Six Pack Stalling on hard braking

[1968rt]

440 six pack starts right up, idles great but has been stalling on hard braking. Im guessing float height or vacuum loss from power booster. What do you guys think? Mild motor .528 cam, 4 speed, 3:54 around 500HP.

 

[PurpleBeeper]

Vacuum leak is my first guess

 

[Rishi]

Vacuum leak is my first guess

this is my guess, too. I resolved a similar leak in a 440-4 car by connecting and watching a vacuum gauge during normal driving, coasting and braking. I did not want to poke a hole through the firewall for the vacuum line (to read the gauge within the cabin) so I zip-tied the gauge to the wiper arm. After a few days of seeing the issue and fiddling around, I found the vacuum leak, which turned out to be the lower (carburetor) flange gasket with the intake.

 

[RJRENTON]

440 six pack starts right up, idles great but has been stalling on hard braking. Im guessing float height or vacuum loss from power booster. What do you guys think? Mild motor .528 cam, 4 speed, 3:54 around 500HP.

View attachment 1864107

Center carb float level too high.......I run mine with fuel level JUST BELOW SIGHT PLUG OPENING......no dribbling....front/rear carbs JUST AT FUEL LEVEL SIGHT PLUG........IT IS NOT A VACUUM LEAK.......just my opinion......
BOB RENTON

 

[1968rt]

Well I guess I will start with the float, then add some timing, then check for a leak. What a pain!!

 

[Aron Gleason]

Best way to check for a vacuum leak is to spray carburetor cleaner around the base of the carburetors and intake, then go from there.

 

[1968rt]

Best way to check for a vacuum leak is to spray carburetor cleaner around the base of the carburetors and intake, then go from there.

I checked the manifold and carbs, they are fine. I suspect the booster though.

 

[68 Sport Satellite]

I checked the manifold and carbs, they are fine. I suspect the booster though.

why do you suspect booster?
Was this a non-issue before on hard braking and now suddenly it is or is this a new or tuned carb setup and suddenly the issue started? When I first installed my Quickfuel 735 SS on my 451, I had the same hard braking stall problem. I lowered the fuel float level to bottom of sight glass and the issue went away.

 

[Dennis H]

Under hard braking, my 440 w/850 Demon would almost stall after panic stop. Only one time. Leaving a 4way stop, Linda screams THEY DONT STOP! Guess what, they DO stop. One other time, I “saved it. Try not to brake hard unless urgent.

 

[1968rt]

why do you suspect booster?
Was this a non-issue before on hard braking and now suddenly it is or is this a new or tuned carb setup and suddenly the issue started? When I first installed my Quickfuel 735 SS on my 451, I had the same hard braking stall problem. I lowered the fuel float level to bottom of sight glass and the issue went away.

Well I had a mother of a time getting these carbs right. Drilled out the idle screws on the end carbs and finally got the mixture right. Then I had a slight vacuum leak, which turned out to be at the base plates. Problem now is that the car was laid up for a year and a half. I lost two close members of my family so the car sat. This is the first time since Feb 2024 that it ran. Maybe I need to check the needle in the center carb also. Could be old gas crap in there also! Never trusted the repo booster. The brakes just didnt feel 100% right.

 

[Budnicks]

My guess would be float levels too high on center carb too

if not

maybe it's a cracked/split or leaking hose, vacuum leak
could be a carb base gasket too

 

[1 Wild R/T]

Are the fuel baffles in place? Top of the metering block..

Or the better design, known as a whistle...

And the hotrodders fix when both of the original Holley designs fail.... is a vent tube extension...

The one shown is the fancy pants version... Usually just a length of 5/16 hose to raise the vent higher prevents fuel sloshing out of the vents which causes the stumble/dying...

 

[eldubb440]

if fuel is sloshing/flooding due to a high float; it could be any of the 3 carbs, no?

 

[68 Sport Satellite]

Under hard braking, my 440 w/850 Demon would almost stall after panic stop. Only one time. Leaving a 4way stop, Linda screams THEY DONT STOP! Guess what, they DO stop. One other time, I “saved it. Try not to brake hard unless urgent.

GEEZ Linda...
Dennis, I can hear your voice now as you explain this. Too funny - I'm laughing out loud now.

 

[1 Wild R/T]

if fuel is sloshing/flooding due to a high float; it could be any of the 3 carbs, no?

Yup... Which is why you extend all three vents

 

[AR67GTX]

The carb bowls are front mounted so on hard braking, if it’s the carbs the fuel is stacking up forward starving the jets. The solution to that is raising the float level slightly or installing jet extensions. Fuel bowl baffles and flooding from a hard stop should not be a factor on six pack carbs.

However, the vacuum brake booster also is a good possibility. Could be as little as a bad vacuum check valve.