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Hemi AFB accelerator pump issue - initial bog on opening throttle

Dragon Slayer said:
Ok Bob your wrong. First, the subject is not AFB but Hemi AFB specifically 66-67. There is no secondary discharge pickup tube. Just a hole in venturi and it is offset to the side. NO WHERE near the center strut, and the strut does not have a strut bleed. Only carbs I have seen with that were Cadilliac AFB. Secondly, the rear carb does not have the cut out on AV. Besides any tubes like that are in the fuel well of the body. So it is not a cutout for the Secondary discharge. It is centered under main strut for main feed. I am sure it allow main fuel to have a clear path into the air stream, but not all AV had it.

Yes the air valve is like vacuum secondary on a holley in that it does not open until demand from the air flow and DP. Difference is it is another throttle valve in the flow. With an offset angle blade.

Here is a hemi secondary venturi. Just the main pickup tube. Notice the discharge port on the wall of the venturi way off center. With the large pickup hole on the opposite side. The smaller hole is actually covered by a lack of transfer hole in body of the carb. Also as I stated earlier it is a separate circuit from the main, so it does not act as an air bleed one main is on. They are not connected. Plus only one venturi on the rear hemi has the discharge. They other does not. Solid. Again distribution issues. The front does have it on both.

I have made it a hobby to study the AFB especially Hemi. So I am not speculating. You may have tuned but did you test on a dyno and measure AF ratio cylinder to cylinder. Chrsyler did. It was all about distribution improvements as they moved from 66 to 71. Also, 68-71 are different. They are AFBs that actually have a secondary idle circuit. Rear carb only.

View attachment 1438823
Click to expand...
I guess it depends on the particular model/application.....i have several models of the Carter AFB 3854S used on the Mopar 1965 273 HP ENGINE, and also consecutive years, that the secondary booster venturii assemblies had the tube as described. I'm also aware of the pull over fuel delivery system discharge orifices on the secondary booster venturii assemblies on other AFB carbs anf how they function. Appreciate your input and comments.
BOB RENTON
 
Resurrecting one of my old threads. I may have found a solution to my Hemi bog/hesitation problem. On a whim I ordered a couple Thompson Power Blast Plates and installed them on my Hemi carbs. Yeah, I know. Could be easily accused of falling in the quack remedy category but seemed to work for me.

thompsonperformance.com

Thompson Performance, LLC

Thompson Performance, LLC
thompsonperformance.com

The best I had been able to achieve by essentially blueprinting my carbs, increasing the shooter size and putting the accelerator pump rod on the inside hole, was to reduce full throttle from a fall-on-its-face bog to a hesitation like a full throttle downshift to first at low speed as the transmission shifts down. Installed these plates the other day and went out and did a couple first gear rolls at 1100-1200 rpm and floor the throttle tests. I had no bog or hesitation at all and just strong engine response. So these can seem to help with a full or part throttle acceleration response issues. They can only affect the accelerator pump circuit so no HP gains here just response improvement possibly, if you need it. My 67 GTX doesn’t need it so not going to waste time or cost on putting one on it.

IMG_2859.jpeg

IMG_2864.jpeg
 
I have slight off idle part throttle bog. Did you have that or did you solve that with carb rebuild? Thanks
 
Fran, in the thread above I tried a lot of stuff. I went through both carbs and part numbers with Dragonslayer and we verified everything seemed original. I had already rebuilt them and returned any deviations in rods and jets to as close as original as I could. I checked all the bleeds and the economizer fuel passages and the economizers seemed a little chocked up with probably residue. I cleaned them out to spec with a micro drill bit set. I bought some Edelbrock shooters in larger sizes and installed and I moved the pump rods to the inside holes. That improved the pretty severe bog I had to more of a distinct hesitation, not unlike kicking an automatic down to first gear from third. But I still had a hesitation when flooring it from low speed.

I tried the same thing twice after installing these plates and didn’t have any hesitation so I think they are helping. What I may try next is moving the rods back to the center hole and see if it will still respond without hesitation.
 
Those ' devices' also soak up some air space, so maybe the engine 'sees' a smaller venturi, which improves throttle response.

AR.
I would try this. It only takes 5 min. Set initial timing to about 25* & test drive. Re-adjust idle speed & mixture if need be. You may get a veeeeeery pleasant surprise & find your stumble has gone. This would be good news. What would be better news is that there is a permanent fix....
 
I’m pretty good with it as is but I am curious as to your suggestion. I have 16 degrees base timing in it now (+17 mechanical) and ported advance. No way I can run 25 degrees base timing. I’m not sure my old direct drive starter would even turn over the engine and it sure as heck would ping like mad if it could. A street Hemi on street gas can usually only tolerate around 33-34 degrees combined timing and 42 degrees would just not work. My bog I was dealing with was going to wide open throttle and not part throttle. Part throttle response has never been a problem. With 25 degrees initial timing I could never floor the throttle anyway.
 
AR,
[1] I maybe seeing things but in the pic #142....am I seeing a snivvy on the lower booster? If so.....that is there to re-direct air flow. Your new addition will likely change the airflow pattern from what Chry intended.
[2] Bog going to WOT. Try tying the velocity valves down so they cannot move.
 
Thinking about [2] above.....
A simpler test: disconnect the link between the pri & sec shafts on both carbs. Now only the primaries will fully open on WOT. If you still get a bog, then the problem is in the primaries. You may still get a bog with the secs working, but trying this is a simple test that could narrow down the cause.
 
Thanks, but it seems to be running good for now so I think I’ll just enjoy that for a while.
 
AR67GTX said:
I’m pretty good with it as is but I am curious as to your suggestion. I have 16 degrees base timing in it now (+17 mechanical) and ported advance. No way I can run 25 degrees base timing. I’m not sure my old direct drive starter would even turn over the engine and it sure as heck would ping like mad if it could. A street Hemi on street gas can usually only tolerate around 33-34 degrees combined timing and 42 degrees would just not work. My bog I was dealing with was going to wide open throttle and not part throttle. Part throttle response has never been a problem. With 25 degrees initial timing I could never floor the throttle anyway.
Click to expand...
If you plug the vacuum advance port at the carb and set the base timing to 21 degrees ....... it will work.
 
Thanks, but that would give me 38 degrees total advance. Not good for a street Hemi on 90 octane non-ethanol. Might not be good for my stock, antiquated, direct drive starter either.
 
K
AR67GTX said:
Resurrecting one of my old threads. I may have found a solution to my Hemi bog/hesitation problem. On a whim I ordered a couple Thompson Power Blast Plates and installed them on my Hemi carbs. Yeah, I know. Could be easily accused of falling in the quack remedy category but seemed to work for me.

[/URL]

The best I had been able to achieve by essentially blueprinting my carbs, increasing the shooter size and putting the accelerator pump rod on the inside hole, was to reduce full throttle from a fall-on-its-face bog to a hesitation like a full throttle downshift to first at low speed as the transmission shifts down. Installed these plates the other day and went out and did a couple first gear rolls at 1100-1200 rpm and floor the throttle tests. I had no bog or hesitation at all and just strong engine response. So these can seem to help with a full or part throttle acceleration response issues. They can only affect the accelerator pump circuit so no HP gains here just response improvement possibly, if you need it. My 67 GTX doesn’t need it so not going to waste time or cost on putting one on it.

View attachment 1856132
View attachment 1856131
Click to expand...
The "device" is nothing more than an add on air deflector plate, mounted in conjunction with the Accelerator pump discharge nozzle assembly. It modifies (bends the venturi air stream) to try and compensate for fuel/air distribution issues.....likely works with stock the manifold but, I have no real experience with the device.......not sure what is trying to be accomplished....web site was vague.....just my opinion......
BOB RENTON
 
Thanks guys, but it’s fixed - running good now. Everything’s good. Well, except for my dinged up oil pan that has started to leak but I have one of those nice, heavy gauge ones to replace it.
 
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