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.