Stalling on Hard Stops

[EngineerDoug]

Hello,

I have a six pack setup on a 493" stroker, with a 727 automatic transmission, and a ~3000 rpm converter. Power disc brakes with a vacuum reservoir.

The cam is a mechanical grind, similar to the Mopar 528 but with 114 degrees lobe separation. Initial advance is 20 degrees, and idle is 900 rpm in gear with 15" vacuum.

All of this is well and good, but when I use the brakes for a "mild" stop, the engine tries to stall on me. I have an AFR gauge and can see the AFR spike up to perhaps 16 when this happens. It is even more noticeable on a downhill grade, say 15 or 20 degrees.

Fuel level at the bottom of the sight plugs on all 3 carbs. Fuel pressure is 6psi (regulated).

I can imagine that the fuel in the bowls will slosh forward when stopping, but this behavior feels unsafe to me. Especially since these are far from panic stops.

Has anybody experienced this before? Is it par for the course with a center hung Holley?

Any thoughts on how to reduce this? Jet extensions? Thanks!

 

[AR67GTX]

Edit - on second thought jet extensions might help but they may interfere with the floats. This may be one case where raising the float level slightly might help.

 

[WileE]

What action does a vacuum gauge do under the conditions you describe ? Not idle, but during this condition you mention.

 

[Potts]

Are you thinking the booster is robbing too much vacuum,
Causing the stall?

 

[PeteyDaMan]

I'm having the same issue on my 512 due to low vacuum. In my case I'm getting a hydroboost setup

 

[Potts]

Okay, so in essence, it's like a vacuum leak, causing the stall.

 

[EngineerDoug]

I can check to see what vacuum does under this condition. Help me understand this theory in the meantime:

When I step on the brake, does the booster draw vacuum from the manifold? If I am not mistaken there is a check valve on the booster. Could this check valve be leaking? I would think having the vacuum canister should isolate the manifold from the vacuum chamber in the booster. Is there a way to check the operation of this without the engine running? Mebbe with a vacuum pump?

As for the theory of fuel level sloshing...if I did experiment with higher float settings, how high is too high? Center of the sight hole?

If it is a fuel level issue, jet extensions might help but cause a problem under acceleration. Under acceleration the fuel would move away from the jet extensions.

 

[Ron H]

Carb float adj was something I encountered eons ago on a 4bbl carb I rebuilt causing this symptom. My goof being the 1st carb I rebuilt. Vacuum check is something I thought of as well, or even possible leak. Installed a reserve tank and the b/a change wasn’t much at all; maybe garnering 2-3 inches, though can be just enough of a back up to maintain what the brakes need. But, symptoms though are usually poor braking action that you’re not describing…

 

[Potts]

Yes. Easily checked with a vacuum pump w Guage.
Basically.
In theory, The check valve is designed to suck out air that is trapped in the brake booster without letting additional air enter the cylinder. ... This part connects the body of the brake booster to the vacuum hose and is a safety solution that still allows the brakes to work – for a few pumps if the engine stalls.

 

[AR67GTX]

I doubt jet extensions will help. You will have to change out your floats to with something like this. https://www.amazon.com/Holley-116-1...ocphy=9025810&hvtargid=pla-572069474861&psc=1

A slight raise of the float level as an experiment will tell you if it’s worth pursuing. If turning the float adjusting nut 2 flats counter-clockwise makes no difference then it’s probably not the problem. Maybe the vacuum issues described above.

 

[EngineerDoug]

I will check out the situation with the PB booster - but if this were the issue, wouldn't I see engine vacuum drop and idle falter stomping on the brake with the car parked?

As for jet extensions....these probably aren't the answer...I think. For one thing, there are only jets in the center carburetor; the end carburetors use jet plates. I have a hard time imagining that the jets are being uncovered under the conditions I have seen. More likely the shift in the fuel level affects the level in the emulsion passages in the metering block, and that affects the mixture.

I will try raising the floats a bit and see what that does.

Thanks for the suggestions/thoughts.

 

[Potts]

In theory, Vacuum should be highest upon de- acceleration. Not sure about your particular readings at idle, vs at a higher rpm during driving conditions.
Would need to see a Guage under the stall condition to know for sure.

 

[6872n73]

Are you running any vacuum advance?

 

[EngineerDoug]

Yes, running vacuum advance from manifold vacuum. Advance can adds about 8 degrees. The advance is "all in" above 11 inches of vacuum.

I will keep an eye on the vacuum when the faltering happens - to see if the advance drops out.

 

[Frustration]

I had this same issue with my '69 6bbl road runner. It was absolutely float related. I found that my floats on the outboard carbs were coming into contact with the jets. I run a Pro Maxx metering block in order to use jets on the outboard carbs instead of plates. You can purchase floats like "AR67" suggested above. I ground the ones I had in order to clear the float when it swings. I saw exactly the same issue as you describe, and the confusing part was that I had the floats adjusted just like you...at the bottom of the eye. While the car was sitting in the garage it was fine. Under power, the float hits the jets and the problem continued. Lots of people told me that if you grind the black, epoxy floats...that they will sink. I've run mine for years with no issues. Good luck.

 

[biomedtechguy]

I have the same problem, but it's sporadic, and NOT vacuum related as I have manual brakes. 70 V-code Roadrunner.
On moderate or harder braking, sometimes, the engine will stall from fuel deprivation (I believe).
I have a dual wideband O2 sensor system from Innovate, and I'll see what the A/F ratio does under those circumstances.
My biggest problem is finding time to drive my car!

 

[WileE]

The condition could very well be due to carb/float/sloshing issues. But, as I've said and seen all too often over the years, don't go directly to the other side of the bridge without figuring out how to get over it. Meaning, why go through changing this or that, only to see zero results. DIAGNOSE the problem first, eliminate the easy fix first BEFORE delving into useless throwing parts and wasting time on unsuccessful attempts.

 

[biomedtechguy]

DIAGNOSE the problem first, eliminate the easy fix first BEFORE delving into useless throwing parts and wasting time on unsuccessful attempts.

ESPECIALLY with a 6bbl!
Another reason for me to get my A/F ratio gauge back in action. It was taken off my dash when I had a ton of work done last year, and I have to get it setup again. It can tie into 2 modules I have for my Dakota Digital RTX gauge system, but I haven't done that yet. Geesh! I hardly have any time to drive it!

 

[EngineerDoug]

On the subject of the floats - I experienced the issue of floats contacting either the jets or the jet adapter plate as well. That was a strange one to be sure.

I had been running the engine with the plates with no problem and then suddenly found one of the end carbs was flooding big time. This was after I had drained the bowl, so the float was at the bottom of its travel. Started the car with the air cleaner off (thank goodness!) and saw the fuel pouring out the vent. Checked the needle & seat; they looked fine. No dirt in the bowl. Note these are new carbs, BTW. I used my inspection camera to peer inside the bowl through the sight hole and saw the float trapped in the "down" position. The float was touching the jet plate. So why did this not happen from the day I assembled the carb with the jet plate? It must have been compression of the bowl gasket over a few months. That was a weird one, to be sure. The jet plates are gone for the time being.

Note that this issue was found and fixed before the "stalling on hard stop" issue I am now confronting. I bring it up just in case anybody else might walk into this trap. I suppose the lesson is, check everything you can anytime you deviate from stock. Don't assume aftermarket = it will fit perfectly.

I have the center carb apart to do a power valve change, but once I get it put back together I should have a better idea of what's going on. The afr guage will probably prove invaluable, and I am glad I added it.

Thanks.

 

[biomedtechguy]

The afr guage will probably prove invaluable, and I am glad I added it.

Here's just one way my Innovate dual wideband O2 sensor system has proven to be the useful tool that I expected. I'm sure most if not all who read or post in the A12 thread have heard of the "lean spike on tip in" of the outboard carbs. I had, although it wasn't "causing a problem" for performance, and I have WAY too many other important things to adjust on the car than my 6bbl, which is running fine...not perfect, but fine.
Anyway, with the wideband system, I have seen the lean spike, basically less than 1 second total duration, upon opening of the outboard carbs, the A/F Ratio goes to about 19.5:1 lean.
For now, all that means to me is if you have a 6bbl and Nitrous, delay the spray for about 1 second after the outboard carbs open.