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Dual Carter AFB Bog and Stall Issue on WOT Acceleration - 413 Max Wedge

Polyhead

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Background:
I have been tuning the induction on a factory spec rebuilt 1964 413 Max Wedge and wanted to pick the brains of those familiar with the Carter AFB and particularly cross-ram AFB setups. The engine is backed by a TorqueFlite 727 with a custom PTC torque converter with 3,500 RPM flash stall. The intake is a factory cross ram 24027261 with two 1965 3871S carburetors off a 413 that I rebuilt to factory specs. These AFBs don't have secondary velocity valves or secondary auxiliary air valves like the AVS. Aside from the secondary venturi clusters, there's nothing in the venturis from the air cleaner to the throttle plates. The secondary linkage is set to factory spec. I have the primary and secondary circuits dialed in to where the engine runs very well under almost all street driving conditions except for a bog/stall issue I'll explain after the background info. The fuel supply is steady with 3/8" diameter line and fittings running from the custom tank pickup through the pump and filter into a distribution block that takes 5/16" lines to each carburetor. I have confirmed that the high-flow pump at a regulated 3 psi are keeping the bowls full and that float level is properly set. The bowls are full when the stall/bog occurs, so fuel supply at the bowls isn't a culprit.

After playing with jets, rods, and springs, the engine runs best with 5 inHG orange springs and the following air-fuel ratios:
Idle: 14:1 - 15:1
Light throttle and Cruising: 11:8 - 13.5:1
WOT: 12.5:1

I experimented with leaning out the light throttle and cruising AFR with the next leanest rod but got two 14.5:1 lean surge spots between 2,300 - 3,000 RPM that I could not tune out until richening to the above range. I tried going down one and then two primary jet sizes but got similar lean surge spots. With the current rods and jets producing the above AFR, I fine tuned the light-throttle AFR via the idle mixture screws to where I get the best overall running condition under light load with the screws 1-1/2 turns out. 1-3/8 turns out leans out the light throttle by about .3, but the engine runs better at the richer 1-1/2 turns out. I'm using factory volume accelerator pumps with the rods set in the middle hole. If I speed up the shots by setting the rods in the first hole from the fulcrum, I get a little popping off idle under load. The carburetors have the factory accelerator pump discharge nozzles.

The ignition system is a rebuild 1974 factory electronic distributor, new matching coil and ballast resistor, blue high-rev ECU, quality wires, and Autolite 85s. I dialed in timing to the following. Everything is working well.
11° initial at 850 RPM
34° with mechanical all in by 2,200 RPM (23° from mechanical)
56° total with vacuum advance (it has a factory 22° vacuum canister)

Issue:
With the background covered, now to the issue I'm struggling to root out.
The throttle response under load is excellent from idle through 6,000 RPM shift point except for the following conditions:
-When driving and mashing the throttle wide open at any point under 4,000 RPM, the engine essentially dies and can be heard pumping air. If I let off the throttle, the engine comes back immediately. If I keep my foot at WOT, the engine pumps air for about 2 seconds before picking back up and is a rocket from there on. The AFR gauge goes all over the place during the stall/bog and isn't any help diagnosing but equalizes at 12.5:1 when the engine picks back up.
-If I'm rolling in 1st or 2nd gear above 4,000 RPM and mash the throttle wide open, I don't get the same stall. At 4,000 RPM, the engine bogs slightly but quickly picks up. At 4,500 RPM and above, I get no bog but steady power.
-If I gently roll into WOT, I get smooth acceleration and no bog/stall.
-Once in WOT past the point of stall/bog, the engine runs great and doesn't starve for fuel.

I have experimented with all the metering rod springs including the 8 inHG to bring in the power step sooner and have sped up and slowed down the accelerator pumps via the linkage holes with absolutely no change in the bog/stall suggesting it's not a fuel shot issue. I don't smell raw fuel during the bog/stall, so I doubt the accelerator pumps are flooding the engine. Mechanical timing is all in by 2,200 RPM suggesting it's not a timing issue since the bog/stall still happens above that RPM.

Here's my thinking: The quick WOT bog/stall is due to a abrupt drop in velocity due to the eight throttle plates opening and the lack of secondary air valves used in the Carter AVS. When the primary and secondary throttle plates quickly open, the velocity falls off hard for a couple seconds until it builds enough where the engine picks back up. I don't get the bog/stall issue when the engine is far up in the RPM range or when rolling into WOT because there is enough velocity to overcome the increase in air volume. I have heard of people experiencing the engine falling off on hard acceleration with the AFB, but I have never had that issue with a single AFB on a low-rise intake manifold. This is the first time I've played with dual AFBs or cross-ram though, and the additional velocity needed for the ram and the additional volume from the second carburetor might be bringing the issue to light. The good thing is that normal street driving won't produce the issue since it's not common to need to mash to WOT at lower RPM. The one place on the street that it's an issue is for transmission kickdown if I'm cruising down the highway at 3,000 RPM in 3rd gear and need to hit the throttle quickly to actuate the kickdown. When I do so, the engine stalls for a second or two, the transmission drops into 2nd, and then the engine picks back up. That's obviously not an ideal situation. The car will see 1/4-mile track time where the issue will harm performance and predictability since even if brake standing at 4,000 RPM (if I can even predictably hold the car from moving) there is still a slight bog when going WOT off the line to where I will need to learn to roll into it at the same timing just enough to stop the bog.

Can anyone with experience running dual AFBs and or/cross-ram induction confirm if this particular bog/stall issue is a normal design side effect? Did the 413 and 426 Max Wedge engines have this issue from the factory? Does anyone have advice on how to improve the bog/stall issue more than I already have? Because the car is a period build and uses the smaller-diameter AFB air cleaners, I don't want to move to Carter AVS or Edelbrock AVS carburetors if I can avoid it.

Thanks for the help.
 
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Which head and which hp?
Have you tried 38*?
Sounds like you're trying to street drive a racecar :lol:
Need a 5500 stall!
 
The AFB carbs do not have an air valve in the secs like the AVS, but they DO have velocity valves between the boosters & the t/blades.

There were a number of different VVs used, that varied in the attached counterweights & 'angle of attack'.
The VV control the sec system. Jon Hardgrove, of the Carb Shop, in Eldon Missouri would likely be able to supply the correct VVs. He bought all the old Carter stuff.
 
3psi sounds low for fuel pressure
Yeah; I would try for 5-6 p.s.i. May not be supplying enough volume at W.O.T. What year car is this in? Late '63 and '64 used Carter 3705's with the bigger tops, like an Edelbrock. '62 and early '63 had the small top 3447's.
 
You are correct in your thought that when the throttle goes open quickly there isn't enough fuel to compenste for the air coming into the manifold. Is it the secondaries open too quickly? Or not enough pump shot? Two things I would try. First try the accelerator link rod hole in the upper arm closest to the air horn. This well help with pump shot. No help? Then cut two 1 1/2" pieces of small vacuum hose. You can jam one each in at the outer edge near the counter weight of the secondary air doors to keep them from opening. It'll run on the front barrels only. This will tell you if its a secondary opening issue.
Doug
 
do i understand that you don't have the velocity valves in the secondaries? are these the small afb's that were used on early 383/413/426 engines? the factory 3447 and 3705 carbs had velocity controlled secondaries.
 
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The AFB carbs do not have an air valve in the secs like the AVS, but they DO have velocity valves between the boosters & the t/blades.

There were a number of different VVs used, that varied in the attached counterweights & 'angle of attack'.
The VV control the sec system. Jon Hardgrove, of the Carb Shop, in Eldon Missouri would likely be able to supply the correct VVs. He bought all the old Carter stuff.
The Max Wedge 3871S, like many AFBs, don't have velocity valves unfortunately.
 
3psi sounds low for fuel pressure
AFB and AVS run just fine on 3 - 5 psi with proper volume.
Yeah; I would try for 5-6 p.s.i. May not be supplying enough volume at W.O.T. What year car is this in? Late '63 and '64 used Carter 3705's with the bigger tops, like an Edelbrock. '62 and early '63 had the small top 3447's.
Fuel supply into the bowls isn't the issue. As I said, the engine runs great at WOT. I've confirmed that the bowls are full of fuel when the stall/bog occurs, so psi and pump volume variables are ruled out from the issue.
 
You are correct in your thought that when the throttle goes open quickly there isn't enough fuel to compenste for the air coming into the manifold. Is it the secondaries open too quickly? Or not enough pump shot? Two things I would try. First try the accelerator link rod hole in the upper arm closest to the air horn. This well help with pump shot. No help? Then cut two 1 1/2" pieces of small vacuum hose. You can jam one each in at the outer edge near the counter weight of the secondary air doors to keep them from opening. It'll run on the front barrels only. This will tell you if its a secondary opening issue.
Doug
I have moved the accelerator pump to all three holes without any change to the stall/bog. Using the 8 inHG metering spring to get the rods into the power step sooner also does nothing to the stall/bog. I have the secondary linkage set to factory specs, so they are opening in the correct sync with the primaries per the manual. I wonder if trying to bend the rods to delay the secondaries will help, although doing so may mean they don't fully open at WOT. These AFBs don't have a secondary air valve to jam shut, so I can't use that as a diagnostic tool unfortunately.
 
do i understand that you don't have the velocity valves in the secondaries? are these the small afb's that were used on early 383/413/426 engines? the factory 3447 and 3705 carbs had velocity controlled secondaries.
Yes, you're following me. No velocity valves or auxiliary air valves in the secondaries on these AFBs. The only things in the secondary venturis between the air filter and the throttle plates are the secondary venturi clusters. Those are free of debris and the vent holes uniform/undamaged and clear.
 
Ok, but are these carbs like the early afb that has the back clusters sitting parallel to the throttle shafts and velocity weights aren't possible?
 
I had a set of early AFB's from a 1962 Chrysler 300H on an inline intake that I put on a 426 Street Wedge in a 1964 Polara 500 convertible I once owned. These both had velocity valves under the secondary venturies, and worked well. Maybe this was the secret to getting 2 carbs to transition smoothly. Some guys tuned these velocity valves by drilling a hole in them, removing weight to open quicker.

45.jpg
 
Ok, but are these carbs like the early afb that has the back clusters sitting parallel to the throttle shafts and velocity weights aren't possible?
Correct. There is no provision for the auxiliary air valve. The secondary venturi clusters are placed like the primary clusters. Here's what they look like from the manual.

afb.jpg
 
I had a set of early AFB's from a 1962 Chrysler 300H on an inline intake that I put on a 426 Street Wedge in a 1964 Polara 500 convertible I once owned. These both had velocity valves under the secondary venturies, and worked well. Maybe this was the secret to getting 2 carbs to transition smoothly. Some guys tuned these velocity valves by drilling a hole in them, removing weight to open quicker.
Thanks. My thinking is that this bog/stall is part of the nature of the beast with this particular Max Wedge induction. I surmise if I purchased and rebuilt a pair of AFBs with velocity valves or AVS with velocity valves and auxiliary air valves that the bog/stall would disappear either from the start or by tuning the additional components. For some reason, Mopar ran these AFBs when GM used auxiliary air valve versions, but Mopar obviously found they were not the best choice since they stopped using them. In my desire to remain correct to the build, I may have baked in the issue. If that's the case, I'm not the only Max Wedge with the bog/stall. With these AFBs, I'm not sure any additional tuning can be done to alleviate the bog/stall.
 
Bear in mind that Chrysler never intended that these cars would be primarily street driven when they designed them. That is a huge intake plenum to work with, and may not be too sensitive to part throttle demands. Still, there must be other guys out there that drive their's to shows, etc. I think your choice of carbs is not helping you. A good buddy of mine has a '62 Dart Max Wedge clone, and he drives it quite a bit in the summer. It has '62 heads and intake/exhaust on a 440 motorhome block, so only has about 8.5 compression. His engine runs 2 Edelbrock 750 cfm. His car has a newer A833 and Dana 60 rear end. I have driven it, and it runs pretty smooth. It even idles pretty good with solid lifters.

P6190054.JPG


P9120055.JPG
 
That manifold with stock carbs was never intended to have a bog or hesitation. The problem is they're the wrong carbs. Original 3447 carbs were larger but had velocity controlled secondaries. The carbs you have were intended to be used with dual plane intakes and small port/small valve. Small cam engines. The s/s manifold is basically a large plenum log ram with extended runners. The small accelerator pump in those carbs just can't cover for 10sqin's of venturi area opening on a bunch of plenum/runner volume. There are some mods that can be done to the secondary boosters but I doubt they'd cover the hole in the fuel curve. If it were me I'd find a pair of edelbrock 1405's and go from there. The 1405 is the same size as a 3447 but has a better accelerator pump circuit plus velocity controlled secondaries.
 
Yes, you're following me. No velocity valves or auxiliary air valves in the secondaries on these AFBs. The only things in the secondary venturis between the air filter and the throttle plates are the secondary venturi clusters. Those are free of debris and the vent holes uniform/undamaged and clear.
I missed the sentence. "These AFBs don't have secondary velocity valves or secondary auxiliary air valves like the AVS." Then there is no method of slowing the air down or any accelerator pump on the secondaries. Not sure if there is any way to fix this issue. Big plenum, no way of adding fuel to the secondaries quickly.
Doug
 
This a copy of the old jetting tips for those carbs, but for use with a single carb dual plane intake. Basically it is showing the user how to add a drizzler to the secondary nozzle discharge tube. A .040" hole is drilled from the under side of the discharge tube with a .110" approach hole. This allows for a quicker discharge but I seriously doubt it will come close to curing the problem. Those carbs also have very small secondary main jets. No longer available. More jet area with the added drizzlers may help a little, but a better carb is the solution.

20240325_145740.jpg
 
sounds like u have the wrong carbs on it,,,carter with chrysler made the 3447 and 3705 specificlly for the crossrams. they r unique in that they r designed for the characteristic's of the application....Those carbs r not. and the fuel pressure should 5-6 lbs...i ran a set of 3705 off al corda's car a few years ago and it was a beast at any rpm range. now i run hemi holleys and its like fuel injection throttle response..But they r Designed for the application.
 
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