Hemi AFB accelerator pump issue - initial bog on opening throttle

[AR67GTX]

I rebuilt the Carbs last year in my 66 Hemi - these are original AFBs - using kits from Mikes Carburetors. Other than setting adjustments per specs I didn’t play around much with the accelerator pump operation. The car drives normally very well but I’ve noticed the few times I’ve eased off in first and then rolled into the throttle I get a big bog (not a backfire) and then the engine catches and I’m off. Finally looked at the accelerator pump on the primary carb and noticed I don’t get any thing out of the nozzles until almost half throttle. I pulled the carb, filled the bowl with some gas and played with the pump. Mikes instructions suggest a pump height of 1/2” with throttle closed. I have to stroke it down to 1/4” height before I feel resistance on the pump link and additional movement squirts gas. That just about buries the pump rod down in the casting.

Not the first one of these I’ve worked on but I must be doing something wrong.

Below I’m manually positioning the plunger where it starts to squirt.

Is it possible I have the wrong pump?

 

[turbine68rt]

Is your float height settings correct and verified? Are you sure you have the correct diameter accelerator pump diaphragm for your particular bore?

 

[AR67GTX]

They are set about 1/32“ lower than spec to minimize percolation effects. I’m feeding the bowl with a squirt bottle for my testing, mimicing the gas level stain on the floats. Plenty of gas to keep it above the wier hole feed to the accelerator pump well.

 

[turbine68rt]

Are you sure you have the correct overall length of your accelerator pump?

 

[AR67GTX]

No, I’m not. I dug out my 66 Hemi FSM supplement and it calls for pump height of 7/16” - still far too high to work. I checked the 67 AFB on my 67 GTX and it’s set at 36/100” or a bit less than 3/8”.

I overlooked one thing which is that the S link on the linkage will slide down into the recessed well the pump rod is centered in so it won’t bottom on the top of the carb lid. It’s got more room to stroke than I realized at first. I guess tomorrow I just need to bend the rod to get it right on the highest point where it still squirts fuel immediately with throttle movement - at 1/4”, 5/16” or whatever - and then see if the carb will open fully without bottoming the pump. If it bottoms I’ll have to get with Mikes Carbs and see if they have a slightly longer pump.

I think that’s the bottom line. At least these carbs seem to have a really good idle transition system as driving the car around normally you would never know the accelerator pump circuit wasn’t working.

 

[RJRENTON]

No, I’m not. I dug out my 66 Hemi FSM supplement and it calls for pump height of 7/16” - still far too high to work. I checked the 67 AFB on my 67 GTX and it’s set at 36/100” or a bit less than 3/8”.

I overlooked one thing which is that the S link on the linkage will slide down into the recessed well the pump rod is centered in so it won’t bottom on the top of the carb lid. It’s got more room to stroke than I realized at first. I guess tomorrow I just need to bend the rod to get it right on the highest point where it still squirts fuel immediately with throttle movement - at 1/4”, 5/16” or whatever - and then see if the carb will open fully without bottoming the pump. If it bottoms I’ll have to get with Mikes Carbs and see if they have a slightly longer pump.

I think that’s the bottom line. At least these carbs seem to have a really good idle transition system as driving the car around normally you would never know the accelerator pump circuit wasn’t working.

Have you checked the operation of the accelerator pump inlet check ball assembly? If this ball check valve does not seat immediately, a portion of the accelerator pump shot will be lost. Also, do you hsve the correct duration spring on the pump plunger assembly and the position of the washer? Just a couple of things to check.....
BOB RENTON

 

[AR67GTX]

I can’t detect any sign of leakage from the check ball. Once the plunger is down fully in the machined bore, it has a firm feel which when overcome by finger pressure results in fuel from the squirted. I have not detected any sign of flow back in the bowl gas from the ball check. And it seems to recharge immediately through the port if I release the plunger slightly (but not too high to leave the bore).

On the other question, I believe the kits from Mikes Carburetors came with fully assembled pump assemblies Just like pictured above. The only thing I reused was the lower spring down in the well. I’ll try to double check to make sure that’s correct.

 

[AR67GTX]

I checked and the kit included the delayer spring, and also the return spring which I forgot.

 

[Geoff 2]

It is hard to tell for sure looking at the pic. The long slot down the side of the pump bore looks like the cut has been cut down further than stock. There will be NO pump shot until the plunger cup has passed this slot, because the fuel will just exit the pump bore via the slot.

 

[Geoff 2]

Just to add. In pics 2 & 3 of post #1. I can see a semi circular mark at the base of the slot, just above the brass one way inlet valve for the pump chamber. It looks like somebody has filed/machined the slot deeper; if this is the case, you would get no pump shot until the plunger is almost bottomed out.

Not familiar with the Hemi carbs, but could this be the scenario. I understand the engine idles/cruises on rear carb; front carb cuts in with more throttle opening. So working acc pump is reqd on rear carb. Could this modification I mentioned above have been factory made, which probably means the carbs are reversed.

 

[AR67GTX]

I sort of thought the same too as the weir slot is deeper than on an Edelbrock AFB I have. But I haven’t found any signs of Bubba mods or parts changes on the carbs other than some messed up jetting that Dragon Slayer helped be correct. I’ll see if I can get them functioning this morning and report back.

 

[dvw]

Here's a pic of a Carter Comp AFB I had apert recently. The area of concern expands pretty clean on a phone. It Only a few issues could cause this problem. As stated, a leaking inlet check. When this happens you cn see the strem flowing back into the bowl when operated manually with the lid removed. Also a leaking outlet check can cause it to suck are on the inlet stroke. This reduces the volume available. Main body groove too deep. Though why would it have been touched? Lastly is the delay spring being too stiff. Or conversely the pump spring on the shaft being to weak. They work in tandem. I'd check both of them against the ones in the other carb.
Doug

 

[AR67GTX]

It looks like yours have the same deep weir notch as my 4139/4140 carbs. And yes, my thoughts too - why would anyone mess with its depth? Pretty easy to see currents in the bowl from the notch before the seal skirt goes below it. I don’t see any disturbance from the inlet when pressing it further down. I think these pump shafts must just be a little short. I’ll adjust the height down this morning and make sure they aren’t bottoming and if so I’ll be good to go.

thanks

 

[mrhemi]

As mentioned, have you checked the pump outlet check valve? was it replaced and re-seated during the rebuild? I have taken more than a few of these carbs apart and found the outlet check has been removed or omitted un-intentionally.

 

[AR67GTX]

Shinning light on the gas surface I think I could detect a bit of backflow. I removed it, cleaned it and blew it out and re-tightened it. Didn’t seem to be doing it anymore but I went ahead and ordered two replacements. Also double checked the squirter gasket and check weight and OK. BUT - can’t adjust it out. Even straightening the rod nearly out and putting it on the outer hole, I don’t get fuel until about 1/4 to 1/3 throttle like this With the top of rod almost at top of lid.

I ordered from Mikes Carburetors and she confirmed my pump length within a few thousands. Their tech support is by email so I sent them a bunch of pictures, measurements and observations. Maybe I need to fine polish the bore a little or the pump umbrella is defective. It doesn’t seem to be recharging through the bottom jet as well as it should - which could be a sealing issue too. I removed the check valve in this pic. Have to see if they have some ideas.

I squirted some fuel in the fuel bowl of the front carb that’s still on the car to see what it does. Same thing, nothing until at least half throttle and the top of the rod is at least below 1/8 off the lid. So, seems to be universal to the 2 carbs.

 

[Geoff 2]

Comments from some one who has worked on these carbs since the 1970s:

- Post #15 is a better pic of the slot. It has DEFINITLEY been machined deeper. Was it factory or previous owner? My comment in post #10 could explain the reason for the factory doing this, in which case the carbs could be reversed. Have you checked this?
- as I said in post #9, repeat, there will be no pump shot until the cup has passed the slot. Since the plunger is almost at the end of it's travel then, the last little bit of movement, there will be very little flow from the nozzles; the very problem you are describing.
- the problem is NOT the inlet valve, as there was pump shot, but right at the end of the pump travel when the cup has passed the slot. If the valve was faulty, there would have been no pump shot at all.

 

[Geoff 2]

There is a simple explanation as to why the factory may have done this modification: cost.
If it was decided as second pump shot was not needed, they could do the mod that we are seeing. To remove the acc pump completely would have needed some sort of cover over the hole.....or a new carb top minus the hole.

 

[AR67GTX]

I‘ve peered at it closely with LED lights and it is definitely all cast and un-modified. It even has some casting flash in the bottom of the “u” and extending a ways up one side. The notch measures .978” from the top. The non-tapered compression portion starts at 1.2” down or nearly 1/4” below the notch.

I think I may be thinking too much about Holleys which have pretty instantaneous response. I probably haven’t been cycling the pump aggressively enough to get a good response. Although the pump well looks good from some research it helps pump performance to polish them up with some 800 or 1000 sandpaper. No response from Mikes Carbs yet.

 

[AR67GTX]

Here are a couple close ups of the notch.

 

[dvw]

I compared my photo #12 close up with photo #15. I'll bet the groove on yours is dropped down 3/16-1/4" lower than mine.
Doug