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

[Geoff 2]

I agree with post #17. Without v/valves in the secondaries too much air. On the carbs that had VVs, Carter incorporated an ingenious starter cct. When the secs went to WOT, the upper edge of the VV was adjacent to a small orifice in the booster that supplied 'start up' fuel for secondaries. Your carbs do not have this. Better off trying 625 Edel AFBs.

Another thing. Fuel pressure & n/seat size BOTH affect the float setting. A low 3 psi fuel pressure is going to need the float level figure reduced.

 

[lewtot184]

The key to geoff's post is the auxiliary discharge tube. It's under the velocity valves when they are closed. To my knowledge it's only used on carbs with the velocity valves. Basically it can eliminate bogs or hesitation at full throttle. I know that on the two carterbrocks I run there's absolutely no flat spots, even with lightened velocity weights.

 

[Polyhead]

The key to geoff's post is the auxiliary discharge tube. It's under the velocity valves when they are closed. To my knowledge it's only used on carbs with the velocity valves. Basically it can eliminate bogs or hesitation at full throttle. I know that on the two carterbrocks I run there's absolutely no flat spots, even with lightened velocity weights.

Thanks for posting the cluster modification details. I found a copy of that modification and intend on modifying four clusters from parts carburetors I have to see if it helps.

 

[Polyhead]

I agree with post #17. Without v/valves in the secondaries too much air. On the carbs that had VVs, Carter incorporated an ingenious starter cct. When the secs went to WOT, the upper edge of the VV was adjacent to a small orifice in the booster that supplied 'start up' fuel for secondaries. Your carbs do not have this. Better off trying 625 Edel AFBs.

Another thing. Fuel pressure & n/seat size BOTH affect the float setting. A low 3 psi fuel pressure is going to need the float level figure reduced.

I've suspected too much air all along, so hopefully bumping up fueling with the post #19 cluster modification coupled with 8 inHG metering springs will be enough to overcome the issue while not throwing things too rich elsewhere. If that doesn't work, I'll likely move away from AFBs and find some AVS to rebuild to make use of both the velocity valves and auxiliary air valves. I'll purchase aftermarket air cleaner bases and modify them for the lids.

Thanks for checking on the float situation. Yes, I have the float level properly set for the 3 psi per Carter's info. In my search to tune out the bog/stall, I experimented with 5 psi along with adjusting the float level accordingly, and there is no difference to either the bog/stall issue or function anywhere during operation between 3 and 5 psi, and I prefer 3 for a few reasons.

 

[lewtot184]

Those carbs you're using have somewhere between. 065 and .068 secondary jets. I'd up that a bunch to try and get fuel flowing quicker.

 

[Polyhead]

Those carbs you're using have somewhere between. 065 and .068 secondary jets. I'd up that a bunch to try and get fuel flowing quicker.

They don't have those jets. Primaries are 92s with 68/57 rods and secondaries are 80s. WOT makes a nice 12.5:1 AFR. I might try 83 secondaries along with the cluster modifications so long as I can stay above 11.5:1 at WOT.

 

[lewtot184]

Something else to think about is ignition timing curve. They had a full centrifugal distributor with 9 degree advance plates and very light springs. Combine the 9 degree plate, 16-18 degrees initial, light springs and you had 34-36 degrees total at 1100-1200rpm. That would definitely boost low to mid-rpm power and efficiency if you're cammed up.

 

[RJRENTON]

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.

Suggest
Racehemi.maxwedge.com
The site explains the different carbs on the Max Wedge plus other nuances re the Max Wedge vehicles. A compendium of information....
BOB RENTON

 

[dvw]

Even with the weighted doors it's tough to make AFB's perfect. There just isn't enough pump shot to overcome the additional air. I've run plenty of them in drag cars. You can make them pretty good. But they'll always be slightly lean at the hit. The lower the rpm you hit them at, the worse it is. I'd look for a pair of used AVS myself. The adjustable air valve will solve the issues for good.
Doug

 

[Geoff 2]

You could also put a piece of thin wire in the secondary high speed [ main circuit ] air bleed to make it smaller. This will bring in the secondary fuel earlier. Very easy to do as a quick test, just secure the wire so that it cannot fall out.

 

[Geoff 2]

To Bob Renton,

Thanks for the Max Wedge link, very interesting.

 

[Geoff 2]

3 psi fuel pressure is really not enough. Yes, the fuel bowls will fill with fuel, idle will be ok. In low gear, that fuel gets used up quicker than 2nd gear because max rpm is reached more quickly. The fuel level is dropping & may not getting replenished quick enough. A symptom would be putting in oversize secondary jets as a 'fix'...when the real problem is fuel starvation.
What size needle & seat do you have?

 

[Polyhead]

3 psi fuel pressure is really not enough. Yes, the fuel bowls will fill with fuel, idle will be ok. In low gear, that fuel gets used up quicker than 2nd gear because max rpm is reached more quickly. The fuel level is dropping & may not getting replenished quick enough. A symptom would be putting in oversize secondary jets as a 'fix'...when the real problem is fuel starvation.
What size needle & seat do you have?

The fuel supply is ruled out as an issue, and for this setup 3 psi is plenty. I have played with the pressure at 3 psi and at 5 psi with absolutely no difference at any point in the RPM range or at any point of sustained throttle. As part of diagnosing the stall/bog issue, I shut the engine off immediately upon the stall/bog, coasted over, and pulled the tops. The float level was at spec. The engine goes WOT up to 120 MPH (the fastest I've had the car so far) with a steady 12.5:1 AFR with no sign of fuel starvation.

 

[Polyhead]

Even with the weighted doors it's tough to make AFB's perfect. There just isn't enough pump shot to overcome the additional air. I've run plenty of them in drag cars. You can make them pretty good. But they'll always be slightly lean at the hit. The lower the rpm you hit them at, the worse it is. I'd look for a pair of used AVS myself. The adjustable air valve will solve the issues for good.
Doug

I appreciate the feedback from a drag racing standpoint. My experience running single AFBs is that they tend to have that lead hesitation no matter how dialed in they are. There's a reason Carter moved to the AVS after all. I'm going to see how things go for a couple test-and-tune days at the track before making my decision on whether to purchase and rebuild some AVSs.

 

[Polyhead]

I'm happy to report that I have resolved the issue to my liking. I went from 80 secondaries to 83 coupled with 8 inHG springs, and the stall I was getting has completely gone away. A very subtle hesitation remains if I transition from low throttle cruise below 3,400 RPM when mashing to WOT, but not unlike the common issue with AFBs. Mashing to WOT rolling above 3,400 RPM and the response is crisp. Sustained WOT AFR is 11.6:1, fatter than the 12.5:1 I was shooting for and likely giving up some power, but safe. Cruising in third gear at 65 MPH, vacuum is higher than 8", so I won't have issues there running the 8" springs.

I may play around with modifying two pairs of spare venturi clusters to see if they make a difference in throttle response and E.T. out of curiosity, but I'm happy with the throttle response and power as-is. If anyone comes across this thread in the future wanting to run dual AFBs without velocity valves, has a WOT transition stall/bog, and has ruled out other variables like I did (fuel supply, timing, etc.), try fattening the secondaries with WOT AFR down into the 11s and going up in spring size.

 

[GTX JOHN]

Do you have the carbs stagger jetted to match
the different length of intake passages.

I would try to find the correct carbs for optimal performance, but they will not
be cheap!

Next best thing would be to find early 625 or 750 Competition
series (They are not very expensive and are close to configuration
of the original carbs. I would then duplicate the jetting and metering rods
of the respective original carbs and start there. There is a LOT more tuning of the
accelerator pumps than just changing to Holes on the arms. There are several
good books on these carbs still availabl

Of Course, If you are not High Dollar Street Racing or at the Track ever week you really
do not worry about squeezing every last Horsepower out of it to enjoy your experience.

 

[Rebelrouser]

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.

I have never run those carbs on a cross ram. I have raced a cross ram in NSS for many years. I never was able to remove the secondary air valves and make the car leave the way I wanted, it always bogged a little. As several suggested I think the root cause of your issue is lag time between when you open the throttles, and the fuel gets pulled up the clusters. My best performance was simply reducing the weights on the air valves and then using a flow bench to set the opening point where I wanted. I kept removing weight until I got the opening equal between carbs and at the vacuum drop I wanted. Only advice I could give if you just don't want to or can't run the secondary air valves is to increase the pump shot to cover up the bog, drill the squirters big. Back in the day guys used to put a screw in the secondaries of a vacuum holley to make it open one to one. It always bogged, then they made a squirter with extension tubes to make the single squirter carb a dual squirter, it fixed the bog.

 

[GTX JOHN]

I was faster with 4224 Holley 660 Center squirters back
in the day 50 years ago. Even faster with 850 Base plates
installed. Even way faster with a 250 Nitrous plate under
each carb ( 1/2 inch stroker 426 Stage III ).