Hemi AFB accelerator pump issue - initial bog on opening throttle
- Thread starter AR67GTX
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Maybe that could explain partly how I managed to put a delay spring in the return spring position. But I’m not sure why they would include two new delay springs in their rebuild kits other than just an error.
But I’m still trying to figure what part dimension makes a 11/16 or 3/4” pump to satisfy my curiosity. The only part I can find that measures as small as 11/16” is the rim on the brass valve body at .617“ but that’s well under 11/16”. Everything else is well over 11/16”. But Mikes Carbs is calling it 11/16” too. I’m pretty sure it’s not pump stroke. It seems to be a case like in 1970 when Chevy increased the 396 to 402 cu in but still called it a 396.
Edit:
I don’t see how the fuel in the bowls can drain back as the height difference between the bowl fuel level and the fuel inlet or needle and seat is a natural siphon break. If the fuel remained in the bowls it would start a week later even if the lines did drain back and take a couple seconds to refill. I think the answer is that modern gas is much different even without ethanol, than what was available when these cars were new. Is overall much more volatile, vaporizes at a lower temp and evaporates quickly. Spill some on the driveway and it’s gone in minutes. From my foggy memory, if we did that back in the 60s, it was there a lot longer in time.
Edit: It’s maybe not typical but it looks like their kit was supposed to include both delay and return springs. So I think they messed up in packaging their kits which they assemble in - house, and accidentally put 2 delay springs in.
Carter AFB Carburetor Rebuild Kit - K618
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Dragon Slayer
Well-Known Member
I never saw carter documentation that gave diameters. Just the pump part number. There were hundreds of different pumps, though only about 5 diameters. Style, compression distance, spring tension, anti perc features, etc... Made the difference. At some point the after market moved to just a handful of pump parts numbers and they produced the diameter data. Some kits provide no spring with the pump and you move your spring over. I have documented many original pumps as the info I provided you in the PM, including spring dia, length and coil count. Same with return spring.
In the AFB world for MOPAR they used the 205, 212, and 279 pump. They are carter numbers. The return springs where the same for all MOPAR carbs 171. Chevy, Pontiac, Buick, Cadillac used other parts.
The check needle under the shooter can be the issue. This hold fuel higher in the body, then float bowl level. When the pump retracts back up it would try to pull the fuel back into the pump well unless the check needle seats. The next shot would have to refill the passage first so there would be a delay in fuel shot.
Too heavy a return spring would absolutely cause a delay in fuel as the pump shaft would collapse first with no movement of the pump seal to push fuel. Some of the Edlebrock ethanal pumps use a very heavy pump shaft spring to ensure the pump moves immediately with very little if any delay or duration of shot.
I am sure some carbs were modified by enlarging the well diameter and using a larger diameter pump to increase the shot for racing. So maybe your carb got modified. I won't be able to measure some know NOS bodies until later in the week.
The 66-67 hemi carbs used the 264 shooter .028" for both carbs. Later years increased the rear carb to the 294 .035" shooter while retaining the 264 in the front.
In the AFB world for MOPAR they used the 205, 212, and 279 pump. They are carter numbers. The return springs where the same for all MOPAR carbs 171. Chevy, Pontiac, Buick, Cadillac used other parts.
The check needle under the shooter can be the issue. This hold fuel higher in the body, then float bowl level. When the pump retracts back up it would try to pull the fuel back into the pump well unless the check needle seats. The next shot would have to refill the passage first so there would be a delay in fuel shot.
Too heavy a return spring would absolutely cause a delay in fuel as the pump shaft would collapse first with no movement of the pump seal to push fuel. Some of the Edlebrock ethanal pumps use a very heavy pump shaft spring to ensure the pump moves immediately with very little if any delay or duration of shot.
I am sure some carbs were modified by enlarging the well diameter and using a larger diameter pump to increase the shot for racing. So maybe your carb got modified. I won't be able to measure some know NOS bodies until later in the week.
The 66-67 hemi carbs used the 264 shooter .028" for both carbs. Later years increased the rear carb to the 294 .035" shooter while retaining the 264 in the front.
well, I have what looks to be an immediate and reasonably strong pump shot now (I have .28 shooters) and it is a little better but still coughed some when laid into the pedal from a slow roll, but better. I guess I could try .035” shooters. I also saw on a video from Turbine68rt about lightening the check weight under shooter to quicken response. I was also trying the car with manifold vacuum at the same time but will probably go back to ported. With the idle air bypass I cant get the idle below 1,000 even unless I shut the valves or lean out the mixture screws. I need a vacuum can with about 10 deg vacuum advance.
Anyone know where to find AFB AP pump squirters now? Lots of them for Holleys, some for Edelbrocks and a few for Rochester but haven’t found any for AFBs. I need to look at the 800 Edelbrock I have hanging on the wall and see if it looks like they will interchange with AFBs.
Dragon Slayer
Well-Known Member
Yes there is. May also find the kit of shooters that Carter and later edelbrock sold.
I ordered a set of Edelbrock shooters to see if they will fit OK. I’m kind of thinking it may be the front carb that may be the contributor to the hesitation at this point. When testing I’m rapidly opening the throttle - but not slamming it to the floor - it seems to briefly respond but then hesitate, which makes me wonder if it might be the front carb opening and not getting a big enough shot. I thought of trying a larger shooter on it first and if that takes care of it fine. And if not, drill out the smaller one in the kit to try it on the rear one. That way I have my unmodified originals for backup.
One site I found browsing around - might have been the Carburetor Shop - said to always replace the metering rod pistons on a rebuild as they are soft aluminum and wear rapidly causing a fuel timing change. I may need to consider that too considering 56 year old carbs. But probably not related to this issue and I suspect the only replacements are Edelbrock ones. Are they interchangeable?
One site I found browsing around - might have been the Carburetor Shop - said to always replace the metering rod pistons on a rebuild as they are soft aluminum and wear rapidly causing a fuel timing change. I may need to consider that too considering 56 year old carbs. But probably not related to this issue and I suspect the only replacements are Edelbrock ones. Are they interchangeable?
Dragon Slayer
Well-Known Member
I have not seen those pistons wear and I am not sure that is critical. I would be worried about the spring. The 264 shooter was common in many afbs. I have them if you needs some. The 294 was a little rarer but still on some carbs.
I was thinking about your question on pump diameter. As I mentioned the brass head measures 5/8" .625. I think if you add the leather cup thickness, 1/16" that gives you the 11/16" dia.
Interesting your not idling under 1000rpm. Do you have good throttle valve seal to body. With the bypass carb, you should have control of air flow. It is iterative to set the rear right, that get contribution from the front. But you should be able to get idle lower. You also need to make sure your ignition is fine too. May not be completely accelerator pump issue.
I was thinking about your question on pump diameter. As I mentioned the brass head measures 5/8" .625. I think if you add the leather cup thickness, 1/16" that gives you the 11/16" dia.
Interesting your not idling under 1000rpm. Do you have good throttle valve seal to body. With the bypass carb, you should have control of air flow. It is iterative to set the rear right, that get contribution from the front. But you should be able to get idle lower. You also need to make sure your ignition is fine too. May not be completely accelerator pump issue.
George, that was with manifold vacuum advance and 37 degrees advance at idle. On ported advance it idles down fine to the 700 - 750 rpm area. Also I probably didn’t have the bypass valves completely closed - maybe an 1/8 th turn out. I wasn’t sure it would be a good idea to shut them completely. Good to hear on the pistons.
dadsbee - I didn’t notice the check weights sticking. They fell out easily and since it’s pumping well now I think they are seating OK.
thanks
dadsbee - I didn’t notice the check weights sticking. They fell out easily and since it’s pumping well now I think they are seating OK.
thanks
Dragon Slayer
Well-Known Member
Dan, I measured a bore on a relatively unused carb. It was .730" diameter. Nice machine type fit of the pump into the well dry. As I said before the top washer is about .720" and the cup expands under it. The pictures are the 623 stamped shaft which I have found in the SMP kits.
So I do think your bore is fine. I did look at movement on a NOS front carb and the pump shaft does move immediately with throttle motion. It will not bottom when full throttle.
The pictures are bottomed but without springs so in the last picture with top on the shaft is collapsed. So it would be right at top with S link if the delay spring installed, and observing when attached would even be higher at full throttle. Which makes sense since the return spring even collapsed has to fit under it.
So it really is not 11/16 nor is it 3/4. More like 23/32" and I have seen that # used by one aftermarket manufacture but it was for a Cadillac pump. I have never seen Carter call out a diameter of the pump.
So I do think your bore is fine. I did look at movement on a NOS front carb and the pump shaft does move immediately with throttle motion. It will not bottom when full throttle.
The pictures are bottomed but without springs so in the last picture with top on the shaft is collapsed. So it would be right at top with S link if the delay spring installed, and observing when attached would even be higher at full throttle. Which makes sense since the return spring even collapsed has to fit under it.
So it really is not 11/16 nor is it 3/4. More like 23/32" and I have seen that # used by one aftermarket manufacture but it was for a Cadillac pump. I have never seen Carter call out a diameter of the pump.
.73“ was what I got too when I got really careful with measuring it. I guess it’s just one of those Carter mysteries.
I ended up setting the pump height at 3/8”. I think I could raise it another 1/16” but I think the 1/2” Mikes Carb recommends might get it up to the notch or close. My 66 Hemi supplement said 7/16 if I remember correctly. Thanks for checking on the bore diameter. I didn’t think my carbs looked modified.
I ended up setting the pump height at 3/8”. I think I could raise it another 1/16” but I think the 1/2” Mikes Carb recommends might get it up to the notch or close. My 66 Hemi supplement said 7/16 if I remember correctly. Thanks for checking on the bore diameter. I didn’t think my carbs looked modified.
Geoff 2
Well-Known Member
[1] All of my Carter AFB/AVS have the nominal 3/4" pump. Actual measurement of the bores varies from 0.730-0.740". I do not think the difference is from wear, just machining tolerance. The design of the leather cup allows for quite a size variation & the ability to still seal.
[2] I have many step up pistons, have measured them, & they do wear. Jon Hardgrove from the Carb Shop claims they should be replaced because they affect the timing of the piston. Never heard of this before, nor of any test to check the operation. I have never had a problem with the step up pistons.
[3] Edel pump plungers are 13/16". So is the TQ. I believe the later 9000 series AFB are also 13/16".
[2] I have many step up pistons, have measured them, & they do wear. Jon Hardgrove from the Carb Shop claims they should be replaced because they affect the timing of the piston. Never heard of this before, nor of any test to check the operation. I have never had a problem with the step up pistons.
[3] Edel pump plungers are 13/16". So is the TQ. I believe the later 9000 series AFB are also 13/16".
Dragon Slayer
Well-Known Member
Dan, 7/16"s is the carter spec too in the middle hole. 1/2" was the spec for the cheater carb but it used a 4 hole arm.
The next time I have them off to change out the pump shooter I will raise them a little more. I received notice that Mikes Carbs sent me a couple of replacement return springs yesterday. I suspect they have seen that packing error of two delay springs in their kits before. They probably figured it was a cheap remedy to get free of me because I think I was wearing their technicians out with e-mail questions. But that’s OK - they seem to be a good outfit with a lot of good stuff for AFBs and AVSs.
Dragon Slayer
Well-Known Member
The current manufacturer call it 11/16" but if you order a 3/4" pump it will not work. Go measure the base of the plunger and the washer above the cup. You will see differences. The larger diameter pumps have a .750" or larger washer. That will bind in the .730" bore and never pump a complete shot. Buick, CAD, Chevy, Pont, Mopar, Ford, all had a different pump part number for the AFB. That is the reason for kit differences. Same with AVS. I have bought vintage original kit to document the actual carter pump dimensions, so I know what I am talking about. You can't just measure the cup which can swell or shrink. The upper washer sets the size and has to smoothly move through the bore.
Why not remove the lower spring and test? If the spring rate difference between upper and lower is the issue, it'll show up immediately.
Doug
Doug
Dragon Slayer
Well-Known Member
I think he did that already and has resolved it. He did have too heavy a return spring which was meant for the pump shaft. Most of this discussion was just validation his carb bore was ok, then the esoteric discussion of pump diameter. If your trying to buy a pump separately you need to trust the current part maker. But after market pumps were consolidated for AFB and they only carry a handful. They also changed the carter part number to their own number.
Dragon Slayer
Well-Known Member
Dan, Here is a good site. They have the shooter kit too if your still looking to experiment.
AFB AVS Accelerator Pumps - Carburetor Parts & Kits for Carter, Ford Motorcraft, Holley & Rochester Carburetors
AFB AVS Accelerator Pumps - Carburetor Parts & Kits for Carter, Ford Motorcraft, Holley & Rochester Carburetors
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