Hemi AFB accelerator pump issue - initial bog on opening throttle

[Dragon Slayer]

The carbs were designed for dual ops. It also has progressive linkage, so the front carb does not open immediately. So the engine is ramping up the rpm before the front carb opens, though flooring it will certainly be a marginal delay. So if his mixture is fine at mid to high rpm, raising level is going to richen it across the rpm range. Not just idle jet/transition.

He bought spare nozzles so he can experiment. Large nozzle means richer sooner, but shorter contribution. Certainly, worth checking the level again and make sure fuel is even in both bowls. I have seen some odd imbalance levels on carbs I have disassembled.

Tuning guide is a little vague. You can balance them both, so they contribute even to idle speed, but that means the rear which does most of the driving and cruising starts with marginal air flow. Since front won't contribute until above about 60% throttle.

Or you get most of the idle from the rear, then add in the front to contribute about 50-100 rpm and back off the rear to get spec idle. These ensures a better mixture in the manifold to prevent any lean cylinders. This works for most street stock hemi's in the 68 up vintage which are not bypass carbs and have idle screws that open throttle blade.

I need to check a 66-67 carb to see how much transition slot is exposed at closed throttle.

 

[AR67GTX]

OK - I was going to fire it up and do some tuning but I’ll pull the carbs instead and bring the fuel levels up to a little over the FSM. Other than putting the .033 squirter in the rear and the .032 in the front, I’ll leave them alone. I thought maybe the front carb primaries flopping open might be the cause of the bog but didn’t seem to help dramatically.

 

[Dragon Slayer]

Chrysler used .028 pump jet on hemi carbs until 68 when emission standard kicked in all states. Since they leaned the idle on the rear carb, plus removed initial timing to meet idle emissions they moved to a .035 pump on rear only. Left the front alone. So I do think you have tried a bigger jet and still have the issue. Plugs all good? Hemi can foul them quickly.

 

[AR67GTX]

The plugs are pretty new Autolite and I believe OK. I haven’t noticed any missing symptoms And the insulators have been a nice light tan color when I had them out.

If the higher float levels and idle tuning make a difference, I’ll see if I can cut back on the AP nozzles then.

 

[AR67GTX]

I set the floats 1/32“ higher than the FSM which was about 3/32” higher than before. I couldn’t get the idle higher than 670 - 680 rpm on the rear carb alone, so I used the front carb to raise it to 800. Retrimming the mixture screws didn’t make any apparent difference at that point. I did notice that the lean roll off and rich roll off range is pretty narrow - not much more than 1/4 turn. Timing still at 15 deg static. Ran out of ooomph today so I’ll test it out tomorrow.

 

[Geoff 2]

A 1/32" fuel level increase is hardly likely to be noticed by the butt-meter & not worth the effort. That is is why I suggested 1/8". You need a sizeable increase to see if 'this' is going to help. It is only a temporary measure that might point you in the right direction.
Raising the fuel level does more than just increasing jet size would do. Increasing jet size richens the mixture but does not change the point at which the system initiates. Raising the fuel level richens the mixture AND starts the system earlier because the spill height has been reduced: the distance the fuel has to be lifted before it enters the booster for delivery to the engine.
If the raised fuel level helps or cures the problem, the fix could be richer jets or a slight reduction in the size of the main air bleed to start the system earlier; or a combination of both. A thin wire poked into the MAB will reduce the area.

 

[AR67GTX]

It’s actually a 3/32” increase from where I was.

 

[Dragon Slayer]

So I am asking. But seems the bog is initial when system is on idle and uses transition circuit. Not necessarily activation of Main HS circuit. Car takes off after the initial bog. Seems his car is pretty stock and he is using OEM carbs. So messing with MAB and when the main comes in doesn't seem to be issue. The economized is behind the idle by pass bleed. Hemi like most Chrysler AFB has a .0492" economizer.
My experience has been the bleeds on the carb tend to be smaller with age. Either corrosion or dirt. So carefully cleaning the bleeds and making sure economizer is clear/clean is important. Attached picture is old reference. Also Not that the term Idle Feed Restriction is not the idle jet (brass tube), but an internal restriction that Carter called economizer in there documentation.

All my carbs I run appropriate bits through to ensure venturi is at spec. I also use plenty of carb cleaner shot through all the venturi bleeds. You also need to clean the hole that leads to the transisiton slot/mixture port under the venturi.

 

[Geoff 2]

The OP says in post #76 that he gets the bog when he 'floors' the throttle. Hence why I was concentrating on the main system, not the idle/transition.
Is the engine actually stock? Or just thought to be?
Stronger step-up piston springs in the carbs might help, especially if what is in the carbs is unknown.

 

[AR67GTX]

When I rebuilt the carbs last year I ran micro drill bits through all the bleeds and passages. None were blocked but I did get a bit of crud out of a couple of them with the bits. The carbs are stock, correct venturis, etc. All jets and rods are by George’s Carter references except one of them did not have stock rods and I got the closest match I could from Mike’s Carburetors which were just a hair richer - very, very slightly. I do not have rebuild info on the engine but I’ve had the pan and intake off and from everything I can determine, it’s a stock street Hemi. It will idle 650 rpm but I keep it higher. Idle vacuum was a steady 17” when I checked it recently. The intake is stock. Stock manifolds. Pistons are probably stock or if not, slightly lower compression. You can’t see much with a camera looking straight down on the top of them from the plug hole directly above. Valves are all adjusted - I set them all .002” tighter than stock. The distributor is a rebuilt dual point with all new parts. Except for the bog hesitation when the throttle is opened from low, near idle rpm, the motor seems healthy.

Rained all day yesterday. Today or tomorrow I’ll test it with the higher float settings.

 

[Dragon Slayer]

Doesn't Main come in based on air flow and DP in venturi booster? So he is at lower rpm making a demand, but the main doesn't come in until rpm rises and airflow increases to circuit activation. I looked at a 4343 67 carb last night, and the transition slot has a hole at the bottom, so looks like a thermometer in shape. The 223 throttle blades are slotted so the hole of the transition slot is exposed. I was adjusting a float on an edlebrock for a 383, and I did notice as soon as I touched the throttle the accelerator pump squirted. Immediate effect. So I am wondering if the brass needle at the top is not seating right and bleeds down so your not getting the immediate enriching. Maybe tap it to help form a seat. Make sure it is free afterwards. You can go back and try to clean the economizer also. The bleed is larger, so you need to go in with .0492 which is odd size metric, or use a slightly smaller one and see if it is tight. It is a natural constriction from the air bleed so it can gather junk let alone any aluminum corrosion from the years. I do think your issue is accelerator or transistion related. Doesn't sound horribly bad, though it sounds like you want to resolve it. Maybe the float change will work.

One last check, I have seen people use the wrong gasket under the primary venturi. Kits have multiple gaskets and while similar looking the alignment is different. If wrong gasket is used it partially blocks the idle passage in the venturi and carb body that feeds mixture screws and transition slot.

 

[AR67GTX]

George, since I figured out the spring issue with the rebuild kits and installed the replacements that Mikes Carbs sent me, the accelerator pump issue seems to be behind me - it’s instantaneous now. My sense is that the transition circuits are working well too. I say this based on the fact that except for this hesitation/bog with full throttle, the car otherwise drives just fine. No hesitation in part throttle take offs, and no hesitation in giving it throttle while cruising down the road. I figure if I wasn’t getting any accelerator pump fuel until half throttle on the rear carb and probably 3/4 throttle on the front - the transition circuit must be doing a good job.

Several years ago I had a problem with the accelerator pump on an Edelbrock 800 on my GTX and found I had to raise the height of the pump top way above their spec to uncover the notch in the pump wall that feeds the pump (no bottom feed & check-valve). In that car, unless I really took it easy leaving a stop, I would almost always get a big bog, sometimes enough to kill the engine when it was cold. Not so with the Hemi unless I really hammer the throttle.

Pouring rain again to day but if I can test it tomorrow and it still has an issue I’ll go back and check the economized tubes again. Also the Venturi gaskets but I was really careful when I replaced those before. I did lightly tap the AP check needle on the seat of the rear carb to make sure it seats, but not the front one. I’ve also removed the squirter and held my finger over the hole and put some pressure on the pump and it seems to hold tight. I have new check valves but the old ones seem to be holding tight.

 

[Dragon Slayer]

Let us know would be interesting to find out what resolved this. I would think the largest demand on the transition circuit is from low speed. At higher speeds more of the transition is exposed and your closer to the main coming online. So working transition at mid speeds doesn't mean you're not at issue at lower rpm.

 

[Geoff 2]

It seems that the bigger pump nozzle helped. It is definitely a lean condition. Mashing the throttle lets in a lot of air into large plenum volumes, & the fuel takes time to get moving. Recipe for a bog.

Since you have some extra pump nozzles, I would try this:
- adjust the pump plunger to the top of the slot in the pump bore so that you get the max stroke from the pump. Use a 0.040" nozzle on the rear carb, pump arm in inner hole. On the front carb, use a 0.030" nozzle, outer hole in pump arm.
- theory is: big pump shot will get used quickly, but there is plenty of fuel to cover the initial lean hole. The smaller nozzle on the front carb will allow the pump shot to last longer & act as back-up.

 

[AR67GTX]

Not much of a test this morning - got to my abandoned piece of road, eased off to 10 mph, hit the throttle, car fell flat on its face, backed off and realized the throttle was hanging at about 3500 rpm. After messing with it on the roadside a while I determined it was the cable and pedal. I relaxed the cable ferrule all the way to get it down to 1400 rpm and limped home. I guess my tach wires that are wrapped into a bundle and run over the top of the cable and under the heater, got knocked out of position and down in the pedal assembly when I was replacing a flasher unit last week and didn’t let the pedal return completely.

But, it was enough test to determine that raising the floats to above spec, emphasizing the rear carb for idle, fine-tuning the idle mixture, bigger pump shooters and setting the accelerator pumps to 7/16” isn’t the immediate solution. Guess I’ll tear down the rear carb “again” this week and go back over everything again.

 

[AR67GTX]

Worn throttle shaft holes in carb?? Seems I would be having other symptoms but maybe not. I’m going to try warming it up and winging the throttle in the driveway and see if I can detect the bog with no load. If so I can try several things pretty quickly.

 

[Geoff 2]

Damn!

 

[AR67GTX]

Let us know would be interesting to find out what resolved this. I would think the largest demand on the transition circuit is from low speed. At higher speeds more of the transition is exposed and closer to the main coming online. So working transition at mid speeds doesn't mean you're not at issue at lower rpm.

I took the back carb apart again to make sure that everything (jets, Venturi, etc) are on the right side and check all the bleeds and passages again. I didn’t find anything out of order and all the passages flow carb cleaner fine. But mabye you can spot something amiss in these photos.

Front venturi and gasket. I didn’t see any other gasket that would match up.

Sec Venturi and gaskets

Continued on next post -

 

[AR67GTX]

Continued - secondary venturi

This is the only other gasket that looked like it would work but other than being narrower across the seat I don’t see any configuration difference.

Here are the rods and jets and Venturi numbers. I think we worked these out last year to conform closely to stock jetting and you also checked the Venturi numbers for me.

While I had the carb apart again I checked shaft play and the primary is pretty loose, especially on the throttle linkage side (.014”). The right side was .010”. I checked the throttle side secondary figuring it doesn’t get as much wear but it was .012”. Do you think that could be a possible cause of the off- idle bog? Unless something obvious jumps out of these photos I think I’ll probably send the carbs off later in the year to have the shafts rebushed and otherwise restored.

thanks

 

[Dragon Slayer]

Primary gaskets are correct, the secondary are wrong. The alternate you show is correct for secondary. Just 5 round holes. They come in 2 sizes make sure it is the larger one and fits properly on venturi.

Shaft tolerances are in spec for carter. The outside dimensions on the body are larger. The inner bore at the throttle blades is reduced and tighter tolerance to shaft. If you have some numbered small drill bits I would check the economize size that I showed in the earlier pictures .0492". Idle jets on primary should be .035".

What do idle mixture screws look like.