Went through this scenario with my 4429 avs. Most aftermarket kits supply a 3/4 pump. Mike's specifically lists the 11/16 for mine. The 3/4 would not allow a full stroke but the smaller one did.
Hemi AFB accelerator pump issue - initial bog on opening throttle
- Thread starter AR67GTX
- Start date
Dragon Slayer
Well-Known Member
Correct. Later in 69 they moved to larger pump for the AVS.
They appear to be selling the same kit I ordered based on the picture and shooter sizes. I’ve read buyers are a little unhappy lately when they receive it because the two larger ones no longer have brass tubes and are just drilled aluminum now. But I guess they will last far longer on the car than I will.
George (others) - what do you think about the shorter dump valve check weights? The Would seem to carry the same advantages and limitations as setting the rod on the inner hole of the pump rocker - faster, stronger discharge but reduced duration.
George (others) - what do you think about the shorter dump valve check weights? The Would seem to carry the same advantages and limitations as setting the rod on the inner hole of the pump rocker - faster, stronger discharge but reduced duration.
Not to rush you 
been following this thread. Waiting for a test run? I gave up on fixing the bog but this thread will make me try again.
been following this thread. Waiting for a test run? I gave up on fixing the bog but this thread will make me try again.Dragon Slayer
Well-Known Member
Are you racing it? I keep stock and have never messed with that mod. 66-67 cars have a richer idle then the 68 up carbs. Do you feel you have adequate contribution towards idle from the front carb?
Not racing - at 71 and 11 months even my street romping days are behind me. About the most I ever do is find a deserted piece of roadway and open a car up in 2nd and 3rd. Things happen too fast in 1rst so I seldom open a car up in it - but if there’s a bog issue that’s pretty much what you have to do. I’m balancing the idle mixture screws by tach or vacuum gage for both carbs and setting the idle air bypass equal turns out.
I did a test run in post #44 Fran but probably didn't say enough to draw attention to it. After fixing the accelerator pumps I tried a 1rst gear, roll into full throttle again. It still hesitated/ bogged for a split second but it was not quite as severe was my impression. I’m kind of surprised as I’m getting a lot more fuel now than before. So now I’m going to try a .035” shooter (or two) and see if that helps.
So does your car have a similar bog/ hesitation?
So does your car have a similar bog/ hesitation?
Not to rush you
Received my set of Edelbrock pump shooters and I was surprised that all 3 were original type with brass tubes. I put the one sized at .033 on the front carb and drilled out the .024 one to .032 and put it on the rear carb. Also received new, replacement accelerator pump return springs from Mikes Carburetors and put them in. The pump circuit is working well now. Better but still a hesitation when I lay into the throttle from a slow roll. So, I plan to put the .033 one on the front and drill out the .032 one to .040 and put on the rear. If that doesn’t take care of it I’m not sure what to do next. It should have enough timing, it’s set at 15 deg initial. The carb linkage is set to where both carbs are synced to reach full throttle at the same time.
Dragon Slayer
Well-Known Member
What type of distributor and do you know the max mechanical? I would step to .035 first. That was the largest mopar used on stock hemi even the cheater carbs.
It’s a dual point - a CAP Hemi unit for 1967 I believe by the numbers. I have an advance limiter plate from FBO in it set for 18 crank degrees (verified with dial back light) and 15 degrees initial for 33 total. I mapped the curve but would need to look through my notes on how fast it comes in. It has a Mopar performance spring set in it that is light but not as light as some spring sets. The vacuum advance unit is a repro - supposedly Hemi one - from Mega Parts I think. I’ve wondered if loping along in first at about 10 mph is opening the throttle enough to bring some of the ported vacuum advance in and then as soon as I lay into the throttle it falls out, and the centrifugal hasn’t spun up yet and can’t make up for it until it picks up rpm again. Once it passes that one little hiccup it pulls strong. If I run a little higher road speed before getting on it, it doesn’t seem to hesitate. I’ll try .035.
Dragon Slayer
Well-Known Member
So are you sure the idle jets on the primary venturi are clean and spec'd right? Transition slot clean.
I blew them all out with carb cleaner and air but I don’t believe I tried to get in those particular areas with a piece of wire or micro drill bit. I can take a piece of copper wire and try to run in the Idle port and slots. I chased all the bleeds out with drill bits.
I have both carbs equally set at idle. I probably need to run through the idle mixture adjustments with a vacuum gage. I have them set now mostly by ear, probably averaging about 1-1/4 turns out. But I don’t think the idle mixture settings play much role in fuel transition. I put a vacuum gage on it a few weeks ago and it had a steady 17 inches at idle.
I have both carbs equally set at idle. I probably need to run through the idle mixture adjustments with a vacuum gage. I have them set now mostly by ear, probably averaging about 1-1/4 turns out. But I don’t think the idle mixture settings play much role in fuel transition. I put a vacuum gage on it a few weeks ago and it had a steady 17 inches at idle.
Dragon Slayer
Well-Known Member
I would think they matter in that if you are too lean at idle your going to be leaner at transition. I am not sure how the 66-67 carbs perform compared to the 68 up. The bypass type made me wonder how well the idle mixture is. Throttle blades closed and air enters from the center of the carb away from the fuel entrance from the transistion and idle port. Opposed to the air entering along the wall of the bore at the fuel entrance. Theory question Seems Fran has the same issue?
Do you have the carbs balance equally for idle, or do you set idle on rear carb, then get the front to pickup 75 to 100 rpm, then reduce rear to get spec idle? If there balanced, you would have the rear in a leaner condition at idle versus having it carry most of the idle. Adding more air with rear carb and more idle fuel to get mixture right, then have some contribution from front carb to ensure no lean cylinders. As you punch it to accelerate the rear should be starting from a richer mixture. That may get rid of the hesitation. Just a thought to try.
Seems Fran has the same issue?Do you have the carbs balance equally for idle, or do you set idle on rear carb, then get the front to pickup 75 to 100 rpm, then reduce rear to get spec idle? If there balanced, you would have the rear in a leaner condition at idle versus having it carry most of the idle. Adding more air with rear carb and more idle fuel to get mixture right, then have some contribution from front carb to ensure no lean cylinders. As you punch it to accelerate the rear should be starting from a richer mixture. That may get rid of the hesitation. Just a thought to try.
George, I can give that a try. I remember reading someone else’s comment some time ago about that method of setting idle. So how do you set the front carb before initially setting the rear carbs idle - air bypass closed? Idle mixture screws closed?I would think they matter in that if you are too lean at idle your going to be leaner at transition. I am not sure how the 66-67 carbs perform compared to the 68 up. The bypass type made me wonder how well the idle mixture is. Throttle blades closed and air enters from the center of the carb away from the fuel entrance from the transistion and idle port. Opposed to the air entering along the wall of the bore at the fuel entrance. Theory question
Do you have the carbs balance equally for idle, or do you set idle on rear carb, then get the front to pickup 75 to 100 rpm, then reduce rear to get spec idle? If there balanced, you would have the rear in a leaner condition at idle versus having it carry most of the idle. Adding more air with rear carb and more idle fuel to get mixture right, then have some contribution from front carb to ensure no lean cylinders. As you punch it to accelerate the rear should be starting from a richer mixture. That may get rid of the hesitation. Just a thought to try.
I can’t help thinking there must be some tuning secret to this. There’s no reason a stock engine with stock carbs, distributor and in good mechanical condition shouldn‘t respond smoothly when the throttle is opened.
edit: I think I‘ve got it figured out - set the idle mixtures in both and then shut down the air bypass on the front, set idle speed with the rear air bypass, open the front to bring it up 100 rpm’s, and close the rear enough to get back to the target idle rpm’s.
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I ease off in first up to 5 - 10 miles mph, and then pretty quickly floor the throttle. There’s an immediate, momentary pause/hiccup/hesitation/bog (no backfire or miss) and then the engine picks right up and off we go, pulling strongly. It does seem the bog has lessened since fixing the acceleration pump problem. I was real surprised to see some of it still there.
In contrast, my 67 GTX with stock 440/AFB/TF from curb idle (no stall torquing) responds instantly, lights up the tires and pulls like a freight train w/o a bit of hesitation.
The one time I’ve tried a dead stock start with the Hemi at maybe 2,000 rpm or a little higher, it’s just fell flat on its face as the clutch engaged and I floored the throttle and then jerked/charged off which felt pretty tough on the drivetrain so I haven't repeated that.
In contrast, my 67 GTX with stock 440/AFB/TF from curb idle (no stall torquing) responds instantly, lights up the tires and pulls like a freight train w/o a bit of hesitation.
The one time I’ve tried a dead stock start with the Hemi at maybe 2,000 rpm or a little higher, it’s just fell flat on its face as the clutch engaged and I floored the throttle and then jerked/charged off which felt pretty tough on the drivetrain so I haven't repeated that.
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Geoff 2
Well-Known Member
There is a fundamental difference between the stock 440 & the hemi induction system when going to WOT:
- 440 has 4 throttle bores & one plenum [ per side, upper & lower ]
- hemi has 8 t/bores & two plenums.
So going to WOT with the hemi, you have a lot more air all of a sudden that needs enough fuel to be added so as not to cause a bog. The fuel is about 550 times heavier than the air, so there is a delay [ bog ] due to the fuels's inertia.
I would raise the float level about 1/8" to richen the mixture & see what it does. Easy to do, no parts reqd.
- 440 has 4 throttle bores & one plenum [ per side, upper & lower ]
- hemi has 8 t/bores & two plenums.
So going to WOT with the hemi, you have a lot more air all of a sudden that needs enough fuel to be added so as not to cause a bog. The fuel is about 550 times heavier than the air, so there is a delay [ bog ] due to the fuels's inertia.
I would raise the float level about 1/8" to richen the mixture & see what it does. Easy to do, no parts reqd.
Dragon Slayer
Well-Known Member
The secondaries do not add air until sufficient air flow based on demand to open the secondary air valves. Both primaries are open adding air and fuel via transition slot until high speed circuit activates. Secondaries have Discharge ports below the main discharge to add fuel as the secondary air valve opens until the main secondary is active. Changing float level with richen the entire fuel curve. The mixture screws can enrichen the primaries and transition.
Geoff 2
Well-Known Member
There is considerable air leakage around the velocity valves in the secondaries, when the sec throttles open.
Raising the fuel level is a quick & easy way to richen the mixture to see if it helps. And if it does not help, nothing has been lost. Meanwhile, if the pump nozzles are drilled & it makes no difference, it is pretty hard to put the metal back....
According to D. Vizard, raising the fuel level 0.100" on a Holley is worth about one jet size.
Raising the fuel level is a quick & easy way to richen the mixture to see if it helps. And if it does not help, nothing has been lost. Meanwhile, if the pump nozzles are drilled & it makes no difference, it is pretty hard to put the metal back....
According to D. Vizard, raising the fuel level 0.100" on a Holley is worth about one jet size.
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