is a 4130S AFB, correct number for a '66 383 4-speed. I had the carb top off a couple weeks ago and made sure the accelerator pump diaphragm is ok, and checked float level (7/32" according to service manual). It sounds like it might be time to tear into the carb though and check that the secondary jets are not boogered up.
Thanks much guys.

 

[Davea Lux]

The carb is a 4130S AFB, correct number for a '66 383 4-speed. I had the carb top off a couple weeks ago and made sure the accelerator pump diaphragm is ok, and checked float level (7/32" according to service manual). It sounds like it might be time to tear into the carb though and check that the secondary jets are not boogered up.
Thanks much guys.

Another item to check would be the base gasket to be sure you do not have a vacuum leak. A leaking power brake booster can also cause a hidden vacuum leak, but I suspect you would have an over lean condition at all operating ranges if that were the case. Our modern crap gasoline with ethanol causes corrosion problems in these older carbs. Be sure to flush the secondary passages with Brake-Kleen or another good solvent to remove any hidden particles.

Dave

 

[Big_John]

My '66 383 came from the factory with a Carter AFB and doesn't have the velocity valve above the secondaries like I was used to on other AFBs I've had way back in the dark ages. The problem is that there is a noticeable bog if I floor the pedal, but if I gradually open the throttle so the secondaries come in slowly, the power comes on better (but still not great due to today's gas - still working on timing).

My question is, does any AFB without the velocity valve have a similar bog "feature" if the secondaries are slammed wide open, or is there an issue with my particular carb? I rebuilt it a few years back, so maybe it's time to do it again. I don't drive the car that much and it's rare that I romp on it, so I don't really know if this bog issue has been there all along.

Just thinking out loud here, I'm curious how Mopar/Carter got away with making some AFB's without the velocity valve, when other variations needed it. Anyone have a thought on that?
Thanks
John

The AFB did not have a air/velocity/flapper valve above the secondaries.

The AVS (used in later 60's cars) did as did the Thermoquad.

The carbs look similar in construction though.

 

[70NEWYORKER]

AFB had valve BELOW secondary clusters.

 

[Davea Lux]

The carb is a 4130S AFB, correct number for a '66 383 4-speed. I had the carb top off a couple weeks ago and made sure the accelerator pump diaphragm is ok, and checked float level (7/32" according to service manual). It sounds like it might be time to tear into the carb though and check that the secondary jets are not boogered up.
Thanks much guys.

As long as the carb is off and the car is sitting, this would be a good time to check the timing chain for excessive play if you have not already done so. A worn timing chain will cause the valve timing to run late and cause performance related issues. (early sign of failing timing chain is farting back thru carb on full throttle application). FSM gives detailed instructions on how to check for excessive wear.

Dave

 

[Big_John]

AFB had valve BELOW secondary clusters.

That is correct and I should have mentioned it rather than having anyone think they didn't have one at all.

 

[mcmlvii]

My '66 383 came from the factory with a Carter AFB and doesn't have the velocity valve above the secondaries like I was used to on other AFBs I've had

The AFB did not have a air/velocity/flapper valve above the secondaries.

When I said the carb didn't have a velocity valve above the secondaries, what was meant was that there was no velocity valve above the secondary throttle plates.
[ETA: AFBs came in two variations, one with a secondary velocity valve and one without. When equipped with velocity valve, the valve was situated above the secondary throttle plates and below the venturis.]

I have put off doing a major rebuild of the engine itself but as mentioned, there may be more going on than just the carb (timing chain, etc). Talking to the original owner when I first got the car, he had some engine work done in the early '70s (didn't remember what), I'm guessing around 80-100k. It has 160k now. It still idles and runs well (other than the full throttle bog) and doesn't burn oil, so I don't really have a desire to pull it. I may just need to bite the bullet though. Or at least check the timing chain.... Thanks again.

 

[CBODY67]

That "manual secondary" AFB is unique to Chrysler. The earlier AFBs (from the earlier '60s) on GM cars had the weighted secondary air valve that modulated the opening of the secondary, effectively, independent of the throttle blades in the rear barrels. To get them to open quicker, you shaved weight from the counterweights and/or changed the shape of the counterweights.

My '67 383 4bbl Newport came with the factory AFB without the counterweighted air valve.

Now, as for that "bog" you mention . . . when mine was still unmolested, I could go WOT from idle with no perceptible bog (with a 3.23 rear axle ratio). After I did some clean-up on the casting flash on the throttle bores and vent. cluster, THEN I could get a slight "over-carbureted" feel to WOT acceleration off-idle.

You have the correct carb for your car and it came that way from the factory. Full "manual secondary" Carter AFB. It took Holley a few years later to get a "full manual secondary" 4bbl, although there was a mod to insert a screw into the slot for the secondary linkage to make a vacuum secondary 4bbl into a manual secondary 4bbl.

Rebuild it, dress the casting flash in the throttle bores, make sure everything is right . . .

Enjoy!
CBODY67

 

[CBODY67]

Best acceleration in a heavier car, can best be achieved by a part-throttle initial punch off idle, then (in drag racing fashion) get it "to the floor" after the car is moving. That "bog" is due to the fact that you're trying to feed the engine MORE air than it needs (at that point in time). Not unlike a spring-loaded secondary air valve having too little spring tension to keep the air valve closed when it needs to be. With "full manual control" of the secondaries, you'll need a little more discretion in applying WOT at lower rpms. In other words, learn to drive the car as it likes rather than try to make it do things it was not meant to do.

The carb is rated at about 550cfm, maybe 575cfm, which is plenty for a stock 383 and is one reason the secondary air valve isn't needed so much. A carb depends upon air flow through the throttle bores to draw fuel out of the float bowl and into the engine. The accel pump helps cover this situation, until fuel metering catches up with air flow. A stronger accel pump shot might help, but as my experiences indicated, it might not cover all of it with a little more air flow into the engine. Key thing is to not expect electronic fuel injection off-idle throttle response from a carburetor.

On our '66 Newport 383 2bbl, the old Stromberg got an off-idle lag if I punched it off-idle. LIke waiting for a hole in traffic to make a lh turn. No amount of accel pump or carb tuning could get rid of it. A new timing chain didn't help, either.

As for that that owner probably had done previously, that mileage you mention tends to coincide with when our Chrysler 383s needed a valve job, both with #7 exhaust valve being "burnt" at about 80K miles. Timing chains (for insurance, rather than waiting for them to fail!) at about that mileage, too.

Rebuild the carb. Fine tune the carb and ignition. Be sure to check the integrity of the motor mounts!!! Enjoy the car for what it is and how it does what it does . . . which is generally better than similar Ford and GM cars did back then.

CBODY67

 

[odcics2]

Hmm, trying to remember if this carb has a secondary air valve of some sort... I'll have to take a look tomorrow.

For Sale - NOS Carter AFB

 

[RJRENTON]

Gentlemen,
The #9 entry shows the counterweighted velocity valve below the secondary booster venturi. Many carbs were built with out these velocity valves. SOMETIMES a transition "bog" can be fixed by increasing the fuel feed circuit located as part of the secondary booster venturi 's emulsion tubes. The emulsion tube starts the innitial mixing of the fuel and air b4 being introduced in the air stream at the booster venturi. Check the air bleeds located in the top of the booster venturii casting. The emulsion tube is pressed into the venturi assembly and is visible when the ventui assembly is removed from the carb. Perhaps checking the small holes drilled in outside diameter of the emulsion tube for any restriction. I believe there are anti-syphon air bleeds drilled in the top of the booster venturii casting....make sure they are not restricted as they affect the initial fuel feed . Some booster venturi assemblies have a small fixed fuel feed restriction which act as an accelerator "pump"...if present, mske sure they are clean. The early AFB carbs are very simple in construction and operation. Just make sure that all fuel feed components are clean....also the gasket under the booster venturi is in the correct orientation.
Bob Renton

 

[RJRENTON]

To continue my thoughts.....
Possibly consider RAISING the fuel level SLIGHTLY. ..by doing so will allow earlier fuel feed (affects primary and secondary circuits) slightly "richening" the mixture. Remember carbs operate on the differential air pressure of the air flowing thru them....the sooner the fuel flow begins the better. Just my thoughts....
Bob Renton

 

[70bigblockdodge]

Set the timing with a vacuum guage. Most carburetor problems are electrical.

 

[RJRENTON]

Set the timing with a vacuum guage. Most carburetor problems are electrical.

Please be aware that this particular AFB has absolutely NO electrical connections, therefore cannot be tuned "electrically". Perhaps you mean setting the timing with timing light and not a vacuum guage...as a vacuum guage has nothing to do with timing, other than measuring manifold vacuum and possibly the vacuum advance signal. Early carbs used very accurately drilled fixed orifices that control both fuel and air feed. Carbs operate on the differential air pressure principals, specifically Boyle's Law which describes the relationship of volume, pressure and temperature.
Bob Renton

 

[70bigblockdodge]

Please be aware that this particular AFB has absolutely NO electrical connections, therefore cannot be tuned "electrically". Perhaps you mean setting the timing with timing light and not a vacuum guage...as a vacuum guage has nothing to do with timing, other than measuring manifold vacuum and possibly the vacuum advance signal. Early carbs used very accurately drilled fixed orifices that control both fuel and air feed. Carbs operate on the differential air pressure principals, specifically Boyle's Law which describes the relationship of volume, pressure and temperature.
Bob Renton

I mean you set your initial timing by usually advancing it to achieve the highest vacuum reading at idle just like setting the idle mixture screws. The initial timing per FSM or sticker on fender is usually more for emissions and reliability than it was for performance. There is some instances where this method can produce kickbacks and such on starting but his engine being stock it should not be a problem.
Reason I say electrical is 9 out 10 times people blame the stock carb on a stock engine, and they have old worn ignition components. In other words they look past the ignition side and go straight to blaming the carburetor because it is attached to their foot and the distributor is not.
Another thing to check is the advances in the distributor mechanical (not sticky and snapping back when released). I'm guessing the vacuum advance is working due to it runs good until he stands on it. Both of these can be checked with the timing light.

 

[CBODY67]

As was stated in my '66 MOTOR Manual, many fuel system problems can act the same as ignition system problems. Making sure that both systems are operating as designed and at peak efficiency is important, on both counts. I suspect that was the intent of the statement above about "electrically tuning".

Some people want things "by the book" ONLY, which can work in many situations, but can also be a little sub-standard in others. You can use a dwell tach to fine-tune the carb mixture, AFTER setting the dwell and base timing (in that order!). There are other methods which might be a little more imprecise (vacuum gauge and such), but the key thing is that the engine vacuum will generally be maximized when the optimum idle speed/mixture is achieved. IF it turns out that the particular engines likes 20 more rpm at idle in "D", for smoothness and response, do it, as it might make a nicer responding car (as long as other issues don't arise). Use the factory specs as a baseline and then see if small deviations might help things along.

From what's been said so far, combined with my own experiences on my '67 Newport 383 4bbl, back in the earlier 1980s, leads me to suspect that NOTHING is really wrong with the carb OR ignition OR timing chain. On some of the Chryslers from that era, WOT off-idle could result in a "spit-back" through the carb, too.

Of course, a Holley with vacuum secondaries would not have such a problem with WOT off-idle as the secondaries would not be opening until the engine was running faster. Same with the larger-cfm AVS or TQ.

CBODY67

 

[dieseldazzle]

To continue my thoughts.....
Possibly consider RAISING the fuel level SLIGHTLY. ..by doing so will allow earlier fuel feed (affects primary and secondary circuits) slightly "richening" the mixture. Remember carbs operate on the differential air pressure of the air flowing thru them....the sooner the fuel flow begins the better. Just my thoughts....
Bob Renton

Bob is absolutely correct here. Thanks Bob.