Hard Braking causing stalls.
 

Hello everyone. I have a Gen IV EM with a FE427 and I am stalling when medium to high braking. I have read it is probably a vacuum leak but the connection from my manifold to brake booster seems to be solid. Is there anything I can do to track down this problem like a vacuum test on the brake booster or such. Thanks for any advice. I don't mind downshifting but if I don't have time it stalls.

What kind of carb do you have? When I was running dual fours with my 427 my engine would stumble with hard breaking in some of the corners at road courses. In my case it was improper float levels and float bowls. Actually ended up changing carbs which had a different type of float bowls. And yes it can be a vaccuum leak.

Stalling due to fuel sloshing under hard braking (and cornering) is a common problem with Holley carbs. Vent whistles can help, and I had to add a 2" length of rubber tubing to the top of my vertical vent tube to keep fuel from sloshing up that tube and into the intake, flooding the engine.

BTW, if you're cornering or braking hard enough to slosh fuel, think about what your oil is doing too. I ruined my first engine when hard cornering and braking led to oil starvation at the pump. There are solutions to that problem too. Until then, don't be too aggressive for too long.

One more thought. If it were me, I'd go to a safe, remote place, block off the vacuum line to the brake and do a hard stop using manual only braking. If it stalled with the vacuum disconnected, its likely the carb. If it didn't stall, then its likely in the brake.

Vacuum is not going to cause fuel sloshing......it's gas flooding......That's why you have to learn heel/toe techniques when driving a real car...��

Thanks Tommy I will try that to narrow it down between the carb and brake booster. I don't think its the carb but I could be wrong. I wasn't sure how to test the brake booster.

Clois my carb is a QFT.

QuickFuel is still a Holley design style carb. The issue is most likely the vents on the secondary bowls. Under hard de-acceleration, the fuel sloshes out of the vent opening into the throats of the carb and stalls the engine. Center pivot bowls are an improvement on this; adding "whistles" to the vents will help some but it remains a constant issue on this style carb.

Adding "road race" floats to the carb will help considerably for the cornering issues.

I fought it for years on my setup and finally went to a weber system. Got rid of the stalling issue - but found twenty other issues %/

Paul%/

QuickFuel is still a Holley design style carb. The issue is most likely the vents on the secondary bowls. Under hard de-acceleration, the fuel sloshes out of the vent opening into the throats of the carb and stalls the engine. Center pivot bowls are an improvement on this; adding "whistles" to the vents will help some but it remains a constant issue on this style carb.

Adding "road race" floats to the carb will help considerably for the cornering issues.

I fought it for years on my setup and finally went to a weber system. Got rid of the stalling issue - but found twenty other issues %/

Paul

Paul is right on all points. :) try this it is cheap, get you some rubber fuel line maybe 3/8" and extend your vents an inch or so. Then find a parking lot where you can try some hard breaking.

The road racing bowls are available for QFT but they might surprise you with their cost.

Clois

The stalling on hard braking can be caused by two different sources. As has been described previously, the rear bowl can be sloshing fuel forward which can cause fuel to slosh up and out of the rear vent and into the main body resulting in momentary flooding of the engine with fuel, thus the stalling. This condition can be cured by lowering the float level, installing "vent whistles", installing road race floats, or buying a 6 inch piece of rubber hose and attaching it to both vent tubes so that if fuel does slosh out of the secondary vent, all it does is go up the tube and into the primary float bowl. If you try this fix, punch a couple of holes at the top of the apex of the hose so there is still a venting function if needed.

The second cause of stalling under hard braking can be from the primary float bowl. When you're hard on the brakes, fuel in the primary bowl flows forward and can cause momentary fuel starvation, as the jets are at the rear of the float bowl and when the fuel sloshes forward, they can be sucking air for a moment. The cure for this is higher fuel level in the bowl, jet extensions, and/or road race floats, or a combination of the above.

Start with the hose connecting the two vents, that will tell you if its the primary bowl sucking air, or the secondary bowl over flowing. Once you know which bowl is causing the problem, and it can be both, you can address each one step by step until the problem is gone.

First, make sure your fuel level is set properly in both bowls before you begin.
Good luck, been there, done that.

Thanks for the suggestions. I have a quick fuel with road race floats that floods the engine under extreme braking.

Thanks everyone I will check this as soon as it stops raining in South Florida. :(

G'Day guys I read this thread as I am having the same issues with my Cobra, I actually agrre with all your suggestions with regards to the vent pipe extension and the vent whistles. I can do the vent extension but can someone help me with the vent whistles as I really am not sure how to install them, any help with directions or a youtube vid would be appreciated, Rob

I reduced my bowl levels but I thought I remembered (and make no claims of motor expertise) that cam overlap and whether I de-clutched early enough to keep RPMs up was a factor too. Mine is a Holley. Usually when I brake 8/10s or more there are "distractions." On the street I just work around it.

Google "Holley vent whistle" for video and watch.

... and you don't necessarily even need the plastic whistle. I use the little black Holley "Metering Body Vent Baffle" that you can see here in black. Part number 85-626. It works surprisingly well. It fits on the two little metering block prongs in between the bowl gasket and the metering block. It's a slip on fit, no installation other than pulling your bowl. Cost is less than three dollars.;)

http://www.clubcobra.com/photopost/d...baffle0011.JPG

I made a cross over vent tube like Holley makes, which made a big improvement on mine. Took a small copper tube (forget the size) and shaped it to fit between the two vent tubes. Then drilled small vent holes on the top side. The tube snugly slides into the carbs vent tubes and the top of the air cleaner keeps it from coming out even if it vibtrates loose.

I plan on adding Metering Body Vent Baffle to my Holley, but was also intrigued by these. They are pricey, but might be worth if they really work.

Has anyone tried them?

https://www.holley.com/products/fuel...s/parts/26-342

dcdoug,
Just curious .... are you planning to do some jumping like the Dukes of Hazard?

A cobra is lighter and should fly farther....:p

I don't plan on getting upside down in the cobra other than when I work on it. ;)

I like the ball check-valve that these valves offer to keep fuel from sloshing out the vents under hard braking. The forward cant of many of our motors in these cars doesn't help this problem in hard braking either. I made my own vented crossover tube which seems to help, but doesn't complete solve the problem either.